EFFECT OF PRESSURE ON THE FORMATION AND DEOXYGENATION KINETICS OF OXYMYOGLOBIN - MECHANISTIC INFORMATION FROM A VOLUME PROFILE ANALYSIS

The effect of pressure on the formation and deoxygenation kinetics of oxymyoglobin was studied by using temperature-jump and stopped-flow techniques, respectively. The corresponding volumes of activation are + 5.2 ± 0.5 and + 23.3 ± 1.8 cm3 mol–1, which result in a reaction volume of –18.1 ± 2.3 cm3 mol–1 for the system Mb + O2 ⇌ MbO2. The latter was also measured directly from the pressure dependence of the equilibrium constant and resulted in a reaction volume of –19.3 ± 1.5 cm3 mol–1. A volume profile analysis indicated that bond breakage during deoxygenation proceeds according to a dissociative mechanism with almost complete Mb-O2 bond cleavage in the transition state. This suggests that almost no bond formation with the heme center occurs in the transition state during the oxygenation of myoglobin. The volume increase observed during this process is ascribed to hydrogen-bonding effects accompanied by desolvation as the oxygen molecule finds its way through the protein. The results are discussed in reference to related studies reported in the literature. © 1990, American Chemical Society. All rights reserved.