Alcohol anaerobic glycolysis

See also Alcoholic Fermentation, Alcohol Dehydrogenase, Fermentation, Anaerobic Glycolysis, see also here... 

Figure 13.6, which depicts the energy profile of anaerobic glycolysis, shows that NADH produced in the oxidation of glyceraldehyde-3-phosphate is used to reduce pyruvate to lactate. Thus, during anaerobic glycolysis or lactic acid fermentation, an overall electron balance is maintained. (Note -alcohol fermentation is another type of anaerobic glycolysis). 

See also Oxidative Phosphorylation (from Chapter 15), Regulation of Glycolysis, Fructose-2,6-Bisphosphate Regulation (from Chapter 16), Reactions/Energies of Glycolysis, Lactic Acid fermentation. Pyruvate Decarboxylase, Alcohol Dehydrogenase, Alcoholic Fermentation, Aerobic vs. Anaerobic Glycolysis... 

Fermentation is defined as an energy-yielding metabolic pathway that involves no net change in oxidation state. Anaerobic glycolysis is a type of fermentation. The lactic acid fermentation (conversion of glucose to lactate) is important in the manufacture of cheese. Another important fermentation involves cleavage of pyruvate to acetaldehyde and C02, with the acetaldehyde then reduced to ethanol by alcohol dehydrogenase in the reaction that follows ... 

The Pathway of NAD and Pyruvate, In order to formulate a continuous process, the reduced pyridine nucleotide NADH2 must be reoxidized and made available again. The reoxidation could be achieved, e.g. through the respiratory chain, if oxygen were available at the site of carbohydrate breakdown. In the absence of oxygen, however, another solution must be found. In the metabolism of vertebrates, in anaerobic glycolysis of muscle, pyruvate is reduced to lactate in yeast the reduction is preceded by a decarboxylation to form acetaldehyde, and the product of the reduction is the much desired ethyl alcohol. 

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. 

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+ ... 

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 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. 

Both the alcohol and lactate fermentations are anaerobic reactions that use the pyruvate and re-oxidize the NADH produced in glycolysis. 

Under anaerobic conditions, NADH produced in glycolysis builds up. This results in a reduction in the amount of NAD+ available to support continuation of glycolysis. Organisms have two pathways for regenerating NAD+ under anaerobic conditions. Animal cells and lactic acid bacteria use the process of lactic acid fermentation. Yeast convert pyruvate to acetaldehyde in a reaction catalyzed by the enzyme pyruvate decarboxylase. This is followed by reduction of acetaldehyde to ethanol catalyzed by alcohol dehydrogenase. The reaction uses NADH and releases NAD+, which is subsequently used in glycolysis. 

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