Redox cycling pathway converting

It is important not to confuse the reactions of Eq. 17-42 as they occur in an aerobic cell with the tightly coupled pair of redox reactions in the homolactate fermentation (Fig. 10-3 Eq. 17-19). Tire reactions of steps a and c of Eq. 17-42 are essentially at equilibrium, but the reaction of step b may be relatively slow. Furthermore, pyruvate is utilized in many other metabolic pathways and ATP is hydrolyzed and converted to ADP through innumerable processes taking place within the cell. Reduced NAD does not cycle between the two enzymes in a stoichiometric way and the "reducing equivalents" of NADH formed are, in large measure, transferred to the mitochondria. The proper view of the reactions of Eq. 17-42 is that the redox pairs represent a kind of redox buffer system that poises the NAD+/NADH couple at a ratio appropriate for its metabolic function. 

The fermentation of 1,3PD from glycerol (see Fig. 8.8c) was discovered in the late 19th century [60]. It has since been found that a considerable number of bacteria can use glycerol as a source of carbon and energy under anaerobic conditions and the reaction pathways have been elucidated [61]. Out of every three molecules of glycerol, one is oxidized, phosphorylated into DHAP and subsequently metabolized via the glycolysis pathway and the TCA cycle the other two are converted into 1,3PD, to maintain redox balance (see Fig. 8.9), via dehydration and NADH-driven reduction. 

Niacin is a water-soluble vitamin. The RDA of niacin for the adult man is 19 mg. Niacin is converted in the bi>dy to the cofactor nicotinamide adenine dinucleotide (NAD). NAD also exists in a phosphorylated form, NADP The phosphate group occurs on the 2-hydrr>xyl group of the AMP half of the coenzyme, NAD and NADP are used in the catalysis of oxidation and reduction reactions. These reactions are called redox reactions. NAD cycles between the oxidized form, NAD, and the reduced form, NADH + H. The coenzyme functions to accept and donate electrons. NADP behaves in a similar fashion. It occurs as NADP and NADPH + HT The utilization of NAD is illustrated in the sections on glycolysis, the malatc-aspartate shuttle, ketone body metabolism, and fatty acid oxidation. The utilization of NADP is illustrated in the sectirrns concerning fatty acid synthesis and the pentose phosphate pathway. 

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