Ionic strength dependence of the reaction between methanol dehydrogenase and cytochrome c-551i: evidence of conformationally coupled electron transfer.
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.(ABSTRACT TRUNCATED AT 250 WORDS)
Harris
T K
TK
Department of Biochemistry, University of Mississippi Medical Center, Jackson 39216.
Davidson
V L
VL
Chen
L
L
Mathews
F S
FS
Xia
Z X
ZX
D000429:Alcohol Oxidoreductases / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism; D003574:Cytochrome c Group / Q000378:metabolism*; D004579:Electron Transport / Q000502:physiology; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D009994:Osmolar Concentration; D010231:Paracoccus denitrificans / Q000378:metabolism; D011487:Protein Conformation
D001426:Bacterial Proteins; D003574:Cytochrome c Group; C017167:cytochrome C(551); D000429:Alcohol Oxidoreductases; C019641:alcohol dehydrogenase (acceptor)