Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy-Yielding Processes
Fermentation
Alcoholic Fermentation

The predominant product of alcoholic Fermentation is ethanol. This type of fermentation is characteristic of certain species of Yeasts, Bacteria, and Molds. There are 3 Main Pathways of alcoholic fermentation, which differ depending on which of the three Catabolic pathways for glucose (and certain other sugars) breakdown is utilized: Glycolysis, the Pentose Phosphate Pathway, or the Entner–Doudoroff pathway. Most commonly, as noted above, Cells use glycolysis, and its end product, Pyruvate, most frequently serves as the substrate for alcoholic fermentation.

Fig. 10.1 illustrates two pathways of alcoholic fermentation. The first pathway involves Glucose Catabolism via the glycolytic route (characteristic of yeasts and the bacterium Sarcina ventriculi), while the second relies on The pentose phosphate pathway (Heterofermentative lactic acid bacteria). Consequently, the Enzymes involved and the range of products vary accordingly. A third pathway of alcoholic fermentation, utilized by Zymomonas mobilis bacteria, is distinguished by the fact that glucose is catabolized via the Entner–Doudoroff pathway. The resulting pyruvate undergoes decarboxylation, just as in the first case, and is stepwise reduced to ethanol. A portion of the pyruvate is reduced to lactic acid.

Class="center">

Fig. 10.1. Pathways of alcoholic fermentation: 1 — based on glycolytic glucose breakdown; 2 — based on the pentose phosphate pathway. Only the number of synthesized, but not consumed, moles of ATP is shown

Fig. 10.1 clearly demonstrates that the number of NAD⁺ (NADP⁺) molecules consumed during glycolysis and the pentose phosphate pathway strictly matches the number of NAD⁺ (NADP⁺) molecules regenerated during alcoholic fermentation. At the same time, Substrate-Level Phosphorylation Reactions leading to ATP synthesis occur exclusively in the catabolic pathways prior to pyruvate formation. In the actual reactions of alcoholic fermentation associated with The conversion of pyruvate into ethanol or lactate, no energy is conserved.

The course of fermentation is heavily dependent on environmental conditions within which cells operate. Furthermore, some microorganisms exhibit certain deviations from the classical schemes of alcoholic fermentation outlined above. As a result, THE SPECTRUM OF fermentation products can vary significantly and, alongside ethanol and lactate, may include trace amounts of glycerol, acetaldehyde, other aldehydes, esters, ketones, higher alcohols, organic acids, etc.