Binding and electron transfer reactions between methanol dehydrogenase and its physiologic electron acceptor cytochrome c-551i: a kinetic and thermodynamic analysis.

The quinoprotein methanol dehydrogenase and cytochrome c-551i form a physiologic complex in which electrons are transferred from pyrroloquinoline quinone to heme. The reoxidation of methanol dehydrogenase by the cytochrome was studied by stopped-flow spectroscopy. The rate constant for the electron transfer reaction and the dissociation constant for 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 yielded values of 1.9 eV for the reorganizational energy and 0.071 cm-1 for the electronic coupling and predicted a theoretical distance between redox centers of 15 A. Kinetically determined dissociation constants correlated well with a Kd of 375 microM which was determined in a direct ultrafiltration binding assay. Thermodynamic analysis of the dissociation constants indicated the importance of the hydrophobic effect in complex formation.