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

Abstract

The rate of reduction of [bovine heart] cytochrome c by ascorbate is decreased in the presence of anions. This decrease is due to 2 factors: nonspecific changes in ionic strength which occur when the total ion concentration or the charge on the anion is altered and 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 sulfonate in isoionic buffer media increases the apparent Kd 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.