Biochemistry_Lippinco

Biochemistry_Lippinco. and a pyridoxal phosphate (PLP)-requiring decarboxylase (see figure at right). RQ4.Answer = B. The exocrine pancreas secretes enzymes required for the digestion of dietary carbohydrate, protein, and fat. The endocrine pancreas secretes the peptide hormones insulin and glucagon. Damage that affects the functions of the pancreas would lead to diabetes (decreased insulin) and steatorrhea (fatty stool), with the latter the consequence of maldigestion of dietary fat. As was seen with the rise of troponins in a myocardial infarction and transaminases in liver damage, loss of cellular integrity (as would be seen in autodigestion of the pancreas) results in proteins that normally are intracellular being found in higher-than-normal concentrations in the blood. Secretin causes the pancreas to release bicarbonate to raise the pH of the chyme coming to the intestine from the stomach. Pancreatic enzymes work best at neutral or slightly alkaline pH. Pancreatitis is seen in individuals with


Biochemistry_Lippinco

Biochemistry_Lippinco. to release bicarbonate to raise the pH of the chyme coming to the intestine from the stomach. Pancreatic enzymes work best at neutral or slightly alkaline pH. Pancreatitis is seen in individuals with hypertriglyceridemia as a result of a deficiency in lipoprotein lipase or its coenzyme, apolipoprotein C-II.


Biochemistry_Lippinco

Biochemistry_Lippinco. Case 4: Answers to Thought Questions TQ1.A. The rise in cytosolic NADH seen with ethanol metabolism inhibits glycolysis. The glyceraldehyde 3-phosphate dehydrogenase step requires NAD+, which gets reduced as glyceraldehyde 3-phosphate gets oxidized. With the rise in NADH, glyceraldehyde 3-phosphate accumulates.


Biochemistry_Lippinco

Biochemistry_Lippinco. B. Glyceraldehyde 3-phosphate from glycolysis is converted to glycerol 3phosphate, the initial acceptor of FA in triacylglycerol (TAG) synthesis (see figure at right). FA are available because of increased synthesis (from acetyl CoA, which is increased as a result of both increased production from the acetate product of acetaldehyde oxidation and decreased use in the TCA cycle), increased availability from lipolysis in adipose tissue, and decreased degradation. The TAG produced in the liver accumulate (due, in part, to decreased production of very-low-density lipoproteins) and cause fatty liver (steatosis). Hepatic steatosis is an early (and reversible) stage in alcohol-related liver disease. Subsequent stages are alcohol-related hepatitis (sometimes reversible) and cirrhosis (irreversible).


Biochemistry_Lippinco

Biochemistry_Lippinco. TQ2.The rise in NADH favors the reduction of pyruvate to lactate by lactate dehydrogenase. Lactate decreases the renal excretion of uric acid, thereby causing hyperuricemia, a necessary step in an acute gouty attack. [Note: The shift from pyruvate to lactate decreases the availability of pyruvate, a substrate for gluconeogenesis. This contributes to the hypoglycemia seen in AK.] TQ3.Prothrombin time (PT) measures the time it takes for plasma to clot after the addition of tissue factor (FIII), thereby allowing evaluation of the extrinsic (and common) pathways of coagulation. In the extrinsic pathway, FIII activates FVII in a calcium (Ca2+)-and phospholipid (PL)-dependent process (see figure at bottom right). FVII, like most of the proteins of clotting, is made by the liver. Alcohol-induced liver damage can decrease its synthesis. Additionally, FVII has a short half-life, and, as a γ-carboxyglutamate (Gla)-containing protein, its synthesis requires vitamin K. Poor nutrition can result


Biochemistry_Lippinco

Biochemistry_Lippinco. liver damage can decrease its synthesis. Additionally, FVII has a short half-life, and, as a γ-carboxyglutamate (Gla)-containing protein, its synthesis requires vitamin K. Poor nutrition can result in decreased availability of vitamin K and, therefore, decreased ability to clot. [Note: Severe liver disease results in prolonged PT and activated partial thromboplastin time, or aPPt.]


Biochemistry_Lippinco

Biochemistry_Lippinco. TQ4.Administration of folate can mask a deficiency in vitamin B12 by reversing the hematologic manifestation (macrocytic anemia) of the deficiency. However, folate has no effect on the neurologic damage caused by B12 deficiency. Over time, then, the neurologic effects can become severe and irreversible. Thus, folate can mask a deficiency of B12 and prevent treatment until the neuropathy is apparent. III. Focused Cases Case 1: Microcytic Anemia Patient Presentation: ME is a 24-year-old man who is being evaluated as a follow-up to a preplacement medical evaluation he had prior to starting his new job. Focused History: ME has no significant medical issues. His family history is unremarkable, but he knows little of the health status of those family members who remain in Greece. Pertinent Findings: The physical examination was normal. Routine analysis of his blood included the following results:


Biochemistry_Lippinco

Biochemistry_Lippinco. Pertinent Findings: The physical examination was normal. Routine analysis of his blood included the following results: Based on the data, hemoglobin (Hb) electrophoresis was performed. The results are as follows: H = High; L = Low. [Note: HbA includes HbA1c.] Diagnosis: ME has β-thalassemia trait (β-thalassemia minor) that is causing a microcytic anemia (see image at right). Ethnicity (such as being of Mediterranean origin) influences the risk for thalassemia. Treatment: None is required at this time. Patients are advised that iron supplements will not prevent their anemia. Prognosis: β-Thalassemia trait does not cause mortality or significant morbidity. Patients should be informed of the genetic nature of their autosomal-recessive condition for family planning considerations because homozygous β-thalassemia (Cooley anemia) is a serious disorder. Case-Related Questions: Choose the ONE best answer.


Biochemistry_Lippinco

Biochemistry_Lippinco. Case-Related Questions: Choose the ONE best answer. Q1. Mutations to the gene for β globin that result in decreased production of the protein are the cause of β-thalassemia. The mutations primarily affect gene transcription or posttranscriptional processing of the messenger RNA (mRNA) product. Which of the following statements concerning mRNA is correct? A. Eukaryotic mRNA is polycistronic. B. mRNA synthesis involves trans-acting factors binding to cis-acting elements. C. mRNA synthesis is terminated at the DNA base sequence thymine adenine guanine (TAG). D. Polyadenylation of the 5′-end of eukaryotic mRNA requires a methyl donor.


Biochemistry_Lippinco

Biochemistry_Lippinco. C. mRNA synthesis is terminated at the DNA base sequence thymine adenine guanine (TAG). D. Polyadenylation of the 5′-end of eukaryotic mRNA requires a methyl donor. E. Splicing of eukaryotic mRNA involves removal of exons and joining of introns by the proteasome. Q2. HbA, a tetramer of 2 α and 2 β globin chains, delivers O2 from the lungs to the tissues and protons and CO2 from the tissues to the lungs. Increased concentration of which of the following will result in decreased O2 delivery by HbA? A. 2,3-Bisphosphoglycerate B. Carbon dioxide C. Carbon monoxide D. Protons Q3. What is the basis for the increase in HbA2 and HbF (fetal Hb) in the βthalassemias? Q4. Why is the allele-specific oligonucleotide (ASO) hybridization technique useful in the diagnosis of all cases of sickle cell anemia but not all cases of β-thalassemia? Case 2: Skin Rash Patient Presentation: KL is a 34-year-old woman who presents with a red, nonitchy rash on her left thigh and flu-like symptoms.


Biochemistry_Lippinco

Biochemistry_Lippinco. Case 2: Skin Rash Patient Presentation: KL is a 34-year-old woman who presents with a red, nonitchy rash on her left thigh and flu-like symptoms. Focused History: KL reports that the rash first appeared a little over 2 weeks ago. It started out small but has gotten larger. She also thinks she is getting the flu because her muscles and joints ache (myalgia and arthralgia, respectively), and she has had a headache for the last few days. Upon questioning, KL reports that she and her husband took a camping trip through New England last month. Pertinent Findings: The physical examination is remarkable for the presence of a red, circular, flat lesion ~11 cm in size that resembles a bullseye (erythema migrans) (see image at right). KL also has a low-grade fever. Diagnosis: KL has Lyme disease caused by the bacterium Borrelia burgdorferi, which is transmitted by the bite of a tick in the genus Ixodes. Infected ticks are endemic in the Northeast region of the United States.


Biochemistry_Lippinco

Biochemistry_Lippinco. Treatment: KL is prescribed doxycycline, an antibiotic in the tetracycline family. Monitoring of KL will continue until all symptoms have completely resolved. Blood is drawn for clinical laboratory tests. Prognosis: Patients treated with the appropriate antibiotic in the early stages of Lyme disease typically recover quickly and completely. Case-Related Questions: Choose the ONE best answer. Q1. Antibiotics in the tetracycline class inhibit protein synthesis (translation) at the initiation step. Which of the following statements about translation is correct? A. In eukaryotic translation, the initiating amino acid is formylated methionine. B. Only the charged initiating transfer RNA goes directly to the ribosomal A site. C. Peptidyltransferase is a ribozyme that forms the peptide bond between two amino acids. D. Prokaryotic translation can be inhibited by the phosphorylation of initiation factor 2.


Biochemistry_Lippinco

Biochemistry_Lippinco. C. Peptidyltransferase is a ribozyme that forms the peptide bond between two amino acids. D. Prokaryotic translation can be inhibited by the phosphorylation of initiation factor 2. E. Termination of translation is independent of guanosine triphosphate hydrolysis. F. The Shine-Dalgarno sequence facilitates the binding of the large ribosomal subunit to eukaryotic messenger RNA (mRNA). Q2. The Centers for Disease Control and Prevention recommends a two-tier testing procedure for Lyme disease that involves a screening enzyme-linked immunosorbent assay (ELISA) followed by a confirmatory western blot analysis on any sample with a positive or equivocal ELISA result. Which of the following statements about these testing procedures is correct? A. Both techniques are used to detect specific mRNA. B. Both techniques involve the use of antibodies. C. ELISA requires the use of electrophoresis.


Biochemistry_Lippinco

Biochemistry_Lippinco. A. Both techniques are used to detect specific mRNA. B. Both techniques involve the use of antibodies. C. ELISA requires the use of electrophoresis. D. Western blots require use of the polymerase chain reaction. Q3. Why are eukaryotic cells unaffected by antibiotics in the tetracycline class? Case 3: Blood on the Toothbrush Patient Presentation: LT is an 84-year-old man whose gums have been bleeding for several months. Focused History: LT is a widower and lives alone in a suburban community on the East Coast. He no longer drives. His two children live on the West Coast and come east infrequently. Since the death of his wife 11 months ago, he has been isolated and finds it hard to get out of the house. His appetite has changed, and he is content with cereal, coffee, and packaged snacks. Chewing is difficult.


Biochemistry_Lippinco

Biochemistry_Lippinco. Pertinent Findings: The physical examination was remarkable for the presence of swollen dark-colored gums (see image at right). Several of LT’s teeth were loose, including one that anchors his dental bridge. Several black and blue marks (ecchymoses) were noted on the legs, and an unhealed sore was present on the right wrist. Inspection of his scalp revealed tiny red spots (petechiae) around some of the hair follicles. Blood was drawn for testing. Results of tests on LT’s blood: The test for blood in his stool (occult blood test) was negative. Results of follow-up tests (obtained several days after the appointment) included the following: H = High; L = Low. Diagnosis: LT has vitamin C deficiency with a microcytic, hypochromic anemia secondary to the deficiency. Treatment: LT was prescribed vitamin C (as oral ascorbic acid) and iron (as oral ferrous sulfate) supplements. He will also be referred to social services. Prognosis: The prognosis for recovery is excellent.


Biochemistry_Lippinco

Biochemistry_Lippinco. Prognosis: The prognosis for recovery is excellent. Case-Related Questions: Choose the ONE best answer. Q1. Which of the following statements about vitamin C is correct? Vitamin C is: A. a competitor of iron absorption in the intestine. B. a fat-soluble vitamin with a 3-month supply typically stored in adipose tissue. C. a coenzyme in several enzymic reactions such as the hydroxylation of proline. D. required for the cross-linking of collagen. Q2. In contrast to the microcytic anemia characteristic of iron deficiency (common in older adults), a macrocytic anemia is seen with deficiencies of vitamin B12 and/or folic acid. These vitamin deficiencies are also common in older adults. Which of the following statements concerning these vitamins is correct? A. An inability to absorb B12 results in pernicious anemia. B. Both vitamins cause changes in gene expression. C. Folic acid plays a key role in energy metabolism in most cells.


Biochemistry_Lippinco

Biochemistry_Lippinco. A. An inability to absorb B12 results in pernicious anemia. B. Both vitamins cause changes in gene expression. C. Folic acid plays a key role in energy metabolism in most cells. D. Treatment with methotrexate can result in toxic levels of the coenzyme form of folic acid. E. Vitamin B12 is the coenzyme for enzymes catalyzing amino acid deaminations, decarboxylations, and transaminations. Q3. How do hemolytic anemias differ from nutritional anemias? Case 4: Rapid Heart Rate, Headache, and Sweating Patient Presentation: BE is a 45-year-old woman who presents with concerns about sudden (paroxysmal), intense, brief episodes of headache, sweating (diaphoresis), and a racing heart (palpitations).


Biochemistry_Lippinco

Biochemistry_Lippinco. Patient Presentation: BE is a 45-year-old woman who presents with concerns about sudden (paroxysmal), intense, brief episodes of headache, sweating (diaphoresis), and a racing heart (palpitations). Focused History: BE reports that the attacks started ~3 weeks ago. They last from 2 to 10 minutes, during which time she feels quite anxious. During the attacks, it feels as though her heart is skipping beats (arrhythmia). At first, she thought the attacks were related to recent stress at work and maybe even menopause. The last time it happened, she was in a pharmacy and had her blood pressure taken. She was told it was 165/110 mm Hg. BE notes that she has lost weight (~8 lbs) in this period even though her appetite has been good.


Biochemistry_Lippinco

Biochemistry_Lippinco. Pertinent Findings: The physical examination was remarkable for BE’s thin, pale appearance. Blood pressure was elevated (150/100 mm Hg), as was the heart rate (110–120 beats/minute). Based on BE’s history, blood levels of normetanephrine and metanephrine were ordered. They were found to be elevated. Diagnosis: BE has a pheochromocytoma, a rare catecholamine-secreting tumor of the adrenal medulla. Treatment: Imaging studies of the abdomen were done to locate the tumor. Surgical resection of the tumor was performed. The tumor was found to be nonmalignant. Follow-up measurement of plasma metanephrines was performed 2 weeks later and was in the normal range. Prognosis: The 5-year survival rate for nonmalignant pheochromocytomas is >95%. Case-Related Questions: Choose the ONE best answer. Q1. Pheochromocytomas secrete norepinephrine (NE) and epinephrine. Which of the following statements concerning the synthesis and degradation of these two biogenic amines is correct?


Biochemistry_Lippinco

Biochemistry_Lippinco. Q1. Pheochromocytomas secrete norepinephrine (NE) and epinephrine. Which of the following statements concerning the synthesis and degradation of these two biogenic amines is correct? A. The substrate for their synthesis is tryptophan, which is hydroxylated to 3,4-dihydroxyphenylalanine (DOPA) by tetrahydrobiopterin-requiring tryptophan hydroxylase. B. The conversion of DOPA to dopamine utilizes a pyridoxal phosphate– requiring carboxylase. C. The conversion of NE to epinephrine requires vitamin C. D. Degradation involves methylation by catechol-O-methyltransferase and produces normetanephrine from NE and metanephrine from epinephrine. E. Normetanephrine and metanephrine are oxidatively deaminated to homovanillic acid by monoamine oxidase. Q2. Which of the following statements concerning the actions of epinephrine and/or NE are correct? A. NE functions as a neurotransmitter and a hormone.


Biochemistry_Lippinco

Biochemistry_Lippinco. Q2. Which of the following statements concerning the actions of epinephrine and/or NE are correct? A. NE functions as a neurotransmitter and a hormone. B. They are initiated by autophosphorylation of select tyrosine residues in their receptors. C. They are mediated by binding to adrenergic receptors, which are a class of nuclear receptors. D. They result in the activation of glycogen and triacylglycerol synthesis. Q3. NE bound to certain receptors causes vasoconstriction and an increase in blood pressure. Why might NE be used clinically in the treatment of septic shock? Case 5: Sun Sensitivity Patient Presentation: AZ is a 6-year-old boy who is being evaluated for freckle-like areas of hyperpigmentation on his face, neck, forearms, and lower legs. Focused History: AZ’s father reports that the boy has always been quite sensitive to the sun. His skin turns red (erythema) and his eyes hurt (photophobia) if he is exposed to the sun for any period of time.


Biochemistry_Lippinco

Biochemistry_Lippinco. Pertinent Findings: The physical examination was remarkable for the presence of thickened, scaly areas (actinic keratosis) and hyperpigmented areas on skin exposed to ultraviolet (UV) radiation from the sun. Small dilated blood vessels (telangiectasia) were also seen. Tissue from several sites on his face was biopsied, and two were later determined to be squamous cell carcinomas. Diagnosis: AZ has xeroderma pigmentosum, a rare defect in nucleotide excision repair of DNA. Treatment: Protection from sunlight through use of sunscreens such as protective clothing that reflect UV radiation and chemicals that absorb it is essential. Frequent skin and eye examinations are recommended. Prognosis: Most patients with xeroderma pigmentosum die at an early age from skin cancers. However, survival beyond middle age is possible. Case-Related Questions: Choose the ONE best answer. Q1. Which of the following statements about DNA repair mechanisms is correct? DNA repair:


Biochemistry_Lippinco

Biochemistry_Lippinco. Case-Related Questions: Choose the ONE best answer. Q1. Which of the following statements about DNA repair mechanisms is correct? DNA repair: A. is performed only by eukaryotes. B. of double-strand breaks is error free. C. of mismatched bases involves repair of the parental strand. D. of UV radiation–induced pyrimidine dimers involves removal of a short oligonucleotide containing the dimer. E. of uracil produced by the deamination of cytosine requires the actions of endo-and exonucleases to remove the uracil base. Q2. Which one of the following statements about DNA synthesis (replication) is correct? Replication: A. in both eukaryotes and prokaryotes requires an RNA primer. B. in eukaryotes requires condensation of chromatin. C. in prokaryotes is accomplished by a single DNA polymerase. D. is initiated at random sites in the genome.


Biochemistry_Lippinco

Biochemistry_Lippinco. B. in eukaryotes requires condensation of chromatin. C. in prokaryotes is accomplished by a single DNA polymerase. D. is initiated at random sites in the genome. E. produces a polymer of deoxyribonucleoside monophosphates linked by 5′→3′-phosphodiester bonds. . What is the difference between DNA proofreading and repair? Case 6: Dark Urine and Yellow Sclerae Patient Presentation: JF is a 13-year-old boy who presents with fatigue and yellow sclerae. Focused History: JF began treatment ~4 days ago with a sulfonamide antibiotic and a urinary analgesic for a urinary tract infection. He had been told that his urine would change color (become reddish) with the analgesic, but he reports that it has gotten darker (more brownish) over the last 2 days. Last night, his mother noticed that his eyes had a yellow tint. JF says he feels as though he has no energy.


Biochemistry_Lippinco

Biochemistry_Lippinco. Pertinent Findings: The physical examination was remarkable for JF’s pale appearance, mild scleral icterus (jaundice), mild splenomegaly, and increased heart rate (tachycardia). JF’s urine tested positive for hemoglobin (hemoglobinuria). A peripheral blood smear reveals a lower-than-normal number of red blood cells (RBC), with some containing precipitated hemoglobin (Heinz bodies; see image at right), and a higher-than-normal number of reticulocytes (immature RBC). Results of the complete blood count (CBC) and blood chemistry tests are pending. Diagnosis: JF has glucose 6-phosphate dehydrogenase (G6PD) deficiency, an X-linked disorder that causes hemolysis (RBC lysis).


Biochemistry_Lippinco

Biochemistry_Lippinco. Diagnosis: JF has glucose 6-phosphate dehydrogenase (G6PD) deficiency, an X-linked disorder that causes hemolysis (RBC lysis). Treatment: G6PD deficiency can result in a hemolytic anemia in affected individuals exposed to oxidative agents. JF will be switched to a different antibiotic. He will be advised that he is susceptible to certain drugs (for example, sulfa drugs), foods (fava or broad beans), and certain chemicals (for example, naphthalene), and must avoid exposure to them. Prognosis: In the absence of exposure to oxidative agents, G6PD deficiency does not cause mortality or significant morbidity. Case-Related Questions: Choose the ONE best answer. Q1. G6PD catalyzes the regulated step in the pentose phosphate pathway. Which of the following statements concerning G6PD and the pentose phosphate pathway is correct? A. Deficiency of G6PD occurs only in RBC. B. Deficiency of G6PD results in an inability to keep glutathione in its functional, reduced form.


Biochemistry_Lippinco

Biochemistry_Lippinco. A. Deficiency of G6PD occurs only in RBC. B. Deficiency of G6PD results in an inability to keep glutathione in its functional, reduced form. C. The pentose phosphate pathway includes one reversible reductive reaction followed by a series of phosphorylated sugar interconversions. D. The reduced nicotinamide adenine dinucleotide phosphate (NADPH) product of the pentose phosphate pathway is utilized in processes such as fatty acid oxidation. Q2. The results of JF's CBC were consistent with a hemolytic anemia. Blood chemistry tests revealed an elevation in the bilirubin level. Which of the following statements concerning bilirubin is correct? A. Hyperbilirubinemia can cause deposition of bilirubin in the skin and sclerae resulting in jaundice. B. The solubility of bilirubin is increased by conjugating it with two molecules of ascorbic acid in the liver. C. The conjugated form of bilirubin increases in the blood with a hemolytic anemia.


Biochemistry_Lippinco

Biochemistry_Lippinco. B. The solubility of bilirubin is increased by conjugating it with two molecules of ascorbic acid in the liver. C. The conjugated form of bilirubin increases in the blood with a hemolytic anemia. D. Phototherapy can increase the solubility of the excess bilirubin generated in the porphyrias. Q3. Why is urinary urobilinogen increased relative to normal in hemolytic jaundice and absent in obstructive jaundice? Case 7: Joint Pain Patient Presentation: IR is a 22-year-old male who presents for follow-up 10 days after having been treated in the Emergency Department (ED) for severe inflammation at the base of his thumb.


Biochemistry_Lippinco

Biochemistry_Lippinco. Patient Presentation: IR is a 22-year-old male who presents for follow-up 10 days after having been treated in the Emergency Department (ED) for severe inflammation at the base of his thumb. Focused History: This was IR’s first occurrence of severe joint pain. In the ED, he was given an anti-inflammatory medication. Fluid aspirated from the carpometacarpal joint of the thumb was negative for organisms but positive for needle-shaped monosodium urate (MSU) crystals (see image at right). The inflammatory symptoms have since resolved. IR reports he is in good health otherwise, with no significant past medical history. His body mass index (BMI) is 31. No tophi (deposits of MSU crystals under the skin) were detected in the physical examination.


Biochemistry_Lippinco

Biochemistry_Lippinco. Pertinent Findings: Results on a 24-hour urine specimen and blood tests requested in advance of this visit reveal that IR is not an undersecretor of uric acid. His blood urate was 8.5 mg/dl (reference = 2.5–8.0). The unusually young age of presentation is suggestive of an enzymopathy of purine metabolism, and additional blood tests are ordered. Diagnosis: IR has gout (MSU crystal deposition disease), a type of inflammatory arthritis. Treatment: IR was given prescriptions for allopurinol and colchicine. The treatment goals are to reduce his blood urate levels to <6.0 mg/dl and prevent additional attacks. He was advised to lose weight because being overweight or obese is a risk factor for gout. His BMI of 31 puts him in the obese category. He was also given written information on the association between diet and gout. Prognosis: Gout increases the risk of developing renal stones. It is also associated with hypertension, diabetes, and heart disease.


Biochemistry_Lippinco

Biochemistry_Lippinco. Prognosis: Gout increases the risk of developing renal stones. It is also associated with hypertension, diabetes, and heart disease. Case-Related Questions: Choose the ONE best answer. Q1. Allopurinol is converted in the body to oxypurinol, which functions as a noncompetitive inhibitor of an enzyme in purine metabolism. Which of the following statements concerning purine metabolism and its regulation is correct? A. As a noncompetitive inhibitor, oxypurinol increases the apparent Michaelis constant (Km) of the target enzyme. B. Colchicine inhibits xanthine oxidase, an enzyme of purine degradation. C. Glutamate provides two of the nitrogen atoms of the purine ring. D. In purine nucleotide synthesis, the ring system is first constructed and then attached to ribose 5-phosphate. E. Oxypurinol inhibits the amidotransferase that initiates degradation of the purine ring system.


Biochemistry_Lippinco

Biochemistry_Lippinco. E. Oxypurinol inhibits the amidotransferase that initiates degradation of the purine ring system. F. Partial or complete enzymic deficiencies in the salvage of purine bases are characterized by hyperuricemia. Q2. Purines are one type of nitrogenous base found in nucleotides. Pyrimidines are the other. Which of the following statements is true of the pyrimidines? A. Carbamoyl phosphate synthetase I is the regulated enzymic activity in pyrimidine ring synthesis. B. Methotrexate decreases synthesis of the pyrimidine nucleotide thymidine monophosphate. C. Orotic aciduria is a pathology of pyrimidine degradation. D. Pyrimidine nucleotide synthesis is independent of 5-phosphoribosyl-1pyrophosphate (PRPP). Q3. IR is subsequently shown to have a form of PRPP synthetase that shows increased enzymic activity. Why does this result in hyperuricemia? Case 8: No Bowel Movement Patient Presentation: DW is a 48-hour-old female who has not yet had a bowel movement.


Biochemistry_Lippinco

Biochemistry_Lippinco. Case 8: No Bowel Movement Patient Presentation: DW is a 48-hour-old female who has not yet had a bowel movement. Focused History: DW is the full-term product of a normal pregnancy and delivery. She appeared normal at birth. DW is the first child of parents of Northern European ethnicity. The parents are both in good health, and their family histories are unremarkable. Pertinent Findings: DW has a distended abdomen. She recently vomited small amounts of bilious (green-colored) material. Diagnosis: Meconium ileus (obstruction of the ileum by meconium, the first stool produced by newborns) was confirmed by abdominal x-rays. About 98% of full-term newborns with meconium ileus have cystic fibrosis (CF). Diagnosis of CF was subsequently confirmed with a chloride sweat test. Treatment: The ileus was successfully treated nonsurgically. For management of the CF, the family was referred to the CF center at the regional children’s hospital.


Biochemistry_Lippinco

Biochemistry_Lippinco. Treatment: The ileus was successfully treated nonsurgically. For management of the CF, the family was referred to the CF center at the regional children’s hospital. Prognosis: CF is the most common life-limiting autosomal-recessive disease in Caucasians. Case-Related Questions: Choose the ONE best answer. Q1. CF is the result of mutations to the gene that encodes the CF transmembrane conductance regulator (CFTR) protein that functions as a chloride channel in the apical membrane of epithelial cells on a mucosal surface. Which of the following statements concerning CF is correct? A. Clinical manifestations of CF are the consequence of chloride retention with increased water reabsorption that causes mucus on the epithelial surface to be excessively thick and sticky. B. Excessive pancreatic secretion of insulin in CF commonly results in hypoglycemia.


Biochemistry_Lippinco

Biochemistry_Lippinco. B. Excessive pancreatic secretion of insulin in CF commonly results in hypoglycemia. C. Genetic testing for CF may involve the use of a set of probes for the most common mutations, a technique known as restriction fragment length polymorphism analysis. D. Some mutations result in premature degradation of the CFTR protein through tagging with ubiquinone followed by proteasome-mediated proteolysis. E. The most common mutation, ∆F508, results in the loss of a codon for phenylalanine (F) and is classified as a frameshift mutation. Q2. The CFTR protein is an intrinsic plasma membrane glycoprotein. Targeting of proteins destined to function as components of membranes: A. includes transport to and through the Golgi. B. involves an amino-terminal signal sequence that is retained in the functional protein. C. occurs after the protein has been completely synthesized (that is, posttranslationally).


Biochemistry_Lippinco

Biochemistry_Lippinco. B. involves an amino-terminal signal sequence that is retained in the functional protein. C. occurs after the protein has been completely synthesized (that is, posttranslationally). D. requires the presence of mannose 6-phosphate residues on the protein. Q3. Why might steatorrhea be seen with CF? Case 9: Elevated Ammonia Patient Presentation: RL is a 40-hour-old male with signs of cerebral edema. Focused History: RL is the full-term product of a normal pregnancy and delivery. He appeared normal at birth. At age 36 hours, he became irritable, lethargic, and hypothermic. He fed only poorly and vomited. He also displayed tachypneic (rapid) breathing and neurologic posturing. At age 38 hours, he had a seizure.


Biochemistry_Lippinco

Biochemistry_Lippinco. Pertinent Findings: Respiratory alkalosis (increased pH, decreased CO2 [hypocapnia]), increased ammonia, and decreased blood urea nitrogen were found. An amino acid screen revealed that argininosuccinate was increased >60fold over baseline, and citrulline was increased 4-fold. Glutamine was elevated, and arginine (Arg) was decreased relative to normal. Diagnosis: RL has a urea cycle enzyme defect with neonatal onset. Treatment: Hemodialysis was performed to remove ammonia. Sodium phenylacetate and sodium benzoate were administered to aid in excretion of waste nitrogen, as was Arg. Long-term treatment will include lifelong limitation of dietary protein; supplementation with essential amino acids; and administration of Arg, sodium phenylacetate, and sodium phenylbutyrate. Prognosis: Survival into adulthood is possible. The degree of neurologic impairment is related to the degree and extent of the hyperammonemia. Case-Related Questions: Choose the ONE best answer.


Biochemistry_Lippinco

Biochemistry_Lippinco. Prognosis: Survival into adulthood is possible. The degree of neurologic impairment is related to the degree and extent of the hyperammonemia. Case-Related Questions: Choose the ONE best answer. Q1. Based on the findings, which enzyme of the urea cycle is most likely to be deficient in this patient? A. Arginase B. Argininosuccinate lyase C. Argininosuccinate synthetase D. Carbamoyl phosphate synthetase I E. Ornithine transcarbamoylase Q2. Why is Arg supplementation helpful in this case? Q3. In individuals with partial (milder) deficiency of urea cycle enzymes, the level of which one of the following would be expected to be decreased during periods of physiologic stress? A. Alanine B. Ammonia C. Glutamine D. Insulin E. pH Case 10: Calf Pain Patient Presentation: CR is a 19-year-old female who is being evaluated for pain and swelling in her right calf.


Biochemistry_Lippinco

Biochemistry_Lippinco. A. Alanine B. Ammonia C. Glutamine D. Insulin E. pH Case 10: Calf Pain Patient Presentation: CR is a 19-year-old female who is being evaluated for pain and swelling in her right calf. Focused History: Ten days ago, CR had her spleen removed following a bicycle accident in which she fractured her tibial eminence, necessitating immobilization of the right knee. She has had a good recovery from the surgery. CR is no longer taking pain medication but has continued her oral contraceptives (OCP). Pertinent Findings: CR’s right calf is reddish in color (erythematous) and warm to the touch. It is visibly swollen. The left calf is normal in appearance and is without pain. An ultrasound is ordered. Diagnosis: CR has a deep venous thrombosis (DVT). OCP are a risk factor for DVT, as are surgery and immobilization. Treatment (Immediate): Heparin and warfarin are administered. Prognosis: In the 10 years following a DVT, about one third of individuals have a recurrence.


Biochemistry_Lippinco

Biochemistry_Lippinco. Treatment (Immediate): Heparin and warfarin are administered. Prognosis: In the 10 years following a DVT, about one third of individuals have a recurrence. Case-Related Questions: Choose the ONE best answer. Q1. A DVT is a blood clot that occludes the lumen of a deep vein, most commonly in the leg. Which of the following statements about the clotting cascade is correct? A. A deficiency in factor (F)IX of the intrinsic pathway results in hemophilia A. B. FIII of the extrinsic pathway is a serine protease. C. Formation of the fibrin meshwork is referred to as primary hemostasis. D. Thrombin proteolytically activates components of the extrinsic, intrinsic, and common pathways. E. Vitamin K is required for the activation of fibrinogen. Q2. Which one of the following would increase the risk of thrombosis? A. Excess production of antithrombin B. Excess production of protein S C. Expression of FV Leiden D. Hypoprothrombinemia


Biochemistry_Lippinco

Biochemistry_Lippinco. A. Excess production of antithrombin B. Excess production of protein S C. Expression of FV Leiden D. Hypoprothrombinemia E. von Willebrand disease Q3. Compare and contrast the actions of heparin and warfarin. Focused Cases: Answers to Case-Based Questions Case 1: Anemia with β-Thalassemia Minor


Biochemistry_Lippinco

Biochemistry_Lippinco. E. von Willebrand disease Q3. Compare and contrast the actions of heparin and warfarin. Focused Cases: Answers to Case-Based Questions Case 1: Anemia with β-Thalassemia Minor Answer = B. Transcription (synthesis of single-stranded RNA from the template strand of double-stranded DNA) requires the binding of proteins (trans-acting factors) to sequences on the DNA (cis-acting elements). Eukaryotic messenger RNA (mRNA) is monocistronic because it contains information from just one gene (cistron). The base sequence TAG (thymine adenine guanine) in the coding strand of DNA is U(uracil)AG in the mRNA. UAG is a signal that terminates translation (protein synthesis), not transcription. It is formation of the 5′-cap of eukaryotic mRNA that requires methylation (using S-adenosylmethionine), not 3′-end polyadenylation. Splicing is the spliceosome-mediated process by which introns are removed from eukaryotic mRNA and exons joined.


Biochemistry_Lippinco

Biochemistry_Lippinco. Answer = C. Carbon monoxide (CO) increases the affinity of hemoglobin (Hb)A for O2, thereby decreasing the ability of HbA to offload O2 in the tissues. CO stabilizes the R (relaxed) or oxygenated form and shifts the O2 dissociation curve to the left, decreasing O2 delivery (see figure at top right). The other choices decrease the affinity for O2, stabilize the T (tense) or deoxygenated form, and cause a right shift in the curve. HbA2 and fetal Hb (HbF) do not contain β globin. As β globin production decreases, synthesis of HbA2 (α2δ2) and HbF (α2γ2) increases.


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Biochemistry_Lippinco. Sickle cell anemia is caused by a single point mutation (A→T) in the gene for β globin that results in the replacement of glutamate by valine at the sixth amino acid position in the protein. Mutational analysis using allele-specific oligonucleotide (ASO) probes for that mutation (βS) and for the normal sequence (βA) is used in diagnosis (see figure at lower right). β-Thalassemia, in contrast, is caused by hundreds of different mutations. Mutational analysis using ASO probes can assess common mutations, including point mutations, in at-risk populations (for example, those of Greek ancestry). However, less common mutations are often not included in the panel and can be detected only by DNA sequencing. Case 2: Skin Rash with Lyme Disease


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Biochemistry_Lippinco. Case 2: Skin Rash with Lyme Disease Answer = C. Peptide-bond formation between the amino acid in the A site of the ribosome and the amino acid last added to the growing peptide in the P site is catalyzed by an RNA of the large ribosomal subunit. Any RNA with catalytic activity is referred to as a ribozyme (see figure on the next page). Formylated methionine is used to initiate prokaryotic translation. The charged initiating transfer RNA (tRNAi) is the only tRNA that goes directly to the P site, leaving the A site available for the tRNA carrying the next amino acid of the protein being made. Eukaryotic translation is inhibited by the phosphorylation of initiation factor 2 (eIF-2). The Shine-Dalgarno sequence is found in prokaryotic messenger RNA (mRNA) and facilitates the interaction of the mRNA with the small ribosomal subunit. In eukaryotes, the cap-binding proteins perform that task.


Biochemistry_Lippinco

Biochemistry_Lippinco. Answer = B. The enzyme-linked immunosorbent assay (ELISA) and western blot are used to analyze proteins. Each makes use of antibodies to detect and quantify the protein of interest. It is western blots that utilize electrophoresis. The polymerase chain reaction (PCR) is used to amplify DNA. Antibiotics in the tetracycline family inhibit protein synthesis by binding to and blocking the A site of the small (30S) ribosomal subunit in prokaryotes. Tetracycline specifically interacts with the 16S ribosomal RNA (rRNA) component of the 30S subunit, inhibiting translation initiation. Eukaryotes do not contain 16S rRNA. Their small (40S) subunit contains 18S rRNA, which does not bind tetracycline. Case 3: Blood on the Toothbrush with Vitamin C Deficiency


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Biochemistry_Lippinco. Case 3: Blood on the Toothbrush with Vitamin C Deficiency Answer = C. Vitamin C (ascorbic acid) functions as a coenzyme in the hydroxylation of proline and lysine in the synthesis of collagen, a fibrous protein of the extracellular matrix. Vitamin C is also the coenzyme for duodenal cytochrome b (Dcytb) that reduces dietary iron from the ferric (Fe3+) to the ferrous (Fe2+) form that is required for absorption via the divalent metal transporter (DMT) of enterocytes (see figure below). With a deficiency of vitamin C, uptake of dietary iron is impaired and results in a microcytic, hypochromic anemia. As a water-soluble vitamin, vitamin C is not stored. Cross-linking of collagen by lysyl oxidase requires copper, not vitamin C.


Biochemistry_Lippinco

Biochemistry_Lippinco. Answer = A. An inability to absorb vitamin B12 leads to pernicious anemia and is most commonly caused by decreased production of intrinsic factor (IF) by the parietal cells of the stomach (see figure at right). Vitamins D and A, in complex with their receptors, bind to DNA and alter gene expression. Thiamine (vitamin B1) is a coenzyme in the oxidative decarboxylation of pyruvate and αketoglutarate and, therefore, is important in energy metabolism in most cells. Methotrexate inhibits dihydrofolate reductase, the enzyme that reduces dihydrofolate to tetrahydrofolate (THF), the functional coenzyme form of folate. This results in decreased availability of THF. It is pyridoxine (vitamin B6) as pyridoxal phosphate that is the coenzyme for most reactions involving amino acids. [Note: Tetrahydrobiopterin is required by aromatic amino acid hydroxylases and nitric oxide synthases.] Nutritional anemias are characterized by either increased red blood cell (RBC) size (folate and B12 deficiencies)


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Biochemistry_Lippinco. is required by aromatic amino acid hydroxylases and nitric oxide synthases.] Nutritional anemias are characterized by either increased red blood cell (RBC) size (folate and B12 deficiencies) or decreased RBC size (iron and vitamin C deficiencies). In hemolytic anemias, such as is seen in glucose 6-phosphate dehydrogenase and pyruvate kinase deficiencies and in sickle cell anemia, RBC size typically is normal, and RBC number is decreased.


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Biochemistry_Lippinco. Case 4: Rapid Heart Rate, Headache, and Sweating with a Pheochromocytoma Answer = D. Degradation of both epinephrine and norepinephrine (NE) involves methylation by catechol-O-methyltransferase (COMT) that produces normetanephrine from NE and metanephrine from epinephrine (see figure at right). Both of these products are deaminated to vanillylmandelic acid by monoamine oxidase (MAO). The substrate for the synthesis of the catecholamines is tyrosine, which gets hydroxylated to 3,4dihydroxyphenylalanine (DOPA) by tetrahydrobiopterin-requiring tyrosine hydroxylase. DOPA is converted to dopamine by a pyridoxal phosphate– requiring decarboxylase. [Note: Most carboxylases require biotin.] NE is converted to epinephrine by methylation, and S-adenosylmethionine provides the methyl group.


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Biochemistry_Lippinco. Answer = A. NE released from the sympathetic nervous system functions as a neurotransmitter that acts on postsynaptic neurons and causes, for example, increased heart rate. It is also released from the adrenal medulla and, along with epinephrine, functions as a counterregulatory hormone that results in mobilization of stored fuels (for example, glucose and triacylglycerols). These actions are mediated by the binding of NE to adrenergic receptors, which are G protein–coupled receptors of the plasma membrane, and not to nuclear receptors like those of steroid hormones or membrane tyrosine kinase receptors like that of insulin. Septic shock is vasodilatory hypotension (low blood pressure caused by blood vessel dilation) resulting from the production of large amounts of nitric oxide by inducible nitric oxide synthase in response to infection. NE bound to receptors on smooth muscle cells causes vasoconstriction and, thus, raises blood pressure.


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Biochemistry_Lippinco. Case 5: Sun Sensitivity with Xeroderma Pigmentosum


Biochemistry_Lippinco

Biochemistry_Lippinco. Answer = D. Pyrimidine dimers are the characteristic DNA lesions caused by ultraviolet (UV) radiation. Their repair involves the excision of an oligonucleotide containing the dimer and replacement of that oligonucleotide, a process known as nucleotide excision repair (NER). (See figure at right for a representation of the process in prokaryotes.) DNA repair systems are found in prokaryotes and eukaryotes. Nothing is error free, but the homologous recombination (HR) method of double-strand break repair is much less prone to error than is the nonhomologous end joining (NHEJ) method because any DNA that was lost is replaced. Mismatched-base repair (MMR) involves identification and repair of the newly synthesized (daughter) strand. In prokaryotes, the extent of strand methylation is used to discriminate between the strands. Base excision repair (BER), the mechanism by which uracil is removed from DNA, utilizes a glycosylase to remove the base, creating an apyrimidinic or apurinic (AP)


Biochemistry_Lippinco

Biochemistry_Lippinco. discriminate between the strands. Base excision repair (BER), the mechanism by which uracil is removed from DNA, utilizes a glycosylase to remove the base, creating an apyrimidinic or apurinic (AP) site. The sugar-phosphate is then removed by the actions of an endo-and exonuclease.


Biochemistry_Lippinco

Biochemistry_Lippinco. Answer = A. All replication requires an RNA primer because DNA polymerases (pol) cannot initiate DNA synthesis. The chromatin of eukaryotes gets decondensed (relaxed) for replication. Relaxation can be accomplished, for example, by acetylation via histone acetyltransferases. Prokaryotes have more than one DNA pol. For example, pol III extends the RNA primer with DNA, and pol I removes the primer and replaces it with DNA. Replication is initiated at specific locations (one in prokaryotes, many in eukaryotes) that are recognized by proteins (for example, DnaA in prokaryotes). Deoxynucleoside monophosphates (dNMP) are joined by a phosphodiester bond that links the 3′hydroxyl group of the last dNMP added with the 5′-phosphate group of the incoming nucleotide, thereby forming a 3′→5′-phosphodiester bond as pyrophosphate is released. Proofreading occurs during replication in the S (synthesis of DNA) phase of the cell cycle and involves the 3′→5′ exonuclease activity possessed by some DNA


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Biochemistry_Lippinco. bond as pyrophosphate is released. Proofreading occurs during replication in the S (synthesis of DNA) phase of the cell cycle and involves the 3′→5′ exonuclease activity possessed by some DNA pol (see figure below). Because repair can occur independently of replication, it can be performed outside of the S phase.


Biochemistry_Lippinco

Biochemistry_Lippinco. Case 6: Dark Urine and Yellow Sclerae with


Biochemistry_Lippinco

Biochemistry_Lippinco. Answer = B. Glutathione in its reduced form (G-SH) is an important antioxidant. The selenium-containing enzyme glutathione peroxidase reduces hydrogen peroxide (H2O2, a reactive oxygen species) to water as glutathionine is oxidized (G-S-S-G). Reduced nicotinamide adenine dinucleotide phosphate (NADPH)requiring glutathionine reductase regenerates G-SH from G-S-S-G (see Figure A). The NADPH is supplied by the oxidative reactions of the pentose phosphate pathway (see Figure B), which is regulated by the availability of NADPH at the glucose 6-phosphate dehydrogenase (G6PD)-catalyzed step (the first step). Deficiency of G6PD occurs in all cells, but the effects are seen in red blood cells where the pentose phosphate pathway is the only source of NADPH. The pathway involves two irreversible oxidative reactions, each of which generates NADPH. The NADPH is used in reductive processes such as fatty acid synthesis (not oxidation) as well as steroid hormone and cholesterol synthesis.


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Biochemistry_Lippinco. Answer = A. Jaundice (icterus) refers to the yellow color of the skin, nail beds, and sclerae that results from bilirubin deposition when the bilirubin level in the blood is elevated (hyperbilirubinemia; see Image C). Bilirubin has low solubility in aqueous solutions, and its solubility is increased by conjugation with uridine diphosphate–glucuronic acid in the liver, forming bilirubin diglucuronide or conjugated bilirubin (CB). In hemolytic conditions, such as G6PD deficiency, both CB and unconjugated bilirubin (UCB) are increased, but it is UCB that is found in the blood. CB is sent into the intestine. Phototherapy is used to treat unconjugated hyperbilirubinemia because it converts bilirubin to isomeric forms that are more water soluble. Bilirubin is the product of heme degradation in cells of the mononuclear phagocyte system, particularly in the liver and the spleen. The porphyrias are pathologies of heme synthesis and, therefore, are not characterized by hyperbilirubinemia.


Biochemistry_Lippinco

Biochemistry_Lippinco. With hemolysis, more bilirubin is produced and conjugated. CB is sent to the intestine where it is converted to urobilinogen, some of which is reabsorbed, enters the portal blood, and travels to the kidney. Because the source of urinary urobilinogen is intestinal urobilinogen, urinary urobilinogen will be low in obstructive jaundice because intestinal urobilinogen will be low as a result of the obstruction of the common bile duct (see Figure D). Case 7: Joint Pain with Gout


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Biochemistry_Lippinco. Case 7: Joint Pain with Gout Answer = F. Salvage of the purine bases hypoxanthine and guanine to the purine nucleotides inosine monophosphate (IMP) and guanosine monophosphate (GMP) by hypoxanthine-guanine phosphoribosyltransferase (HGPRT) requires 5-phosphoribosyl-1-pyrophosphate (PRPP) as the source of the ribose 1phosphate. Salvage decreases the amount of substrate available for degradation to uric acid. Therefore, a deficiency in salvage results in hyperuricemia (see figure at right). Noncompetitive inhibitors such as oxypurinol have no effect on the Michaelis constant (Km) but decrease the apparent maximal velocity (Vmax).


Biochemistry_Lippinco

Biochemistry_Lippinco. Colchicine is an anti-inflammatory drug. It has no effect on the enzymes of purine synthesis or degradation. Glutamine (not glutamate) is a nitrogen source for purine ring synthesis. In purine nucleotide synthesis, the purine ring system is constructed on the ribose 5-phosphate provided by PRPP. Allopurinol and its metabolite, oxypurinol, inhibit xanthine oxidase of purine degradation. The amidotransferase is the regulated enzyme of purine synthesis. Its activity is decreased by purine nucleotides and increased by PRPP.


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Biochemistry_Lippinco. Answer = B. Methotrexate inhibits dihydrofolate reductase, decreasing the availability of N5,N10-methylene tetrahydrofolate needed for synthesis of deoxythymidine monophosphate (dTMP) from deoxyuridine monophosphate (dUMP) by thymidylate synthase (see figure at right). Carbamoyl phosphate synthetase (CPS) II is the regulated enzymic activity of pyrimidine biosynthesis in humans. CPS I is an enzyme of the urea cycle. Orotic aciduria is a rare pathology of pyrimidine synthesis caused by a deficiency in one or both enzymic activities of bifunctional uridine monophosphate synthase. Pyrimidine nucleotide synthesis, like purine synthesis and salvage, requires PRPP. Increased activity of PRPP synthetase results in increased synthesis of PRPP. This results in an increase in purine nucleotide synthesis beyond need. The excess purine nucleotides get degraded to uric acid, thereby causing hyperuricemia. Case 8: No Bowel Movement with Cystic Fibrosis


Biochemistry_Lippinco

Biochemistry_Lippinco. Answer = A. The clinical manifestations of cystic fibrosis (CF) are the consequence of chloride retention with increased water absorption that causes mucus on an epithelial surface to be excessively thick and sticky. The result is pulmonary and gastrointestinal problems such as respiratory infection and impaired exocrine and endocrine pancreatic functions (pancreatic insufficiency). Impaired endocrine pancreatic function can result in diabetes with associated hyperglycemia. The genetic testing technique described, and one used in the diagnosis of CF, is the use of allele-specific oligonucleotides (ASO). Some mutations do result in increased degradation of the CF transmembrane conductance regulator (CFTR) protein, but degradation is initiated by tagging the protein with ubiquitin. Frameshift mutations alter the reading frame through the addition or deletion of nucleotides by a number not divisible by three. Because the ∆F509 mutation is caused by the loss of three nucleotides that code


Biochemistry_Lippinco

Biochemistry_Lippinco. mutations alter the reading frame through the addition or deletion of nucleotides by a number not divisible by three. Because the ∆F509 mutation is caused by the loss of three nucleotides that code for phenylalanine (F) at position 509 in the CFTR protein, it is not a frameshift mutation. Answer = A. Targeting of proteins destined to function as components of the plasma membrane is an example of cotranslational targeting. It involves the initiation of translation on cytosolic ribosomes; recognition of the amino (N)terminal signal sequence in the protein by the signal recognition particle; movement of the protein-synthesizing complex to the outer face of the membrane of the endoplasmic reticulum (ER); and continuation of protein synthesis, such that the protein is threaded into the lumen of the ER and packaged into vesicles that travel to and through the Golgi and eventually fuse with the plasma membrane. The N-terminal signal sequence is removed by a peptidase in the lumen of the ER.


Biochemistry_Lippinco

Biochemistry_Lippinco. the ER and packaged into vesicles that travel to and through the Golgi and eventually fuse with the plasma membrane. The N-terminal signal sequence is removed by a peptidase in the lumen of the ER. Mannose 6-phosphate is the signal that cotranslationally targets proteins to the matrix of the lysosome where they function as acid hydrolases.


Biochemistry_Lippinco

Biochemistry_Lippinco. The pancreatic insufficiency seen in some patients with CF results in a decreased ability to digest food, and digestion is required for absorption. Dietary fats move through the intestine and are excreted in the stool (see figure at right), which is foul-smelling and bulky and may float. Patients are at risk for malnutrition and deficiencies in fat-soluble vitamins. Oral supplementation of pancreatic enzymes is the treatment. Case 9: Hyperammonemia with a Urea Cycle Defect


Biochemistry_Lippinco

Biochemistry_Lippinco. Answer = B. Argininosuccinate lyase (ASL) cleaves argininosuccinate to arginine (Arg) and fumarate. The increase in argininosuccinate and citrulline and the decrease in Arg seen in RL indicate a deficiency in ASL (see figure below). With arginase deficiency, Arg would be increased, not decreased. Additionally, with arginase deficiency, the hyperammonemia would be less severe because two nitrogens are excreted. Deficiency of argininosuccinate synthetase (ASS) would also cause an increase in citrulline, but argininosuccinate would be low to absent. Deficiency of carbamoyl phosphate synthetase (CPS) I is characterized by low levels of Arg and citrulline. Deficiency of ornithine transcarbamoylase (OTC), the only X-linked enzyme of the urea cycle, would result in low levels of Arg and citrulline and elevated levels of urinary orotic acid. [Note: The orotic acid is elevated because the carbamoyl phosphate (CP) substrate of OTC is being used in the cytosol as a substrate for pyrimidine


Biochemistry_Lippinco

Biochemistry_Lippinco. and elevated levels of urinary orotic acid. [Note: The orotic acid is elevated because the carbamoyl phosphate (CP) substrate of OTC is being used in the cytosol as a substrate for pyrimidine synthesis.]


Biochemistry_Lippinco

Biochemistry_Lippinco. Arg supplementation is helpful because the Arg will be hydrolyzed to urea + ornithine by arginase. The ornithine will be combined with CP to form citrulline (see figure above). With ASL (and ASS) deficiency, citrulline accumulates and is excreted, thereby carrying waste nitrogen out of the body. Answer = D. In individuals with milder (partial) deficiencies in the enzymes of the urea cycle, hyperammonemia may be triggered by physiologic stress (for example, an illness or prolonged fasting) that decreases the insulin/counterregulatory hormone ratio. [Note: The degree of the hyperammonemia is usually less severe than that seen in the neonatal onset forms.] The shift in the ratio results, in part, in skeletal muscle proteolysis, and the amino acids that are released get degraded. Degradation involves transamination by pyridoxal phosphate–requiring aminotransferases that generate the α-keto acid derivative of the amino acid + glutamate. The glutamate undergoes oxidative deamination to


Biochemistry_Lippinco

Biochemistry_Lippinco. involves transamination by pyridoxal phosphate–requiring aminotransferases that generate the α-keto acid derivative of the amino acid + glutamate. The glutamate undergoes oxidative deamination to α-ketoglutarate and ammonia (NH3) by glutamate dehydrogenase (GDH; see figure at right). [Note: GDH is unusual in that it uses both nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP) as coenzymes.]


Biochemistry_Lippinco

Biochemistry_Lippinco. The NH3, which is toxic, can be transported to the liver as glutamine (Gln) and alanine (Ala). The Gln is generated by the amination of glutamate by ATP-requiring glutamine synthetase. In the liver, the enzyme glutaminase removes the NH3, which can be converted to urea by the urea cycle or excreted as ammonium (NH4+) (see figure at right). Gln, then, is a nontoxic vehicle of NH3 transport in the blood. Ala is generated in skeletal muscle from the catabolism of the branched-chain amino acids (BCAA). In the liver, Ala is transaminated by alanine transaminase (ALT) to pyruvate (used in gluconeogenesis) and glutamate. Thus, Ala carries nitrogen to the liver for conversion to urea (see figure below). Therefore, defects in the urea cycle would result in an elevation in NH3, Gln, and Ala. The elevated NH3 drives respiration, and the hyperventilation causes a rise in pH (respiratory alkalosis). [Note: Hyperammonemia is toxic to the nervous system. Although the exact mechanisms are not


Biochemistry_Lippinco

Biochemistry_Lippinco. The elevated NH3 drives respiration, and the hyperventilation causes a rise in pH (respiratory alkalosis). [Note: Hyperammonemia is toxic to the nervous system. Although the exact mechanisms are not completely understood, it is known that the metabolism of large amounts of NH3 to Gln (in the astrocytes of the brain) results in osmotic effects that cause the brain to swell. Additionally, the rise in Gln decreases the availability of glutamate, an excitatory neurotransmitter.]


Biochemistry_Lippinco

Biochemistry_Lippinco. Case 10: Swollen, Painful Calf with Deep Venous Thrombosis Answer = D. Thrombin, a serine protease, is activated by the prothrombinase complex of factor (F)Xa + FVa. Once formed, activated thrombin (FIIa) proteolytically activates components of the extrinsic (FVII) and intrinsic (FXI, FVIII) pathways, generating FXa. Thrombin can also activate FV, FI, and FXIII of the common pathway (see figure below). Hemophilia A is caused by a deficiency in FVIII. FIX deficiency results in hemophilia B. FIII, also known as tissue factor (TF), is a transmembrane glycoprotein of the vascular endothelium. It functions as an accessory protein and not a protease. Formation of the platelet plug is primary hemostasis, and formation of the fibrin meshwork is secondary hemostasis. Vitamin K is required for the activation (γ-carboxylation) of FII, FVII, FIX, and FX (proteases that require calcium [Ca2+] and phospholipids [PL]) but not for FI (fibrinogen).


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Biochemistry_Lippinco. Answer = C. FV Leiden is a mutant form of FV that is resistant to proteolysis by the activated protein C complex. Decreased ability to degrade FV allows continued production of activated thrombin and leads to an increased risk of clot formation or thrombophilia. Antithrombin III (ATIII) and protein S are proteins of anticoagulation. Increased, not decreased, production of prothrombin would result in thrombophilia. Deficiency of von Willebrand factor causes a coagulopathy or a deficiency in clotting through effects on FVIII and platelets. Heparin and warfarin are anticoagulants. Heparin, a glycosaminoglycan, increases the affinity of ATIII for thrombin. Binding of ATIII removes thrombin from the blood and prevents it from converting fibrinogen to fibrin. Warfarin, a synthetic analog of vitamin K, inhibits vitamin K epoxide reductase and prevents the regeneration of the functional hydroquinone form of the vitamin that is required for the γ-carboxylation of glutamate residues to


Biochemistry_Lippinco

Biochemistry_Lippinco. of vitamin K, inhibits vitamin K epoxide reductase and prevents the regeneration of the functional hydroquinone form of the vitamin that is required for the γ-carboxylation of glutamate residues to γ-carboxyglutamate (Gla) residues in FII, FVII, FIX, and FX (see figures below).