NADPH-NADP+, hydrogen transfer

Why are there four major hydrogen transfer coenzymes, NAD+, NADP+, FAD, and riboflavin phosphate (FMN), instead of just one Part of the answer is that the reduced pyridine nucleotides NADPH and NADH are more powerful reducing agents than are reduced flavins (Table 6-7). Conversely, flavin coenzymes are more powerful oxidizing agents than are... 

Whereas redox reactions on metal centres usually only involve electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer, but hydrogen transfer as well — hence the frequently used denomination dehydrogenase . Note that most of these dehydrogenase reactions are reversible. Redox reactions in biosynthetic pathways usually use NADPH as their source of electrons. In addition to NAD and NADP+, which intervene in redox reactions involving oxygen functions, other cofactors like riboflavin (in the form of flavin mononucleotide, FMN, and flavin adenine dinucleotide, FAD) (Figure 5.3) participate in the conversion of [—CH2—CH2— to —CH=CH—], as well as in electron transfer chains. In addition, a number of other redox factors are found, e.g., lipoate in a-ketoacid dehydrogenases, and ubiquinone and its derivatives, in electron transfer chains. 

NADP+/NADPH + H+ redox reactions and hydrogen transfer vitamin PP, (niacin)... 

Fig. 10. Hydrogen donor systems for ribonucleotide reduction. Enzyme reactions are I thioredoxin reductase (EC 1.6.4.5) II ribonucleotide reductase (EC 1.17.4) III glutathione reductase (EC 1.6.4.2). GSH, GSSG reduced and oxidized glutathione NADPH, NADP reduced and oxidized nicotinamide adenine dinucleotide phosphate coenzymes. The hydrogen transfer chain is continued in Fig. II...

Nicotinamide functions in the animal body as the active group of two important coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP).These coenzymes are involved in the mechanism of hydrogen transfer in living cells (see Chapter 9) NAD is involved in the oxidative phosphorylation system, the tricyclic acid (TCA) cycle and the metabolism of many molecules, including pyruvate, acetate, (3-hydroxy-butyrate, glycerol, fatty acids and glutamate NADPH is the hydrogen acceptor in the pentose phosphate pathway. 

This observation apparendy could not be explained without postulating hydrogen transport between NADP and NAD. However, in contrast to the mitochondria, the respective enzyme enabling the direct transfer of hydrogen from NADH to NADP in the cytoplasm was not so far described. In addition, as we have already stated, the ratios of the reduced forms of the coenzymes to the oxidized ones are in favor of NADPH and the transfer of hydrogen from NADH to NADP is energy dependent, even if it is an indirect transfer (Flatt and Ball, 1965). There is no absolute evidence, however, that NADH could not be direcdy utilized in the liver for the fatty acid synthesis. 

NAD PI I gives up hydrogen atoms to the flavo protein NADPH— cytochrome P450 reductase and becomes NADP+. The reduced flavo protein transfers these reducing equivalents to cytochrome P450. The reducing... 

An important aspect of enzymatic oxidation-reduction reactions involves the transfer of hydrogen atoms. This transfer is mediated by coenzymes (substances that act together with enzymes) nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). These two species pick up H atoms to produce NADH and NADPH, respectively, both of which can function as hydrogen atom donors. Another pair of species involved in oxidation-reduction processes by hydrogen atom transfer consists of flavin adenine triphosphate (FAD) and its hydrogenated form FADH2. The structural formulas of NAD and its cationic form, NAD+, are shown in Figure 4.7. 

The vast majority of alcohol dehydrogenases require nicotanimide cofactors, such as nicotinamide adenine dinucleotide (NADH) and its respective phosphate NADPH. The structure of NAD/NADP is shown in Fig. 3.39. Hydrogen and two electrons are transferred from the reduced nicotinamide to the carbonyl group to effect a reduction of the substrate (see Fig. 3.39). 

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