Rate of electron transfer between plastocyanin, cytochrome ƒ, related proteins and artificial redox reagents in solution

The rate of electron transfer between reduced cytochrome ? and plastocyanin (both purified from parsley) has been measured as k = 3.6 · 10⁷ M^?1 · s^?1, at 298 °K and pH 7.0, with activation parameters ΔH^? = 44 kJ · mole^?1 and ΔS^? = +46 J · mole^?1 · °K^?1. Replacement of cytochrome ? with red algal cytochrome c-553, Pseudomonas cytochrome c-551 and mammalian cytochrome c gave rates at least 30 times slower: k = 5 · 10⁵, 7.5 · 10⁵ and 1.0 · 10⁶ M^?1 · s^?1, respectively.

Similar measurements made with azurin instead of plastocyanin gave k = 6 · 10⁶ and approx. 2 · 10⁷ M^?1 · s^?1 for reaction of reduced azurin with cytochrome ? and algal cytochrome respectively.

Rate constants of 115 and 80 M^?1 · s^?1 were found for reduction of plastocyanin by ascorbate and hydroquinone at 298 °K and pH 7.0. The rate constants for the oxidation of plastocyanin, cytochrome ?, Pseudomonas cytochrome c-551 and red algal cytochrome c-553 by ferricyanide were found to be between 3 · 10⁴ and 8 · 10⁴ M^?1 · s^?1.

The results are discussed in relation to photosynthetic electron transport.