Oxidation-reduction properties of trimethylamine dehydrogenase: effect of inhibitor binding.

The redox potentials of trimethylamine dehydrogenase from the bacterium W3A1 have been determined by means of uv-visible spectroelectrochemistry. In the presence of the inhibitor tetramethylammonium chloride a shift of +0.2 V was observed in the midpoint redox potential for conversion of the oxidized 6-S-cysteinyl-FMN to the flavin radical form. The pH-independent value was +0.23 V vs the standard hydrogen electrode. The pH-dependent conversion of this radical to fully reduced flavin was shifted negative by 0.1 V in the presence of the inhibitor to -0.05 V at pH 7.0 and -0.15 V at pH 8.4. Tetramethylammonium chloride also caused moderate negative shifts (0.03-0.05 V) in the midpoint redox potential for the Fe4S(+2)4/Fe4S(+1)4 couple of trimethylamine dehydrogenase. The midpoint potentials are +0.06 V at pH 7.0 and +0.04 V at pH 8.4. Therefore, in the presence of tetramethylammonium chloride, electron transfer from the flavin radical to the Fe4S(+2)4 group is energetically unfavorable and trimethylamine dehydrogenase is trapped in the flavin radical state. The redox potential changes provide a thermodynamic basis for inhibition by tetramethylammonium chloride. Spectroelectrochemical titrations of trimethylamine dehydrogenase which had been inactivated by phenylhydrazine revealed heterogeneity in the redox behavior which had not been observed in other laboratories. The reason for this heterogeneity was not determined, but the midpoint redox potential for the Fe4S(+2)4/Fe4S(+1)4 couple of the main fraction of the inactivated enzyme was the same as that of active trimethylamine dehydrogenase.