Glycolysis alcoholic fermentation

K. Burton and H. A. Krebs, The free energy changes associated with the individual steps of the tricarboxylic acid cycle, glycolysis, alcoholic fermentation, and with the hydrolysis of the pyrophosphate group of adensosine triphosphate, Biochem.. /. 54, 94-107 (1953). 

A C. in the narrow sense enters the reaction stoi-chiometrically, in that it reacts sequentially with two enzyme proteins, and therefore catalyses substrate turnover. An example is NAD, the active group of dehydrogenases and reductases. It first forms an active complex with a dehydrogenase (enzyme I) and accepts the hydrogen removed from the substrate. The resulting NADH then dissociates from enzyme I and associates with a reductase (enzyme II), then donates the hydrogen to the substrate of this enzyme (Fig.). Since the C. acts as a second substrate, it is sometimes called a cosubstrate. It must be able to react reversibly with 2 different apoenzymes. Many examples of NAD/ NADH as a cosubstrate are found in Glycolysis, Alcoholic fermentation and the Tricarboxylic acid cycle. 

The reducing power of NADH, produced by glycolysis, must be transferred to an electron acceptor to regenerate NAD" ". In alcoholic fermentation, it is not pyruvate but rather acetaldehyde, its decarboxylation product, that serves as the terminal electron acceptor. With respect to glycolysis, alcoholic fermentation contains two additional enzymatic reactions, the first of which (catalyzed by pyruvate decarboxylase), decarboxylates pyruvic acid. 

Terms in bold are defined glycolysis 522 fermentation 522 lactic acid fermentation hypoxia 523 ethanol (alcohol) fermentation 523 isozymes 526... 

Anaerobic Oxidation of ducose. Historically, the first system of carbohydrate metabolism to be studied was the conversion by yeast of glucose to alcohol (fermentation) according to the equation CnH,Of,2CH)CH,OH + 2CO . The biochemical process is complex, involving the successive catalytic actions of 12 enzymes and known as the Emhden-Meyerhof pathway This series of reactions is summarized in the entry on Glycolysis. 

The biochemistry of alcoholic fermentation involves a series of internal enzyme-mediated oxidation-reduction reactions m which glucose is degraded via the Embden-Meyerhof-Parnas pathway See also Carbohydrates and Glycolysis. 

When plants experience anoxic conditions there is a shift in carbohydrate metabolism from an oxidative to a fermentative pathway (Fig. 1). In the absence of oxygen, ATP is generated not by the Krebs cycle but by alcoholic fermentation, i.e. glycolysis and ethanol synthesis. 

Alcoholic fermentation occurs when the end product is ethanol, as shown in Figure 4.11. In this process the pyruvate is first converted enzymatically to acetaldehyde. The conversion of acetaldehyde to ethanol produces NAD+ from NADH + H+, and the NAD+ is cycled through the glycolysis process. As with lactic acid fermentation, the glycolysis process produces usable energy contained in two molecules of ATP produced for each molecule of glucose metabolized. 

Figure 4.11 Alcoholic fermentation in which the conversion of pyruvate to ethanol through an acetaldehyde intermediate is coupled with glycolysis to produce energetic ATP.

Conversion to ethanol. In yeast and some other microorganisms under anaerobic conditions, the NAD+ required for the continuation of glycolysis is regenerated by a process called alcoholic fermentation. The pyruvate is converted to acetaldehyde (by pyruvate decarboxylase) and then to ethanol (by alcohol dehydrogenase), the latter reaction reoxidizing the NADH to NAD+ ... 

As was quoted above, when fermenting grape juice, Saccharomyces cerevisiae mainly directs the pyruvate to produce of ethanol in order to regenerate the NAD+ consumed by glycolysis. This process, called alcoholic fermentation, is shown in Fig. 1.4. 

The first metabolic pathway that we encounter is glycolysis, an ancient pathway employed by a host of organisms. Glycolysis is the sequence of reactions that metabolizes one molecule of glucose to two molecules ofpyruvate with the concomitant net production of two molecules of ATP. This process is anaerobic (i.e., it does not require O2) inasmuch as it evolved before the accumulation of substantial amounts of oxygen in the atmosphere. Pyruvate can be further processed anaerobically (fermented) to lactate (lactic acidfermentation) or ethanol (alcoholic fermentation). Under aerobic conditions, pyruvate can be completely oxidized to CO2, generating much more ATP, as will be discussed in Chapters 17 and 18. 

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. The regeneration of NAD + in the reduction ofpyruvate to lactate or ethanol sustains the continued operation of glycolysis under anaerobic conditions. 

In alcoholic fermentation, each three-carbon molecule that is produced during glycolysis is split to form a two-carbon molecule—the alcohol ethanol—and a one-carbon molecule, carbon dioxide. Two molecules of ATP are produced during glycolysis. ... 

In the overview of glycolysis we noted that the pyruvate produced must be used up in some way so that the pathway will continue to produce ATP. Similarly, the NADH produced by glycolysis in step 6 (see Figure 21.8) must be reoxidized at a later time, or glycolysis will grind to a halt as the available NAD+ is used up. If the cell is functioning under aerobic conditions, NADH will be reoxidized, and pyruvate will be completely oxidized by aerobic respiration. Under anaerobic conditions, however, different types of fermentation reactions accomplish these purposes. Fermentations are catabolic reactions that occur with no net oxidation. Pyruvate or an organic compound produced from pyruvate is reduced as NADH is oxidized. We will examine two types of fermentation pathways in detail lactate fermentation and alcohol fermentation. 

Under anaerobic conditions the NADH produced by glycolysis is used to reduce p)mivate to lactate in skeletal muscle (lactate fermentation) or to convert acetaldehyde to ethanol in yeast (alcohol fermentation). 

When glycolysis occurs under anaerobic conditions, it is followed by fermentation reactions, such as the lactate and alcohol fermentations. These reactions reduce pyruvate—or a molecule produced from pyruvate—and simultaneously oxidize the NADH produced in glycolysis. As a result, the net energy yield from glycolysis under anaerobic conditions is only two ATP. No further ATP energy is harvested from the oxidation of the NADH. It is simply reoxidized in the fermentation reactions. 

This child must have the enzymes to carry out the alcohol fermentation. When the child exercised hard, there was not enough oxygen in the cells to maintain aerobic respiration. As a result, glycolysis and the alcohol fermentation were responsible for the majority of the ATP production by the child. The accumulation of alcohol (ethanol) in the child caused the symptoms of drunkenness. 

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