Kinetic and Equilibrium Studies of the Ligand Binding Reactions of Eight Electrophoretic Components of Sperm Whale Ferrimyoglobin

The reactions of eight electrophoretic components of sperm whale ferrimyoglobin with fluoride, azide, and cyanide have been studied. There do not appear to be significant differences in rate constants or equilibrium constants among the various components. We conclude that at pH 7.0 in 0.05 M potassium phosphate these ligand binding kinetics and equilibria are insensitive to the net charge on the protein. The variation of the azide equilibrium constant with ionic strength from μ 0.01 M to 0.11 M is not in accord with the predictions of the Debye-Hückel theory. On the other hand, azide association kinetic and equilibrium constants are respectively six- and threefold greater for beef ferrimyoglobin than for the isoelectric whale ferrimyoglobin (band V). An examination of the data for whale, horse, and beef myoglobins reveals that large differences in azide (but not CO) association rate constants are associated with amino acid substitutions at residues 45 and 99 in the heme cavity.