title: Model studies for molybdenum enzymes. The reduction of cytochrome c by molybdenum(V)-cysteine complexes.
abstract: The reduction of ferricytochrome c by two molybdenum(V)-cysteine complexes has been investigated as a model for electron transfer in the molybdenum enzymes sulfite oxidase and nitrate reductase. The reduction by the dioxo-bridged Mo(V)-cysteine complex, di-mu-oxo-bis-[oxo(L-cysteinato)molybdate(V)] (I), is relatively slow and its rate is first order in cyt cIII and zero order in I (k = (1.09 +/- 0.10) times 10(-3) sec minus 1, pH 7.5, 20 degrees). The reduction by the monoxo-bridged complex, mu-oxo-bis[oxodihydroxo(L-cysteinato)molybdate(V)] (II), is extremely rapid and its rate is first order in both reactants (k = (2.6 +/- 0.7) times 10(7) M minus 1 sec minus 1, pH 7.0, 25 degrees). Above pH 7.5, the reduction by II follows biphasic kinetics due to the fast reduction of a low pH form of cyt cIII and a slower reduction of a high pH form (at pH 10.0, 25 degrees, k = 2.9 times 10(6) M minus 1 sec minus 1 for the low pH form and k = 7.2 times 10(4) M minus 1 sec minus 1 for the high pH form). Reaction mechanisms for reductions by both I and II are proposed and the biological implications of the results, both for sulfite oxidase and mechanisms of electron transfer to cytochrome c, are discussed.
mesh_terms: D000818:Animals; D001665:Binding Sites; D003545:Cysteine; D003574:Cytochrome c Group; D004579:Electron Transport; D006736:Horses; D006863:Hydrogen-Ion Concentration; D008433:Mathematics; D008982:Molybdenum; D009206:Myocardium / Q000201:enzymology; D009565:Nitrate Reductases; D010084:Oxidation-Reduction; D010088:Oxidoreductases; D011485:Protein Binding; D011487:Protein Conformation; D013447:Sulfites
chemical_list: D003574:Cytochrome c Group; D013447:Sulfites; D008982:Molybdenum; D010088:Oxidoreductases; D009565:Nitrate Reductases; D003545:Cysteine