Fermentation Embden-Meyerhof pathway

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. 

Schroder, C., Selig, M., and Schonheit, P. 1994. Glucose fermentation to acetate, C02 and H2 in the anaerobic hyperthermophilic eubacterium Thermotoga maritima involvement of the Embden-Meyerhof pathway. Arch. Microbiol., 767,460-470. 

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. 

The Embden-Meyerhof pathway via pyruvic acid is the normal route to fermentation products from biomass carbohydrates (Chapter 11). Enteric bacteria appear to have the unique capability of converting pyruvic acid directly to formic acid (Section I1,D) ... 

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). 

Figure 4.1 Homolactic fermentation. The fermentation of one mole of glucose yields two moles of lactic acid via the Embden-Meyerhof pathway.

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). 

Biochemical pathway used by acetogens for fermentation of sugars and utilization of one-carbon precursors for biomass and acetate production 7 hexose oxidation to pyruvate by the Embden-Meyerhof pathway, 2 pyruvate-ferredoxin oxidoreductase,... 

The industrial strains of lactic acid bacteria can completely ferment a medium of 12-15 % sugar in 2-4 days with greater than a 90 % yield of lactic acid. All of these bacteria are considered homofermentative in that they utilize a homolactic fermentation pathway, producing two molecules of lactic acid for each molecule of hexose. As diagrammed in O Fig. 1.10, the Embden-Meyerhof pathway is used in the homolactic fermentation. 

Homolactic fermentation pathway of giucose (giycoiysis, Embden-Meyerhof pathway)... 

The metabolic pathway for bacterial sugar fermentation proceeds through the Embden-Meyerhof-Paranas (EMP) pathway. The pathway involves many catalysed enzyme reactions which start with glucose, a six-carbon carbohydrate, and end with two moles of three carbon intermediates, pyruvate. The end pyruvate may go to lactate or be converted to acetyl CoA for the tricarboxylic acid (TCA) cycle. The fermentation pathways from pyruvate and the resulting end products are shown in Figures 9.7 and 9.8. 

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.

Figure 17-9 Reaction sequences in fermentation based on the Embden-Meyerhof-Parnas pathway. Oxidation steps (producing NADH + H+) are marked "O" reduction steps (using NADH + H+) are marked "R."...

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. 

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