Adrenodoxin reductase and adrenodoxin. Mechanisms of reduction of ferricyanide and cytochrome c.
Adrenodoxin reductase, the flavoprotein moiety of the adrenal cortex mitochondrial steroid hydroxylating system, participates in adrenodoxin-dependent cytochrome c and adrenodoxin-independent ferricyanide reduction, with NADPH as electron donor for both of these 1-electron reductions. For ferricyanide reduction, adrenodoxin reductase cycles between oxidized and 2-electron-reduced forms, reoxidation proceeding via the neutral flavin (FAD) semiquinone form (Fig. 9). Addition of adrenodoxin has no effect upon the kinetic parameters of flavoprotein-catalyzed ferricyanide reduction. For cytochrome c reduction, the adrenodoxin reductase-adrenodoxin 1:1 complex has been shown to be the catalytically active species (Lambeth, J. D., McCaslin, D. R., and Kamin, H. (1976) J. Biol. Chem. 251, 7545-7550). Present studies, using stopped flow techniques, have shown that the 2-electron-reduced form of the complex (produced by reaction with 1 eq of NADPH) reacts rapidly with 1 eq of cytochrome c (k approximately or equal to 4.6 s-1), but only slowly with a second cytochrome c (k = 0.1 to 0.3 s-1). However, when a second NADPH is included, two more equivalents of cytochrome are reduced rapidly. Thus, the adrenodoxin reductase-adrenodoxin complex appears to cycle between 1- and 3-electron reduced states, via an intermediate 2-electron-containing form produced by reoxidation by cytochrome (Fig. 10). For ferricyanide reduction by adrenodoxin reductase, the fully reduced and semiquinone forms of flavin each transfer 1 electron at oxidation-reduction potentials which differ by approximately 130 mV. However, adrenodoxin in a complex with adrenodoxin reductase allows electrons of constant potential to be delivered from flavin to cytochrome c via the iron sulfur center...
D000302:Adrenal Cortex / Q000032:analysis; D000325:Adrenodoxin / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002417:Cattle; D003574:Cytochrome c Group / Q000378:metabolism*; D004579:Electron Transport; D005287:Ferredoxin-NADP Reductase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D005292:Ferricyanides / Q000378:metabolism*; D007700:Kinetics; D008928:Mitochondria / Q000032:analysis; D008954:Models, Biological; D009247:NADH, NADPH Oxidoreductases / Q000378:metabolism*; D009249:NADP / Q000378:metabolism; D010084:Oxidation-Reduction; D011485:Protein Binding; D013816:Thermodynamics
D003574:Cytochrome c Group; D005292:Ferricyanides; D000325:Adrenodoxin; D009249:NADP; D005287:Ferredoxin-NADP Reductase; D009247:NADH, NADPH Oxidoreductases