Relaxation analysis of ligand binding to the myoglobin reconstituted with cobaltic heme.

Myoglobin reconstituted with oxidized Co(III) deuteroheme was found to exhibit relatively large affinities to CN(-), N3(-), SCN(-), pyridine, and imidazole contrary to the early proposal. The ligand-induced changes in electronic spectra were less obvious than those of Fe(III) myoglobin owing to the absence of accompanied spin-state transition on the ligand binding. The relaxation kinetic analysis revealed that the ligand association rates were small and that the dissociation rates were still much smaller. The relatively large ligand affinities in Co(III) myoglobin were found due to the compensation of small association rates with fairly smaller dissociation rates. This is in marked contrast with the ligation profile in Fe(III) myoglobin where large association rates and small dissociation rates are generally observed. The rationale for the characteristic ligand-binding behavior of Co(III) myoglobin was provided on the basis of the properties of Co(III) which has an additional negative charge and forms stronger metal-ligand bonds than Fe(III).