Electron transfer from copper to heme within the methylamine dehydrogenase--amicyanin--cytochrome c-551i complex

Methylamine dehydrogenase (MADH), amicyanin, and cytochrome c-551i are soluble redox proteins that form a complex in solution [Chen, L., Durley, R., Mathews, F. S., & Davidson, V. L. (1994) Science 264, 86-90] which is required for the physiologic electron transfer from the tryptophan tryptophylquinone cofactor of MADH to heme via the copper center of amicyanin. The electron transfer reaction from copper to heme within the protein complex has been characterized by transient kinetic and thermodynamic analysis. The rate of this electron transfer reaction is 87 s-1 at 30 degrees C, and it varied with temperature. The reaction exhibited a reorganizational energy (lambda) of 1.1 eV and an electronic coupling (H(AB)) of 0.3 cm-1. The results of these analyses also predict an electron transfer distance, depending upon the value of beta which is used, of 13-24 A. The larger value approximates the direct copper to heme distance observed in the crystal structure of the complex. The most efficient pathways for electron transfer were predicted from the crystal structure using the Greenpath program, and these predictions were correlated with the results of the solution studies of the electron transfer reaction. It is concluded that electron transfer is, in fact, rate limiting for the observed electron transfer reaction in solution and that the two redox centers are strongly coupled, given the distance which separates them.