Conversion of, pyruvate to ethanol

The conversion of pyruvate to ethanol occurs by the two reactions summarized in Figure 8.24. The decarboxylation of pyruvate by pyruvate decarboxylase occurs in yeast and certain microorganisms, but not in humans. The enzyme requires thiamine pyrophosphate as a coenzyme, and catalyzes a reaction similar to that described for pyruvate dehydrogenase (see p. 108). 

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 of pyruvate to ethanol by certain yeast strains occurs in two steps. It is first decarboxylated to acetaldehyde by pyruvate decarboxylase, which utilizes thiamine pyrophosphate (TPP) as coenzyme. 

Although there are a number of notable differences between the metabolic networks of C. thermocellum and T. saccharolyticum [56], the potential pathways for conversion of pyruvate to ethanol is similar in both organisms. Both have PFOR... 

An overall reaction for alcoholic fermentation is obtained by combining the reaction for the conversion of pyruvate to ethanol and the reactions of glycolysis. Notice that because NAD is used up in glycolysis but regenerated in ethanol formation, it does not appear in the overall equation ... 

A further step in the D-xylose catabolism is believed to be phosphorylation of D-xylulose to D-xylulose-5-phosphate [2, 98, 99]. The steps after formation of xylulose phosphate appear to use a combination of pentose phosphate and EMP pathways to the key intermediate pyruvate. This process is followed by the conversion of pyruvate to ethanol by the action of pyruvate decarboxylase and alcohol dehydrogenase. D-xylulose-5-phosphate, after converting into D-glyce-raldehyde-3-phosphate via the enzymes of the pentose phosphate cycle, is also... 

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 10.3 Possible pathways involved in conversion of pyruvate to end products. (a) Homoacetate fementation, (b) mixed acetate/ethanol fermentation, and (c) homoethanol fermentation C. ther-mocellum. ATP yields per hexose equivalent are provided. Pyr, pyruvate Ac-CoA, acetyl-Coenzyme A Fd, ferredoxin PFL,...

Cocoa bean fermentation is a mixed-culture process, consisting initially of fermentations by yeast and lactic acid bacteria followed by oxidation of the fermentation products ethanol and lactic acid into acetic acid and acetoin by several Acetohacter strains, of which /I. pasteurianus is the prominent one (Moens et al. 2014). A C-based carbon flux analysis of Acetohacter during cocoa pulp fermentation-simulating conditions revealed a functionally separated metabolism during co-consumption of ethanol and lactate. Acetate was almost exclusively derived from ethanol, whereas lactate served for formation of acetoin and biomass building blocks. This switch was attributed to the lack of phosphoenolpyruvate carboxykinase and malic enzyme activities, which prevents conversion of oxalo-acetate and malate formed by acetate metabolism in the TCA cycle to PEP and pyruvate and subsequently to acetoin (Adler et al. 2014). Lactate, on the other hand, can be converted to pyruvate, which is then used for acetoin formation or, after conversion to PEP by pymvate phosphate dikinase, for gluconeogenesis. The inability of conversion of TCA cycle intermediates to PEP resembles the situation in G. oxydans, where in addition no enzyme for conversion of pyruvate to PEP is present. 

Variations of the alcoholic and homolactic fermentations. The course of a fermentation is often affected drastically by changes in conditions. Many variations can be visualized by reference to Fig. 17-9, which shows a number of available metabolic sequences. We have already discussed the conversion of glucose to triose phosphate and via reaction pathway a to pyruvate, via reaction c to lactate, and via reaction d to ethanol. 

Answer The first step in the synthesis of glucose from lactate in the liver is oxidation of the lactate to pyruvate like the oxidation of ethanol to acetaldehyde, this requires NAD+. Consumption of alcohol forces a competition for NAD+ between ethanol metabolism and gluconeogenesis, reducing the conversion of lactate to glucose and resulting in hypoglycemia. The problem is compounded by strenuous exercise and lack of food because at these times the level of blood glucose is already low. 

Figure 31. The biocatalyzed conversion of pyruvic acid (21) to lactic acid (22) by an LDH AlcDH system and the use of the NAD+/NADH cofactor. The transformation proceeds with the oxidation of ethanol, and is reversed in a high concentration of lactic acid.

After biochemical conversion of glucose to pyruvic acid intermediate, the next step in ethanol synthesis is nonoxidative decarboxylation and acetaldehyde formation catalyzed by a native decarboxylase, and then acetaldehyde reduction to ethanol catalyzed by a native dehydrogenase. 

The net reaction is the conversion of 1 mole of glucose to 2 moles each of ethanol and CO2 with balanced consumption of the reducing power generated during the formation of pyruvic acid. The analogous biochemical conversion of glucose to methanol would involve formation of formaldehyde and its reduction to methanol ... 

Alcohol taken in excess tends to prevent gluconeogenesis from lactate in the liver, because oxidation of ethanol to acetaldehyde competes for the NAD" that is necessary for the conversion of lactate to pyruvate. Severe acidosis, such as diabetic ketoacidosis, may suppress lactate conversion and cause a shift in the lactate-pyruvate equilibrium with the accumulation of H. This shift may, in part, be responsible for the lactic acidosis seen in diabetics. 

Recall that molecular oxygen does not participate directly in the citric acid cycle. However, the cycle operates only under aerobic conditions because NAD and FAD can be regenerated in the mitochondrion only by the transfer of electrons to molecular oxygen. Glycolysis has both an aerobic and an anaerobic mode, whereas the citric acid cycle is strictly aerobic. Glycolysis can proceed under anaerobic conditions because NAD is regenerated in the conversion of pyruvate into lactate or ethanol. 

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