The kinetics and specificity of electron transfer from cytochromes and copper proteins to P700.

The rates of electron transfer to P700 from plastocyanin and cytochrome f have been compared with those from three other c-type cytochromes and azurin, a copper protein resembling plastocyanin. Three different disruptive techniques were used to expose P700; digitonin, Triton X-100 and sonication. The following rate constants were measured at 25 degrees C, pH 7.0, with digitonin-treated chloroplasts: plastocyanin, 8 x 10(7)M(-1) x s(-1); red-algal cytochrome c-553, 1.9 x 10(7)M(-1) x s (-1); Pseudomonas cytochrome c-551, 8 x 10(6)M(-1) x s (-1); azurin, less than or = 3 x 10(5)M(-1) x s (-1); cytochrome f, less than or = 2 x 10(4)M(-1) x s (-1); mammalian cytochrome c, less than or = 2 x 10(4)M(-1) x s (-1). For electron transfer from plastocyanin, the effects of ionic strength, pH and temperature were also studied, and saturation effects found in earlier work were avoided by a full consideration of the various secondary reactions and inclusion of superoxide dismutase. The relative rates are discussed in relation to photosynthetic electron transport.