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B1. Amino Acid Analysis and Chemical Sequencing


Figure: Protein Analysis from low to high resolution.

In the last chapter section, we learned about the charge and chemical reactivity properties of isolated amino acids and amino acids in proteins. The analysis of a whole protein is complicated since each different amino acid might be represented many times in the sequence. Each protein has an N-terminal and C-terminal amino acid and secondary structure. Some proteins exists biologically as multisubunit proteins, which adds to the complexity of the analyses since now the proteins would have multiple N- and C-terminal ends. In addition, isolated proteins might have chemically modifications (post-translational) which add to the functionalities of the proteins but also add to the complexities of the analyses. To illustrate some of these issues, view the structure of the RhoA program below.

 Updated RhoA - a cytoplasmic protein - The complexity of protein analysis Jmol14 (Java) | JSMol (HTML5)

Amino Acid Composition

At a low level of resolution, we can determine the amino acid composition of the protein by hydrolyzing the protein in 6 N HCl, 100oC, under vacuum for various time intervals. After removing the HCl, the hydrolysate is applied to an ion-exchange or hydrophobic interaction column, and the amino acids eluted and quantitated with respect to known standards. A non naturally- occurring amino acid like norleucine is added in known amounts as an internal standard to monitor quantitative recovery during the reactions. The separated amino acids are often derivitized with ninhydrin or phenylisothiocyantate to facilitate their detection. The reaction is usually allowed to procedure for 24, 36, and 48 hours, since amino acids with OH (like ser) are destroyed. A time course allows the concentration of Ser at time t=0 to be extrapolated. Trp is also destroyed during the process. In addition, the amide links in the side chains of Gln and Asn are hydrolyzed to form Glu and Asp, respectively.

N- and C-Terminal Amino Acid Analysis


Analysis for Specific Amino Acids


Figure: amino acid absorbance profiles

Amino Acid Sequence - Edman Degradation


Figure: Edman Degradation

In this technique, a protein adsorbed to a solid phase reacts with phenylisothiocyanate. An intramolecular cyclization and cleavage of the N-terminal amino acid results, which can be washed from the adsorbed protein and detected by HPLC analysis. The yields in this technique are close to 100%. However, with time, more chains accumulate in which an N-terminal amino acid has not been removed. If it is removed on the next step, two amino acids will elute, creating increasing "noise" in the elution step - i.e. more than 1 amino acid derivative will be detected. Hence the maximal length of the peptide which can be sequenced is about 50 amino acids. Most proteins are larger than that. Hence, before the protein can be sequenced, it must be cleaved with specific enzymes called endoproteases which cleave proteins after specific side chains. For example, trypsin cleaves proteins within a chain after Lys and Arg, while chymotrypsin cleaves after aromatic amino acids, like Trp, Tyr, and Phe. Chemical cleavage by small molecules can be used as well. Cyanogen bromide, CNBr, cleaves proteins after methionine side chains. The individual proteins must be cleaved using two different methods, and each peptide fragment isolated and sequenced. Then the order of the cleaved peptides with known sequence can be pieced together by comparing the peptide sequences obtained using different cleavage methods. Many proteins also have disulfide bonds connecting Cys side chains distal to each other in the polypeptide chain. Proteolytic or chemical cleavage of the protein would lead to the formation of a fragment containing two peptides linked by disulfides. Edman degration would release two amino acids from such fragments. To avoid this problem, the protein is oxidized with performic acid, which irreversibly oxidizes free Cys, or Cys-Cys disulfides to cysteic acid residues. A summary of the steps involved in protein sequencing are shown below:


  1. If the protein contains more than one polypeptide chain, the chains are separated and purified. If disulfide bonds connect two different chains, the S-S bond must be cleaved (as described in step 2) and each peptide independently purified.
  3. The amino acid composition of each chain is determined