Transient spectroscopy of the reaction of cyanide with ferrous myoglobin: Effect of distal side residues

The reaction of cyanide metmyoglobin with dithionite conforms to a two-step sequential mechanism with formation of an unstable intermediate, identified as cyanide bound ferrous myoglobin. This reaction was investigated by stopped-flow time resolved spectroscopy using different myoglobins, i.e. those from horse heart, Aplysia limacina buccal muscle, and three recombinant derivatives of sperm whale skeletal muscle myoglobin (Mb) (the wild type and two mutants). The myoglobins from horse and sperm whale (wild type) have in the distal position (E7) a histidyl residue, which is missing in A. limacina Mb as well as the two sperm whale mutants (E7 His → Gly and E7 His → Val). All these proteins in the reduced form display an extremely low affinity for cyanide at pH < 10. The differences in spectroscopy and kinetics of the ferrous cyanide complex of these myoglobins indicate a role of the distal pocket on the properties of the complex. The two mutants of sperm whale Mb are characterized by a rate constant for the decay of the unstable intermediate much faster than that of the wild type, at all pH values explored. Therefore, we envisage a specific role of the distal His (E7) in controlling the rate of cyanide dissociation and also find that this effect depends on the protonation of a single ionizable group, with pK = 7.2, attributed to the E7 imidazole ring. The results on A. limacina Mb, which displays the slowest rate of cyanide dissociation, suggests that a considerable stabilizing effect can be exerted by Arg E10 which, according to Bolognesi et al. (Bolognesi, M., Coda, A., Frigerio, F., Gatti, C., Ascenzi, P., and Brunori, M. (1990) J. Mol. Biol. 213, 621-625), interacts inside the pocket with fluoride bound to the ferric heme iron. A mechanism of control for the rate of dissociation of cyanide from ferrous myoglobin, involving protonation of the bound anion, is discussed.