16.2 NAD+ Is Regenerated from the Metabolism of Pyruvate

✓ 2 Explain why the regeneration of NAD+ is crucial to fermentations.

The conversion of glucose into two molecules of pyruvate results in the net synthesis of ATP. However, an energy-converting pathway that stops at pyruvate will not proceed for long, because redox balance has not been maintained. This imbalance is caused by the activity of glyceraldehyde 3-phosphate dehydrogenase, which leads to the reduction of NAD+ to NADH when glyceraldehyde 3-phosphate is oxidized. In the cell, there are limited amounts of NAD+, which is derived from the vitamin niacin, a dietary requirement for human beings. Consequently, NAD+ must be regenerated for glycolysis to proceed. Thus, the final process in the pathway is the regeneration of NAD+ through the metabolism of pyruvate.

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Fermentations Are a Means of Oxidizing NADH

NUTRITION FACTS

Niacin Also called vitamin B3, niacin is a component of coenzymes NAD+ and NADP+, which are used in electron-transfer reactions. There are many sources of niacin, including chicken breast. Niacin deficiency results in the potentially fatal disease pellagra, a condition characterized by dermatitis, dementia, and diarrhea.

The sequence of reactions from glucose to pyruvate is similar in most organisms and most types of cells. In contrast, the fate of pyruvate is variable. Three reactions of pyruvate are of primary importance: conversion into ethanol, lactate, or carbon dioxide and water (Figure 16.4). The first two reactions are fermentations that take place in the absence of oxygen. Fermentations are ATP-generating processes in which organic compounds act as both donors and acceptors of electrons. In the presence of oxygen, the most common situation in multicellular organisms and for many unicellular ones, pyruvate is metabolized to carbon dioxide and water through the citric acid cycle and the electron-transport chain (Sections 8 and 9). In these circumstances, oxygen accepts electrons and protons to form water. We now take a closer look at these three possible fates of pyruvate.

Figure 16.4: Diverse fates of pyruvate. Ethanol and lactate can be formed by reactions that include NADH. Alternatively, a two-carbon unit from pyruvate can be coupled to coenzyme A (see Chapter 18) to form acetyl CoA.

  1. Ethanol is formed from pyruvate in yeast and several other microorganisms. The first step is the decarboxylation of pyruvate. This reaction is catalyzed by pyruvate decarboxylase, which requires the coenzyme thiamine pyrophosphate. This coenzyme is derived from the vitamin thiamine (B1). The second step is the reduction of acetaldehyde to ethanol by NADH, in a reaction catalyzed by alcohol dehydrogenase. Acetaldehyde is thus the organic compound that accepts the electrons in this fermentation. This reaction regenerates NAD+:

    The conversion of glucose into ethanol is an example of alcoholic fermentation. The net result of this anaerobic process is

    Note that NAD+ and NADH do not appear in this equation, even though they are crucial for the overall process. NADH generated by the oxidation of glyceraldehyde 3-phosphate is consumed in the reduction of acetaldehyde to ethanol. Thus, there is no net oxidation-reduction in the conversion of glucose into ethanol (Figure 16.5). The ethanol formed in alcoholic fermentation is a key ingredient in brewing and winemaking, and the carbon dioxide formed accounts for some of the carbonation in beer and champagne.

    Figure 16.5: Maintaining redox balance in alcoholic fermentation. The NADH produced by the glyceraldehyde 3-phosphate dehydrogenase reaction must be reoxidized to NAD+ for the glycolytic pathway to continue. In alcoholic fermentation, alcohol dehydrogenase oxidizes NADH and generates ethanol.

    NUTRITION FACTS

    Thiamine Also called vitamin B1, thiamine is a component of the coenzyme thiamine pyrophosphate (TPP), which is used in decarboxylation reactions. Pork and legumes are good sources of thiamine. A deficiency of thiamine results in beriberi, the symptoms of which include muscle weakness, anorexia, and an enlarged heart.

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  2. Lactate is formed from pyruvate in a variety of microorganisms in a process called lactic acid fermentation. Pyruvate accepts the electrons from NADH to form lactate in a reaction catalyzed by lactate dehydrogenase:

    The overall reaction in the conversion of glucose into lactate is

    Figure 16.6: Maintaining redox balance in lactic acid fermentation. In lactic acid fermentation, lactate dehydrogenase oxidizes NADH to produce lactic acid and regenerate NAD+.

    As in alcoholic fermentation, there is no net oxidation-reduction. The NADH formed in the oxidation of glyceraldehyde 3-phosphate is consumed in the reduction of pyruvate (Figure 16.6). The regeneration of NAD+ in the reduction of pyruvate to lactate or ethanol sustains the continued process of glycolysis under anaerobic conditions.

    Certain types of skeletal muscles in most animals can function anaerobically for short periods. For example, a specific type of muscle fiber, called fast twitch or type IIb fibers, perform short bursts of intense exercise. The ATP needs rise faster than the ability of the body to provide oxygen to the muscle. The muscle functions anaerobically until fatigue sets in, which is caused, in part, by lactate buildup. Indeed, the pH of resting type IIb muscle fibers, which is about 7.0, may fall to as low as 6.3 during the bout of exercise. The drop in pH inhibits phosphofructokinase. A lactate/H+ symporter allows the exit of lactate and H+ from the muscle cell.

  3. Only a fraction of the energy of glucose is released in its anaerobic conversion into ethanol or lactate. Much more energy can be extracted aerobically by means of the citric acid cycle and the electron-transport chain, which combust, or oxidize, glucose all the way to CO2 and H2O. The entry point to this oxidative pathway is acetyl coenzyme A (acetyl CoA), which is formed from pyruvate inside mitochondria:

    This reaction will be considered in detail in Chapter 18. The NAD+ required for this reaction and for the oxidation of glyceraldehyde 3-phosphate is regenerated in the electron-transport chain in mitochondria (Chapter 20).

!quickquiz! QUICK QUIZ 2

Lactic acid fermentation and alcoholic fermentation are oxidation-reduction reactions. Identify the ultimate electron donor and electron acceptor.

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DID YOU KNOW?

Fermentation is an ATP-generating process in which organic compounds act as both donors and acceptors of electrons. Fermentation can take place in the absence of 02. Louis Pasteur, who discovered fermentation, described it as la vie sans l’air (“life without air”).

!bio! BIOLOGICAL INSIGHT: Fermentations Provide Usable Energy in the Absence of Oxygen

Fermentations yield only a fraction of the energy available from the complete combustion of glucose. Why is a relatively inefficient metabolic pathway so extensively used? The fundamental reason is that fermentations do not require oxygen. The ability to survive without oxygen affords a host of living accommodations such as soils, deep water, and skin pores. Some organisms, called obligate anaerobes, cannot survive in the presence of O2, which is a highly reactive compound. The bacterium Clostridium perfringens, the cause of gangrene, is an example of an obligate anaerobe. Other pathogenic obligate anaerobes are listed in Table 16.2. Some organisms, such as yeast, are facultative anaerobes that metabolize glucose aerobically when oxygen is present and perform fermentation when oxygen is absent.

Table 16.2 Examples of pathogenic obligate anaerobes

Many food products, including sour cream, yogurt, various cheeses, beer, wine, and sauerkraut, result from fermentation. Yogurt is produced by the fermentation of lactose in milk to lactate by a mixed culture of Lactobacillus acidophilus and Streptococcus thermophilus. Sour cream begins with pasteurized light cream, which is fermented to lactate by Streptococcus lactis. The lactate is further fermented to ketones and aldehydes by Leuconostoc citrovorum. The second fermentation adds to the taste and aroma of sour cream. Yeast, Saccharomyces cerevisiae, ferments carbohydrates to ethanol and carbon dioxide, providing some of the ingredients for an array of alcohol beverages. Although we have considered only lactic acid and alcoholic fermentation, microorganisms are capable of generating a wide array of molecules as end points to fermentation.