BINDING AND ELECTRON-TRANSFER REACTIONS BETWEEN METHANOL DEHYDROGENASE AND ITS PHYSIOLOGICAL ELECTRON-ACCEPTOR CYTOCHROME-C-551I - A KINETIC AND THERMODYNAMIC ANALYSIS

The quinoprotein methanol dehydrogenase and cytochrome c-55li form a p hysiologic complex in which electrons are transferred from pyrroloquin oline quinone to heme. The reoxidation of methanol dehydrogenase by th e cytochrome was studied by stopped-flow spectroscopy. The rate consta nt for the electron transfer reaction and the dissociation constant fo r complex formation were each determined at temperatures ranging from 20 to 50-degrees-C. The electron transfer rates varied from 1.4 to 4.6 s-1. Analysis of the electron transfer reaction by Marcus theory yiel ded values of 1.9 eV for the reorganizational energy and 0.071 cm-1 fo r the electronic coupling and predicted a theoretical distance between redox centers of 15 angstrom. Kinetically determined dissociation con stants correlated well with a K(d) of 375 muM which was determined in a direct ultrafiltration binding assay. Thermodynamic analysis of the dissociation constants indicated the importance of the hydrophobic eff ect in complex formation.