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What happens to the molecules of NADH and pyruvate produced in glycolysis? The answer depends on the particular kind of cell, and whether the rest of the metabolic pathway is aerobic or anaerobic. In both aerobic and anaerobic metabolism, NADH must be converted back to its oxidized state, NAD, or the cell will eventually run out of this coenzyme.
Under aerobic conditions, NAD is regenerated when the electrons from NADH molecules are shuttled into the mitochondria and the electron transport chain. The electrons from NADH eventually make their way to molecular oxygen, which is reduced to water. The energy released when NADH molecules transfer electrons is ultimately used to make ATP.
Under aerobic conditions, pyruvate molecules are transported to the mitochondria, where they enter the tricarboxylic acid or TCA cycle and are eventually oxidized to carbon dioxide. The TCA cycle generates more NADH molecules, which are used to produce ATP.
Under anaerobic conditions, the regeneration of NAD is coupled to the reduction of pyruvate. The anaerobic regeneration of NAD is called fermentation. In animal tissues, fermentation reduces pyruvate to lactate, as NADH transfers electrons to pyruvate. This enzyme-catalyzed reaction occurs whenever the available oxygen has been consumed.
Strenuous exercise may occur anaerobically and can lead to the buildup of lactate in muscle tissues. When lactate is converted to lactic acid, the pH in these tissues to drops, producing muscle fatigue and soreness. Anaerobic fermentation is used for rapid ATP production. It produces ATP about 100 times faster than passage through the electron transport chain.
Sprinters have a higher percentage of muscle tissue suited for generating ATP through fermentation, whereas marathon runners have more muscle tissue suited for generating ATP aerobically. Cells that are suited for aerobic metabolism have more mitochondria than other cells.
In yeast, the regeneration of NAD from NADH also takes place under anaerobic conditions. In yeast fermentation, pyruvate is converted into ethanol in a two-step reaction. In the first step, carbon dioxide is removed from pyruvate, producing acetaldehyde. In the second step, acetaldehyde is reduced by the enzyme alcohol dehydrogenase.
While our cells don’t ferment pyruvate to ethanol, many of us do have the enzyme alcohol dehydrogenase that catalyzes the second step. Recall that enzymes catalyze reactions in both the forward and reverse directions. Individuals who have the enzyme alcohol dehydrogenase metabolize ethanol to acetaldehyde. When individuals lack this enzyme, they have a low tolerance for ethanol.
Alcohol dehydrogenase also catalyses the reaction with methanol. However, methanol is much more toxic than ethanol because its metabolism produces formaldehyde and formic acid. Ethanol is sometimes given to treat methanol poisoning, because alcohol dehydrogenase would rather bind to ethanol than methanol in its active site. Ethanol displaces methanol from the enzyme’s active site and prevents the enzyme from converting methanol into toxic formaldehyde.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education