Substrate binding affinity changes in mitochondrial energy-linked reactions.

The effects of uncouplers and valinomycin plus nigericin (in the presence of K+) were studied on the apparent Km for substrates and apparent Vmax of the following energy-linked reactions catalyzed by submitochondrial particles: oxidative phosphorylation, NTP-33Pi exchange, ATP-driven electron transfer from succinate to NAD, and respiration-driven transhydrogenation from NADH to 3-acetylpyridine adenine dinucleotide phosphate. In all cases, partially uncoupling (up to 90%) concentrations of uncouplers of valinomycin plus nigericin were found to decrease apparent Vmax and to increase apparent Km. Results plotted as ln (Vmax/Km) versus the concentration of uncouplers or ionophores showed a linear decrease of the former as a function of increasing perturbant concentration (i.e., decreasing free energy). Because Vmax/Km may be considered as a measure of the apparent first-order rate constant for enzyme-substrate interaction and reflects the affinity between enzyme and substrate to form a complex, the results are consistent with the interpretation that membrane energization leads to a change in enzyme conformation with the resultant increase in enzyme-substrate affinity and facilitation of the reaction rate under consideration. The significance of these findings with respect to the mechanism of action of the energy-transducing systems studied is discussed.