The Interaction of Cyanide with Cytochrome Oxidase

Abstract

The interaction of cyanide with the oxidised and reduced forms of cytochrome‐c oxidase has been investigated by kinetic and equilibrium measurements at 20 °C and pH 7.4. The inhibition by cyanide of the oxidation of cytochrome c has also been studied under different conditions. When the oxidised form of cytochrome oxidase is mixed with cyanide, the heme‐absorption bands are changed extremely slowly in a process whose rate is independent of the concentrations of cyanide and protein. Thus, the primary binding site does not appear to be one of the cytochrome components of the oxidase. With reduced cytochrome oxidase, the interaction can be described as a simple secondorder process involving the a32+ form and HCN. The stability constant for the complex is 1.8×103 M−1 and the rate constant for its formation 1.3X102M−1sec−1. The inhibition of the enzyme may occur in two ways. One involves the reduction of the enzyme by cytochrome c and a subsequent reaction between a32+ and the inhibitor. However, if the oxidase is preincubated with cyanide, inhibition has been reported at much lower concentrations of cyanide. This cannot be related to the much slower changes in the heme‐absorption bands observed in the reaction of cyanide with the oxidised form of the enzyme, and it is suggested that it involveds binding of cyanide to one of the copper ions in the oxidase.

Deciphering the Structural Role of Histidine 83 for Heme Binding in Hemophore HasA

“…The pK a of the transition from the fivecoordinated form and the six-coordinated form is ϳ8.4 (Scheme 1). Transitions involving deprotonation of bound water have been reported to occur with a wide range of pK a values (56,57). In the present case, exchange broadening prevents us from an accurate estimate of the pK a for the transition between the 6c HS and the 6c LS form (Scheme 1).…”

Quantitation and Characterization of CytochromecOxidase in Complex Systems

“…The method is based upon the distinctive optical signal of the cyanide-ferroheme a 3 compound in the visible region (12,13). Hemoglobin-like proteins do not contribute to the cyanide-binding spectrum because their affinity for cyanide in their reduced states is very poor.…”

A model for cytochrome oxidase.

“…This is in contrast to the ready reaction of partially reduced oxidase with azide which results in the conversion of a high-spin EPR signal to a low-spin one and demonstrates that the reaction is indeed with a heme iron under these conditions. Cyanide (8) and sulfide (9) behave similarly and it seems credible that the heme sites in the resting oxidase are blocked, but, upon reduction, a structural change occurs which exposes the liganding site to exogenous reagents. With very long incubation times, reaction of exogenous ligands with the small, steady-state concentration of conformationally perturbed molecules might occur.…”