NADH redox transfer mechanisms

C. Cyclopropane Models for NADH Redox Transfer Mechanisms... 

But in mammalian species, several mechanisms exist for transferring NADH reducing equivalents, i.e., the malate-aspartate cycle (10) and the a-glycerophosphate-dihydroxy acetone phosphate cycle (16). It seems unlikely that the magnitude of NADH redox transfers due to the interconversions of proline - P5C would alter the N AD+/NADH balance. The oxidation of NADPH by P5C reductase, on the other hand, may play a major role in regulating NADP+/NADPH. Importantly, the of P5C reductase for NADPH is markedly lower than that for NADH (86,117). Although the V .. activities are higher with NADH than with NADPH the conversion of P5C to proline by PC reductase would affect NADP / NADPH more than NAD+/NADH if one considers the respective in vivo concentrations and the respective redox ratios of the two pyridine nucleotides. 

However, some data have been more difficult to incorporate into the mechanism shown in Figs. 8 and 9. As reported 21) in Section II,B the Fe protein can be reduced by two electrons to the [Fe4S4]° redox state. In this state the protein is apparently capable of passing two electrons to the MoFe protein during turnover, although it is not clear whether dissociation was required between electron transfers. More critically, it has been shown that the natural reductant flavodoxin hydroquinone 107) and the artificial reductant photoexcited eosin with NADH 108) are both capable of passing electrons to the complex between the oxidized Fe protein and the reduced MoFe protein, that is, with these reductants there appears to be no necessity for the complex to dissociate. Since complex dissociation is the rate-limiting step in the Lowe-Thorneley scheme, these observations could indicate a major flaw in the scheme. 

Because reduced redox cofactors, NADH and FADH2, are produced in the mitochondria, there is no need for shuttle mechanisms to reoxidize them via oxidative phosphorylation. NADH is reduced directly by complex I. FADH2 is reduced by the electron transfer flavoprotein, which then reduces ubiquinone. See Chapter 17 for details. 

With the exception of reduced -nicotinamide adenine dinucleotide (NADH), substrates interact at the Mo centre and two electrons are transferred from the substrate to Mo(VI), reducing the metal to Mo(IV). The substrate residue reacts with an oxo ligand of Mo and a proton also reduces a terminal sulphide ligand of Mo. Hydrolysis of the Mo-substrate complex releases oxidized product, while the Mo(IV) is reoxidized by intramolecular transfer to other redox centres. The catalytic mechanism as described by Bray is depicted below [23], Aldehyde oxidase and xanthine oxidase can each take up to six... 

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