CATALYTIC ROLE OF MONOVALENT CATIONS IN THE MECHANISM OF PROTON-TRANSFER WHICH GATES AN INTERPROTEIN ELECTRON-TRANSFER REACTION

Within the methylamine dehydrogenase (MADH)-amicyanin protein complex, long range intermolecular electron transfer (ET) occurs between trypt ophan tryptophylquinone (TTQ) of MADH and the type I copper of amicyan in. The reoxidations of two chemically distinct reduced forms of TTQ w ere studied, a quinol (O-quinol) generated by reduction by dithionite and the physiologically relevant aminoquinol (N-quinol) generated by r eduction by methylamine. The latter contains a substrate-derived amino group which displaces the C6 carbonyl oxygen on TTQ. ET from N-quinol MADH to amicyanin is gated by the transfer of a solvent exchangeable proton [Bishop, G. R., & Davidson, V. L. (1995) Biochemistry 34, 12082 -12086]. The factors which influence this proton transfer (PT) reactio n have been examined. The rate of PT increases with increasing pH and with increasing salt concentration. The salt effect is due to specific monovalent cations and is not a general ionic strength effect. The ra te enhancements by pH and cations do not reflect an elimination of the PT step that gates ET. Over the range of pH from 5.5 to 9.0 and with cation concentrations from 0 to 200 mM, the observed rate of the redox reaction is still that of PT. This is proven by kinetic solvent isoto pe effect studies which show that a primary isotope effect persists ev en at the highest values of pH and cation concentration. A model is pr esented to explain how specific cations contribute to catalysis and in fluence the rate of PT in this reaction. The pH dependence is attribut ed to an ionizable group that is involved in cation binding. The effec t of the cation is stabilization of a negatively charged reaction inte rmediate that is formed during the deprotonation of the N-quinol, and from which rapid ET to the copper of amicyanin occurs. The relevance o f these findings to other enzymes which exhibit reaction rates that ar e influenced by monovalent cations is also discussed.