KINETIC AND THERMODYNAMIC ANALYSIS OF A PHYSIOLOGICAL INTERMOLECULAR ELECTRON-TRANSFER REACTION BETWEEN METHYLAMINE DEHYDROGENASE AND AMICYANIN

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 reoxidati on of MADH by amicyanin has been studied by stopped-flow spectroscopy. The rate constant for the electron-transfer (ET) reaction and the dis sociation constant for the complex have been determined at different t emperatures. Marcus theory was used to calculate the distance, reorgan izational energy, and electronic coupling for the intermolecular ET re action. The ET reaction exhibited a large apparent reorganizational en ergy of approximately 225 kJ mol(-1) (2.3 eV) and a coupling of approx imately 11.7 cm(-1) From X-ray crystallographic studies of an actual c omplex of these proteins from Paracoccus denitrificans [Chen, L., et a l. (1992) Biochemistry 31, 4959-4964], it was possible to infer putati ve pathways of ET. The ET distance predicted by Marcus theory from kin etic data correlated reasonably well with the structural information. Thus, it has been possible to correlate ET theories with data from sol ution studies and a known structure for a naturally occurring ET react ion between soluble proteins.