Electron transfer between soluble and immobilized mammalian cytochrome c. Equilibrium and kinetic studies on immobilized cytochrome c

Relation of the structure and function of ferricytochrome c bound to the phosphoprotein phosvitin

The relationship between the structure and function of ferricytochrome c bound to the phosphoprotein phosvitin was investigated. The rates of reduction of phosvitin-bound ferricytochrome c by cytochrome b2, ascorbate and the superoxide radical generated by xanthine oxidase wer repressed where the binding ratio was less than half the maximum, but at higher ratios they were restored gradually with increase in the ratio. The affinity of cytochrome b2 for cytochrome c was not affected by binding of cytochrome c to phosvitin. The redox potential of the bond form was lower than that of the free form and only decreased with decrease in the ratio. The conformatin around the heme moiety and the electronic structure of the heme group of bound ferricytochrome c were similar to those of free ferricytochrome c, but the conformational stability in the vicinity of the prosthetic group was related to the binding ratio as ratios above half the maximum and was well correlated with the reduction rate. Since the binding of cytochrome c to phosvitin is much stronger at binding ratios below half the maximum, these results suggest that this binding strength exclusively affects the conformational flexibility of the heme crevice in the cytochrome molecule, thus altering the reduction rate.

Properties of cytochrome c modified by attachment to a carbohydrate polymer

By enzymic digestion of the polysaccharide part of the covalent complex between cytochrome c and Sephadex G-200, a new water-soluble cytochrome c derivative is obtained (called cytochrome cr). Measurement of the free amino groups of this derivative indicates that on average the molar ratio between cytochrome c and polysaccharide is close to 1. Chemical determination of the sugar content gives a value of approx. 24000 for the molecular weight of cytochrome cr. On these bases the soluble cytochrome cr complex may be thought of as a folded protein to which a long polysaccharide tail is covalently bound. The functional behaviour of cytochrome cr is much more similar to that of the native molecule than to that of the insoluble complex (cytochrome ci). In particular the kinetics of the reaction of cytochrome cr and cytochrome cn (native) with ascorbate, ferrocyanide-ferricyanide, O2 and cytochrome c oxidase were investigated in considerable detail. The results of these experiments, together with the observation that the insoluble complex of cytochrome c is a very poor substrate of cytochrome c oxidase [Colosimo, Brunori & Antonini (1976) Biochem. J. 153, (657-661], indicate that hindrance effects constraining the approach between cytochrome cr and its oxidase are of greater importance than specific chemical modifications in determining the functional behavior of the protein.