Oxidative decarboxylation

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Oxidative decarboxylation is the name given to the preparation of pyruvate, a product of glycolysis, for entrance into the citric acid cycle (a.k.a. Krebs cycle). Oxidative decarboxylation is sometimes referred to as "the transition reaction". In this reaction, pyruvate breaks down into carbon dioxide and acetaldehyde, reducing NAD⁺ to NADH. Then, the acetaldehyde binds to a sulfur molecule attached to Coenzyme A, forming Acetyl CoA inside the mitochondrion. Coenzyme A is released during the citric acid cycle, allowing oxidative decarboxylation to recur indefinitely under aerobic respiration.

Enzyme activity

Pyruvate dehydrogenase catalyzes this reaction. Its inhibitors have the overall effect of slowing this reaction when there is either little oxygen, or when the cell has a lot of energy (as characterized by the ratios ATP/ADP, NADH/NAD⁺ and acetyl-CoA/CoASH). An alternate name for this reaction, the "pyruvate dehydrogenase reaction", is derived from the name of this enzyme.

Setting

Oxidative decarboxylation occurs only inside the mitochondria. The pyruvate enters from the cytosol via a transport protein, consuming energy. It cannot diffuse across the membrane because it is a polar molecule. The acetyl-CoA molecule is very large and cannot leave the mitochondrion. Under normal circumstances, the acetyl-CoA is consumed by the citric acid cycle and the Coenzyme A is regenerated, allowing oxidative decarboxylation to occur again. The carbon dioxide is nonpolar and small, so it can diffuse out of the mitochondria and out of the cell. Note that since oxidative decarboxylation occurs in the mitochondria or their prokaryotic infolding analogs, most prokaryotes do not undergo this process.

Under aerobic conditions, NADH may be oxidized by the electron transport chain into NAD⁺, renewing this reactant for use in oxidative decarboxylation. Note that this requires oxygen. In anaerobic conditions, NAD⁺ can be regenerated by fermentation; however, the reaction quotient of reactants and products is not large enough that the reaction will proceed, because since the citric acid cycle does not run, product is not consumed.

Other

There is some disagreement as to whether oxidative decarboxylation should be considered a part of the citric acid cycle or not. This is irrelevant to the understanding of the process, but should be noted.

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