Embden-Meyerhof pathway, ethanol

Yeast (qv) metabolize maltose and glucose sugars via the Embden-Meyerhof pathway to pymvate, and via acetaldehyde to ethanol. AH distiUers yeast strains can be expected to produce 6% (v/v) ethanol from a mash containing 11% (w/v) starch. Ethanol concentration up to 18% can be tolerated by some yeasts. Secondary products (congeners) arise during fermentation and are retained in the distiUation of whiskey. These include aldehydes, esters, and higher alcohols (fusel oHs). NaturaHy occurring lactic acid bacteria may simultaneously ferment within the mash and contribute to the whiskey flavor profile. 

As previously mentioned and in the earlier discussion of fermentation methanol, bacteria of the genus Zymomonas such as Z. mobilis are known to convert hexoses to ethanol at high yields and short residence times. These bacteria are facultative anaerobes that have fermentative capacity and convert only glucose, fructose, and sucrose to equimolar quantities of ethanol and CO2 the pentoses are not converted. The Entner-Doudoroff pathway is utilized instead of the Embden-Meyerhof pathway, and a net yield of 1 mol of ATP is generated, not 2 mol as in bakers yeast. But pyruvate is the same key intermediate. In Z. mobilis, it is decarboxylated by pyruvate decarboxylase to yield acetaldehyde which is then reduced to ethanol by alcohol dehydrogenase. 

LAB are non-respiring microorganisms, principally generating ATP by fermentation of carbohydrates coupled to substrate-level phosphorylation. The two major pathways for the metabolism of hexoses are homofermentative or glycolysis (Embden-Meyerhof pathway), in which lactic acid is virtually the only end-product, and heterofermentative (phosphoketolase pathway), in which other end-products such as acetic acid, C02, and ethanol are produced in addition to lactic acid (Axelsson et al., 1989 Kandler, 1983 Zourari et al., 1992). 

Under anaerobic conditions, pyruvic acid is converted either into lactic acid, as in muscle action, or to ethanol and acetaldehyde as occurs in fermentation reactions. The anaerobic route from glucose to lactic acid is sometimes known as the Embden-Meyerhof pathway. 

A variety of monosaccharides (hexoses or pentoses) can be fermented to produce 2,3-BD (Syu, 2001). In bacterial metabolism, monosaccharides must first be converted to pyruvate before generation of major products. From glucose, pyruvate is formed in a relatively simple manner via the Embden-Meyerhof pathway (glycolysis). In contrast, the production of pyruvate from pentoses must proceed via a combination of the pentose phosphate and Embden-Meyerhof pathways (Jansen and Tsao, 1983). In addition to 2,3-BD, the pyruvate produced from the monosaccharides is then channeled into a mixture of acetate, lactate, formate, succinate, acetoin, and ethanol, through the mixed acid-2,3-BD fermentation pathway (Ji et al., 2011a). 

The Pasteur Effect. It is an old observation that cells, e.g. of muscle or yeast, transform much glucose (to lactate or ethanol) imder anaerobic conditions. If oxygen is supplied, however, then not only is anaerobic glycolysis inhibited, but much less glucose actually enters the Embden-Meyerhof pathway. Evidently, the oxidative processes of mitochondria exert some influence back on the cytoplasm. This seems rather sensible physiologically, since in aerobiosis a great quantity of ATP is produced in the respiratory chain and the demands for energy can w ell be met with less substrate. However, this does not explain the mechanism responsible for the effect, which is known as the Pasteur effect. ... 

In contrast with the well-known Embden-Meyerhof-Pamass glycolysis pathway for the conversion of hexose sugars to alcohol, the steps in conversion of ethanol to acetic acid remain in some doubt. Likely, ethanol is first oxidized to acetaldehyde and water (39). For further oxidation, two alternative routes are proposed more likely, hydration of the acetaldehyde gives CH2CH(OH)2, which is oxidized to acetic acid. An alternative is the Cannizzaro-type disproportionation of two molecules of acetaldehyde to one molecule of ethanol and one molecule of acetic acid. Jicetobacter... 

Figure 10-3 Coupling of the reactions of glycolysis with formation of lactic acid and ethanol in fermentations. Steps a to g describe the Embden-Meyerhof-Parnas pathway. Generation of 2 ATP in step b can provide all of the cell s energy.

The generic dynamic model for the bioparticle, where the kinetic expressions are adapted of Rotboll and Jorgensen [9] is given by Eq. 3. In the model, [ ] represents a generic concentration. Substrate conversion in the pellet through the reactions of the Embden-Meyerhof-Pamas pathway forms intermediates such as acetaldehyde [A] and pyruvate [P] that are reduced to ethanol [ ] or oxidized in the steps of the TCA pathway. 

The best-known glycolytic pathway is that studied especially in muscle tissue and yeast cells. This pathway, sometimes known as the Embden-Meyerhof-Parnas (EMP) scheme, is shown in Fig. 8. The reactions take place under anaerobic conditions. In yeast the end-products are ethanol and carbon dioxide, whereas in muscle tissue the end-product is L-lactic acid. The EMP scheme is operative in a great many tissues and organisms and apparently represents the major pathway of carbohydrate breakdown. 

The study of these metabolic steps is quite active at the present time. As noted above, Dickens found that ribose-5-phosphate was fermented anaerobically to ethanol, a 2-carbon compound, inorganic phosphate, and CO2. Racker observed that extracts of E. colt converted ribose-5-phosphate to a triose phosphate, which could be analyzed in the presence of triose phosphate isomerase as dihydroxyacetone phosphate.Therefore, the products of the oxidative pathway eventually join the Embden-Meyerhof scheme at the triose phosphate stage, the major difference being the formation of 2 moles of triose phosphate in the latter pathway and only 1 mole via the phosphogluconate pathway. 

Ethanol is formed via glycolysis (the Embden-Meyerhof-Parnes pathway). The overall reaction starting from glucose can be written as follows ... 

---