NADH reduction

There are other reactions apart from NADH reduction (Sect 4.1) where the hydride equivalent shifts between electron donors and acceptors without bond formation between the n bonds. The hydride equivalent transfer must be reactions in the transfer band. In fact, a photochemical reaction between donors and acceptors is similar to thermal reactions between strong donors and acceptors. This further supports the mechanistic spectrum (Scheme 32). 

As explained in detail in Chapter 3, the flavimNADH reductase catalyzes the NADH reduction reaction, i.e. ... 

All parasitic flatworms capable of anaerobic metabolism favour malate as the primary mitochondrial substrate and the oxidative decarboxylations of first malate and then pyruvate generate intramitochondrial reducing power in the form of NADH (Fig. 20.1). In contrast, the pathways used to reoxidize intramitochondrial NADH are quite diverse and depend on the stage or species of parasite under examination, but in all cases, redox balance is maintained and electron-transport associated ATP is generated by the NADH-reduction of fumarate to succinate. In the cestode, hi. diminuta, succinate and acetate are the major end products of anaerobic malate dismutation and are excreted in the predicted 2 1 ratio. In the trematode F. hepatica, succinate is then further decarboxylated to propionate with an additional substrate level phosphorylation coupled to the decarboxylation of methylmalonyl CoA. F. hepatica forms primarily propionate and acetate as end products, again in a ratio of 2 1 to maintain redox balance. 

The reaction mechanism is similar to many NADH reductions (Fig. 13-24) it is approximately the reverse of steps 2 and 3 of Figure 14-7. The transition state involves a strongly polarized carbonyl group of the pyruvate molecule as shown below ... 

The active enzyme may be regenerated during this process or by a successive reactivation step such as NADH reduction. In this way, the oxidation also proceeds catalytically. 

In Nox, NADH reduction of the thiolate form involves the stabilization of the four-electron reduced enzyme ... 

Even if pulse radiolysis has been used [13], most studies of the initial electron entry into cytochrome oxidase are based on the finding that at low ionic strength cytochrome c forms a strong electrostatic complex with the oxidase [14]. Cytochrome c itself cannot be photoactivated, but in the complex with the enzyme it can be reduced by photochemically generated radicals, such as that of 5-diazariboflavin [15]. Another approach has been to employ a photoinduced uroporphyrin/NADH reduction system [16]. 

Coenzymes and vitamins are Nature s versions of common organic reagents Reductions with NADH Reductive amination, deamination, and decarboxylation with pyridoxal Enol chemistry with lysine enamines, with coenzyme A, and with phosphoenolpyruvate Umpolung chemistry with thiamine as a d reagent... 

Reduction of Cu and Fe organic complexes is saturable, and is inhibited by cell-impermeable probes, including polyclonal antibodies raised to Chlorella nitrate reductase (Jones and Morel, 1987). The model proposed suggests that a fraction of a membrane-bound nitrate reductase (the so-called diaphorase component that catalyzes NADH reduction), spans the cell membrane (Fig. 4). Organic metal complexes intercept electrons (destined for nitrate reduction intracellularly) at the outer cell surface. 

The induction in B. megaterium by barbiturates, and the fact that the enzyme is unusual in having a single 119-kD protein coupling NADH reduction to oxygenation (Narhi and Fulco 1986). 

Fe+2-Fe+V 424 409 185 Assayed after NADH reduction of microsomal samples or... 

Figure 10. Microsome-catalyzed NADH reduction of Cl(NHo)r,Ru(lll) cmd subsequent formation of (Isonicotinamide)(NH3)sRu(II) A, reaction run in air B, reaction run under Ar C, reaction run under Ar in presence of metyrapone D, reaction run under N2 (3S)...

The mechanism for NADH reduction of the carbonyl group of an aldehyde or a ketone follows. First, the carbonyl-containing compound and NADH are positioned on the surface of the enzyme catalyst in a highly specific relationship to each other. Then follows a redistribution of valence electrons, one part of which is the transfer of a hydrogen atom with its pair... 

Sun, I., Morre, D. J., Crane, F. L., Safranski, K., and Croze, E. M., 1984, Monodehydroascorbate as an electron acceptor for NADH reduction by coated vesicle and Golgi apparatus fractions of rat liver, Biochim. Biophys. Acta 797 266-275. 

The transformation may look complicated because you re probably unfamiliar with molybdenum. Note, though, that the reactions taking place on xanthine are familiar and we ve seen them numerous times. Thus, the initial nucleophilic addition of an oxygen anion to C=N is similar to what occurs in step 2 when water adds to guanine, and the subsequent expulsion of hydride ion by the adjacent nitrogen atom is analogous to what occurs during NADH reductions (Section 14.10). 

The COOH functional group of carboxylic acids is not reduced. Write the products of the NADH reduction of each of the molecules below. 

For some anaerobic organisms, such as yeasts, pyruvate is decarboxylated and NAD+ is regenerated by NADH reduction of acetaldehyde to ethanol. 

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