Effect of citrate on the reactions of cytochrome c with reductants and cyanide.

The rate of reduction of cytochrome c by ascorbate is decreased in the presence of anions. This decrease is due to two factors: (a) nonspecific changes in ionic strength which occur when the total ion concentration or the charge on the anion is altered and (b) "specific" binding of anions to a site on cytochrome c which directly inhibits the reaction. The reaction between cyanide ion and cytochrome c is also affected by anions: increasing the ionic strength decreases the apparent association rate constant for cyanide binding. Substitution of citrate for morpholinopropane sulphonate in isoionic buffer media increases the apparent dissociation constant for cyanide suggesting citrate stabilizes the cytochrome c heme crevice. Binding of cytochrome c to cytochrome aa3 also affects the Kd for the cyanide - cytochrome c complex indicating that cytochrome c bound to cytochrome aa3 does not react with cyanide as readily as does free cytochrome c.