Myoglobin interspecies structural differences: effects on autoxidation and oxygenation.

Thermodynamic and kinetic parameters of the oxygenation and autoxidation reactions were determined for yellowfin tuna (Thunnus albacares), Pacific green sea turtle (Chelonia mydas caranigra), and sperm whale myoglobin. These proteins have quite similar heme contact residues, but vary in residues which are involved in globin stabilization and in postulated ligand paths to the heme iron. Oxygen equilibria measurements from 10 to 40 degrees C reveal that green sea turtle myoglobin has a lower oxygen affinity and lower enthalpy and entropy for oxygen-binding than the other proteins. Yellowfin tuna myoglobin has the most rapid oxygen dissociation rate and the highest susceptibility to autoxidation under all conditions studied. However, the dependence of the autoxidation rate on ionic strength is the same for the three proteins, despite substantial differences in their dynamic stabilities. These results suggest that the autoxidation reaction is more dependent on ligand accessibility than on the dynamic stability of the myoglobin.