Biochemistry - Chemical Reactions in Living Cells, Volume 2 - D. Metzler 1980

Types of reactions catalyzed by enzymes
Brief overview of some metabolic pathways
Fermentation

While The conversion of Pyruvate to acetyl-CoA and the subsequent oxidation of the latter leads to the complete combustion of glucose to carbon dioxide and Water, there is an alternative glycolytic pathway in which sugar fermentation occurs in the absence of oxygen. For example, lactic acid Bacteria can reduce pyruvate to lactate using NADH (see Fig. 7-1, lower left). Note that this reduction reaction is perfectly balanced with the preceding oxidation step—namely, The oxidation of glyceraldehyde 3-phosphate to 3-phosphoglycerate. With a balanced sequence of oxidation followed by reduction, the conversion of glucose to lactate, i.e., fermentation, can proceed anaerobically without electron transfer in the Respiratory Chain.

Living organisms cannot exist without energy, and therefore, among the reactions of fermentation, the most crucial one is that responsible for ATP generation. In Lactic acid fermentation and most other Types of fermentation, this reaction is the oxidation of glyceraldehyde 3-phosphate to 3-phosphoglycerate. The oxidation of an aldehyde to a carboxylic acid is a strongly exergonic reaction coupled with ATP synthesis. Because each glucose molecule yields two triose phosphate molecules, fermentation produces two molecules of ATP per molecule of glucose consumed. This is entirely sufficient to sustain bacterial life, provided there is an adequate supply of fermentable sugar. The anaerobic conversion of glucose to lactate is just one example of the many diverse fermentation processes that we will examine in Chapter 9.

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FIG. 7-1. Brief Overview of selected metabolic pathways. Bold lines indicate some of the most important Catabolic pathways, while dashed lines represent biosynthetic pathways. Several sites of ATP synthesis and utilization are shown, as well as several Reactions Involving the formation or consumption of reduced hydrogen carriers (NADH, NADPH, and FADH₂).