Abstract
We have undertaken a kinetic study of ubiquinol cytochrome c reductase, aimed at developing a better understanding of the interaction of its water-soluble substrate, cytochrome c, in bovine heart submitochondrial particles.We have confirmed that the enzyme kinetics is of a ping-pong two-site mechanism and is not very sensitive to ionic strength; nevertheless, the kcat/Km ratio, representing the minimal value of the association rate constant of cytochrome c, is decreased by increasing ionic strength. Several lines of evidence suggest that the association of cytochrome c with its active site is diffusion limited, including the high value of the kcat/Km ratio (greater than 108 M−1 s−1), its sensitivity to the viscosity of the medium, and its low activation energy.A break in the Arrhenius plot of the enzymic rate, obtained at cytochrome c concentrations considered saturating for the solubilized enzyme (about 15 μM), is artifactual and due to the steep increase in Km for cytochrome c at increasing temperatures. Accordingly, the Arrhenius plot becomes linear when the cytochrome c concentration is increased to over 50 μM.
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