In this article we will discuss about the re-oxidation process of NADH during fermentation.
NADH is not oxidized by the electron transport chain (i.e., the oxidative phosphorylation does not occur) in the absence of aerobic or anaerobic respiration because, in that condition, no external electron acceptor becomes available. Yet NADH generated in glycolysis during the oxidation of glyceraldehyde 3-phosphate to 1, 3- bisphosphoglyceric acid must still be oxidized back to NAD+.
If NAD+ is not regenerated, the oxidation of glyceraldehyde 3-phosphate to 1, 3- bisphosphoglyceric acid will not take place and, as a result, the glycolysis will stop. Variety of microorganisms, which would have faced this problem, evolved the solution.
They solved the problem by slowing or stopping the activity of enzyme pyruvate dehydrogenase and using pyruvic acid (the end-product of glycolysis) or one of its derivatives as an electron or hydrogen acceptor for the oxidation of NADH to NAD+ in a fermentation process (Fig. 26.5) resulting in the production of more ATP.
Fermentation process is so effective, however, that some chemotrophic microorganisms do not carry out respiration even when oxygen or another external electron acceptor is available.