Comparison of Myoglobins from Harbor Seal, Porpoise, and Sperm Whale

Denaturation of dilute solutions of ferrimyoglobins at pH 6.50 and 50° by a 7- to 10-fold molar excess for copper(II) is completely reversible on addition of excess chelating agent. The renaturation reaction exhibits first order kinetics. In the ultracentrifuge, the copper(II)-denatured proteins show little aggregation except for an increase in sedimentation constant for the sperm whale protein from 1.9 to 2.8 to 2.9. Copper(II) denaturation of sperm whale protein does not result in liberation of the heme group from the protein. Bound copper(II) ion inhibits alkylation of ferrimyoglobin histidyl residues by iodoacetamide. First order rate constants for renaturation of porpoise, harbor seal, and alkylated sperm whale ferrimyoglobins have different pH-independent values under mildly acidic and mildly basic conditions; the transition between the two can be accounted for by the ionization of a single group with a value of pKa near 6.5. Renaturation of native sperm whale protein is nearly pH independent over the accessible range. In all cases, the entropy of activation for the renaturation process is near zero. The rate constants for ferrimyoglobin renaturation show little sensitivity to variations in ionic strength but are increased about 2-fold in the presence of 0.01 m benzene.