Intermolecular electron transfer from substrate-reduced methylamine dehydrogenase to amicyanin is linked to proton transfer.

Within the methylamine dehydrogenase-amicyanin complex, intermolecular electron transfer (ET) occurs between tryptophan tryptophylquinone (TTQ) and copper. The ET reactions from two chemically distinct reduced forms of TTQ were studied. The quinol form of TTQ was generated by reduction by dithionite. An aminoquinol form of TTQ, in which an amino group displaces the carbonyl oxygen, was generated by reduction by the substrate methylamine. Thermodynamic analysis of the ET reactions suggested that the ET event was rate-limiting for the redox reaction between quinol TTQ and copper, but not for the ET reaction from aminoquinol TTQ to copper. Solvent kinetic isotope effect studies indicated that proton transfer was involved in the rate-limiting reaction step for the ET from the substrate-reduced enzyme, but not the dithionite-reduced enzyme. Solvent deuterium kinetic isotope effects of 1.5 and 12.2 were obtained, respectively, for the ET reactions from dithionite-reduced and substrate-reduced methylamine dehydrogenase. These results demonstrate that application of ET theory to the analysis of thermodynamic data for the intermolecular protein ET reactions can potentially be used to distinguish between true ET reactions and those which are gated or attenuated by adiabatic events. Kinetic models are presented to explain how the incorporation of the substrate-derived amino group into TTQ may alter the rate-limiting step for ET.