Kinetics of cyanide binding to Chromatium vinosum ferricytochrome c'

The kinetics of the reversible binding of cyanide by the ferric cytochrome c' from Chromatium vinosum have been studied over the pH range 6.9-9.6. The reaction is extremely slow at neutral pH compared to the reactions of other high-spin ferric heme proteins with cyanide. The observed bimolecular rate constant at pH 7.0 is 2.25 X 10(-3) M-1 s-1, which is approximately 10(7)-fold slower than that for peroxidases, approximately 10(5)-fold slower than those for hemoglobin and myoglobin, and approximately 10(2)-fold to approximately 10(3)-fold slower than that recently reported for the Glycera dibranchiata hemoglobin, which has anomalously slow cyanide rate constants of 4.91 X 10(-1), 3.02 X 10(-1), and 1.82 M-1 s-1 for components II, III, and IV, respectively [Mintorovitch, J., & Satterlee, J. D. (1988) Biochemistry 27, 8045-8050; Mintorovitch, J., Van Pelt, D., & Satterlee, J. D. (1989) Biochemistry 28, 6099-6104]. The unusual ligand binding property of this cytochrome c' is proposed to be associated with a severely hindered heme coordination site. Cyanide binding is also characterized by a nonlinear cyanide concentration dependence of the observed rate constant at higher pH values, which is interpreted as involving a change in the rate-determining step associated with the formation of an intermediate complex between the cytochrome c' and cyanide prior to coordination. The pH dependence of both the binding constant for the formation of the intermediate complex and the association rate constant for the subsequent coordination to the heme can be attributed to the ionization of HCN, where cyanide ion binding is the predominant process.(ABSTRACT TRUNCATED AT 250 WORDS)