A new spectral intermediate in cyanide binding with the oxidized cytochrome c oxidase

Reaction of cyanide with oxidized cytochrome c oxidase at a low concentration of the ligand and pH > 8 reveals an initial phase, not reported earlier, associated with a small blue shift of the absorption spectrum, which is followed by a conventional red shift of the heme a₃³⁺. The initial blue shift resembles the spectral changes induced under the same conditions by low concentrations of azide and it is not observed in the presence of 0.3 mM azide. It is suggested that, similarly to NO, cyanide and HN₃ cannot only bind to heme a₃ but to Cu²⁺_B as well, perturbing the spectrum of a³⁺₃ indirectly. A rapid binding to Cu²⁺_B could provide the long-sought intermediate in the cyanide reaction with heme a³⁺₃, the existence of which is implied by the Michaelis-Menten type kinetics of the latter process.