Fates of Pyruvate under Anaerobic Conditions: Fermentation:- Ethanol Is the Reduced Product in Ethanol Fermentation
Yeast and other microorganisms ferment glucose to ethanol and CO2, rather than to lactate. Glucose is con verted to pyruvate by glycolysis, and the pyruvate is converted to ethanol and CO2 in a two-step process:
In the first step, pyruvate is decarboxylated in an irreversible reaction catalyzed by pyruvate decarboxylase. This reaction is a simple decarboxylation and does not involve the net oxidation of pyruvate. Pyruvate de carboxylase requires Mg2+ and has a tightly bound coenzyme, thiamine pyrophosphate, discussed below. In the second step, acetaldehyde is reduced to ethanol through the action of alcohol dehydrogenase, with the reducing power furnished by NADH derived from the dehydrogenation of glyceraldehyde 3-phosphate. This reaction is a well-studied case of hydride transfer from NADH (Fig. 14–12). Ethanol and CO2 are thus the end products of ethanol fermentation, and the overall equation is two CO2 (combined H:C ratio=12/6=2). In all fermentations, the H:C ratio of the reactants and products remains the same.
MECHANISM FIGURE 14–12 The alcohol dehydrogenase reaction. AZn2 at the active site polarizes the carbonyl oxygen of acetaldehyde, allowing transfer of a hydride ion (red) from the reduced cofactor NADH. The reduced intermediate acquires a proton from the medium (blue) to form ethanol.
Pyruvate decarboxylase is present in brewer’s and baker’s yeast and in all other organisms that ferment glucose to ethanol, including some plants. The CO2 produced by pyruvate decarboxylation in brewer’s yeast is responsible for the characteristic carbonation of champagne. The ancient art of brewing beer involves a number of enzymatic processes in addition to the reactions of ethanol fermentation (Box 14–2). In baking, CO2 released by pyruvate decarboxylase when yeast is mixed with a fermentable sugar causes dough to rise. The en zyme is absent in vertebrate tissues and in other or ganisms that carry out lactic acid fermentation.
Alcohol dehydrogenase is present in many organ isms that metabolize ethanol, including humans. In hu man liver it catalyzes the oxidation of ethanol, either ingested or produced by intestinal microorganisms, with the concomitant reduction of NAD+ to NADH.
