Differential coupling through Val-344 and Tyr-442 of trimethylamine dehydrogenase in electron transfer reactions with ferricenium ions and electron transferring flavoprotein

Modeling studies of the trimethylamine dehydrogenase - electron transferring flavoprotein (TMADH - ETF) electron transfer complex have suggested potential roles for Val-344 and Tyr-442, found on the surface of TMADH, in electronic coupling between the 4Fe - 4S center of TMADH and the FAD of ETF. The importance of these residues in electron transfer, both to ETF and to the artificial electron acceptor, ferricenium (Fc+), has been studied by site-directed mutagenesis and stopped-flow spectroscopy. Reduction of the 6-(S)-cysteinyl FMN in TMADH is not affected by mutation of either Tyr-442 or Val-344 to a variety of alternate side chains, although there are modest changes in the rate of internal electron transfer from the 6-(S)-cysteinyl FMN to the 4Fe-4S center. The kinetics of electron transfer from the 4Fe - 4S center to Fc+ are sensitive to mutations at position 344. The introduction of smaller side chains (Ala-344, Cys-344, and Gly-344) leads to enhanced rates of electron transfer, and likely reflects shortened electron transfer 'pathways' from the 4Fe - 4S center to Fc+. The introduction of larger side chains (Ile-344 and Tyr-344) reduces substantially the rate of electron transfer to Fc+. Electron transfer to ETF is not affected, to any large extent, by mutation of Val-344. In contrast, mutation of Tyr-442 to Phe, Leu, Cys, and Gly leads to major reductions in the rate of electron transfer to ETF, but not to Fc+. The data indicate that electron transfer to Fc+ is via the shortest pathway from the 4Fe - 4S center of TMADH to the surface of the enzyme. Val-344 is located at the end of this pathway at the bottom of a small groove on the surface of TMADH, and Fc+ can penetrate this groove to facilitate good electronic coupling with the 4Fe - 4S center. With ETF as an electron acceptor, the observed rate of electron transfer is substantially reduced on mutation of Tyr-442, but not Val-344. We conclude that the flavin of ETF does not penetrate fully the groove on the surface of TMADH, and that electron transfer from the 4Fe - 4S center to ETF may involve a longer pathway involving Tyr-442. Mutation of Tyr-442 likely disrupts electron transfer by perturbing the interaction geometry of TMADH and ETF in the productive electron transfer complex, leading to less efficient coupling between the redox centers.