Kinetic and thermodynamic analysis of a physiologic intermolecular electron-transfer reaction between methylamine...

The quinoprotein methylamine dehydrogenase (MADH) and a type I copper protein, amicyanin, form a physiologic complex in which electrons are transferred from tryptophan tryptophylquinone to copper. The reoxidation of MADH by amicyanin has been studied by stopped-flow spectroscopy. The rate constant for the electron-transfer (ET) reaction and the dissociation constant for the complex have been determined at different temperatures. Marcus theory was used to calculate the distance, reorganizational energy, and electronic coupling for the intermolecular ET reaction. The ET reaction exhibited a large apparent reorganizational energy of approximately 225 kJ mol-1 (2.3 eV) and a coupling of approximately 11.7 cm-1. From X-ray crystallographic studies of an actual complex of these proteins from Paracoccus denitrificans [Chen, L., et al. (1992) Biochemistry 31, 4959-4964], it was possible to infer putative pathways of ET. The ET distance predicted by Marcus theory from kinetic data correlated reasonably well with the structural information. Thus, it has been possible to correlate ET theories with data from solution studies and a known structure for a naturally occurring ET reaction between soluble proteins.