The reduction of cytochrome c by iron EDTA-like complexes: implications on charge effect corrections to mediator-protein rate constants

Kinetics and mechanisms of the oxidation of myoglobin by Fe(III) and Cu(II) complexes

Two distinct mechanisms by which sperm whale myoglobin reduces, respectively, complexes of Fe(III) and Cu(II) and, in turn, is oxidized to metmyoglobin have been characterized. For both mechanisms, deoxymyoglobin is the active reductant. An outer sphere electron transfer, probably at the edge of the heme, is involved for Fe(III)NTA (NTA is nitrilotriacetic acid). This pathway does not involve ionic binding of the Fe(III) complex to the protein. The most reactive species of Fe(III)NTA is uncharged. No inhibition is observed with Ni(II) or Zn(II). An outer sphere site specific electron transfer is operative for reduction of Cu(II) complexes. The site has been characterized using NMR spectroscopy and involves one or more histidines. There is an initial binding of the Cu(II) chelate. The ternary complex of chelator-Cu(II)-deoxymyoglobin is a mandatory intermediate. Ni(II) and Zn(II) compete with Cu(II) for the binding site. A scheme for the participation of either or both of these mechanisms in reduction reactions of heme proteins is proposed. Both the overall redox potential, delta E0, and the stability constant for the ternary complex, K, govern the pathway and the reaction rate.

Reactions of mitochondrial cytochrome c with iron-polyaminocarboxylate complexes

The binding of Fe(III)-polyaminocarboxylate (pac) complexes to mitochondrial ferricytochrome c and ferrocytochrome c has been studied by NMR and four binding sites on cytochrome c have been defined. These sites have different binding preferences for different pac complexes and these are not correlated with the kinetic classification of Wei and Ryan [J. Inorg. Biochem. 17, 237-246 (1982]. It is suggested that the classification of Wei and Ryan is faulty because an assumption made by these authors is incorrect. This assumption is that the reduction of ferricytochrome c by Fe(II)-(pac) complexes proceeds in all cases with similar electron transfer mechanisms with each of the pac complexes having similar self-exchange rates.