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Consider the role of the coenzyme NAD in glucose oxidation. In the process of glycolysis the formation of pyruvate involves the reduction of the coenzyme NAD to NAD.2H.

(a) Which other stages of glucose oxidation produce molecules of the reduced coenzyme, NAD.2H?

(b) Explain what happens to NAD.2H produced during glycolysis in:

i) an animal cell respiring aerobically
ii) an animal cell respiring anaerobically

(c) The oxidation of NAD.2H is an energy-releasing process. Name the energy-requiring process it is coupled to and why this is important for the cell.

Solution Preview

(a) NADH is produced in the oxidation of glucose to CO2 (carbon dioxide). Essentially, NAD is reduced to NADH while glucosed is oxidized to CO2. But that oxidation to carbon dioxide doesn't take place in just one step. It occurs in a multitude of steps through glycolysis and the Krebs cycle. NADH is produced in the conversion of glyceraldehyde-3-phosphate to glycerate-1,3-bisphosphate. This is the single step in glycolysis where NADH is produced that is mentioned in the question. But, in addition to that, NADH is also produced in the Krebs cycle (tricarboxylic acid cycle, TCA) and in the preparatory step to make acetyl-CoA.
<br>Essentially, when pyruvate is converted to acetyl-CoA, NAD is reduced to NADH.
<br>Also, in the Krebs cycle itself, NADH is produced when ...