Volume changes associated with cytochrome c oxidase-porphyrin cytochrome c equilibrium

The binding of a fluorescent derivative of cytochrome c to cytochrome c oxidase has been studied by use of pressure to perturb the equilibrium. delta Vo for the reaction oxidase-porphyrin cytochrome (formula; see text) was small and favored dissociation of the complex. Pressure-induced dissociation is to be expected if the major forces governing the equilibrium are electrostatic in nature. The dependence of log Kd on pressure is not linear but biphasic; high pressures lead to a decrease in Kd and association of the reactants. The latter fact indicates that the net compressibility of the complexes is greater than that of the free reactants, an unexpected result.