COMPLEX-FORMATION WITH METHYLAMINE DEHYDROGENASE AFFECTS THE PATHWAY OF ELECTRON-TRANSFER FROM AMICYANIN TO CYTOCHROME C-551I

Methylamine dehydrogenase (MADH), amicyanin, and cytochrome c-551i are soluble redox proteins that form a complex in solution (Chen, L., Dur ley, R., Mathews, F. S., and Davidson, V. L. (1994) Science 264, 86-90 ), which is required for the physiologic electron transfer from the tr yptophan tryptophylquinone cofactor of MADH to heme via the copper cen ter of amicyanin. The reduction of cytochrome by amicyanin within the complex in solution has been demonstrated using rapid scanning stopped -flow spectroscopy. Electron transfer from free, uncomplexed, amicyani n to cytochrome c-551i occurs much more rapidly but only to a very sma ll extent because the reaction is thermodynamically much less favorabl e when amicyanin is not associated with MADH (Gray, K. A., Davidson, V . L., and Knaff, D. B. (1988) J. Biol. Chem. 263, 13987-13990). These kinetic data suggest that amicyanin binding to cytochrome c-551i occur s at different sites when amicyanin is free and when it is in complex with MADH. A model for the interactions of these proteins is presented .