The quinoprotein methanol dehydrogenase and cytochrome c-551i are two soluble acidic proteins that form a physiological complex in which electrons are transferred from pyrroloquinoline quinone to heme. The oxidation of methanol dehydrogenase by the cytochrome was studied as a function of ionic strength using stopped-flow spectroscopy. The dissociation constant (Kd) for complex formation decreased 2-fold with increasing ionic strength from 0.21 to 1.3 M and increased at higher ionic strengths. The rate constant for the electron transfer reaction (kET) increased 2-fold with increasing ionic strength from 0.21 to 1.3 M and decreased at higher ionic strengths. The variation of Kd and kET over this range of ionic strengths was described by Van Leeuwen theory, which takes into account monopole-dipole and dipole-dipole forces, in addition to the monopole-monopole force, to predict the interactions between large molecules. Analysis of the kinetic results in terms of these electrostatic interactions indicated the probable orientations for protein-protein binding and electron transfer. To explain the ionic strength dependence of the observed kET, a model is presented in which the true kET is reduced by a factor Kc, an equilibrium constant that describes some rearrangement of the proteins after a nonoptimal collision to produce the most efficient orientation for electron transfer. This model is consistent with the notion that the large reorganizational energy obtained from temperature-dependence studies of this electron transfer reaction [Harris, T. K., & Davidson, V. L. (1993) Biochemistry 32, 14145-14150] is due to such an intracomplex rearrangement. Kinetic schemes are presented that distinguish between an electron transfer reaction that is absolutely gated and one that is conformationally coupled, such as that between methanol dehydrogenase and cytochrome c-551i.
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