Revealing the membrane-bound catalytic oxidation of NADH by the drug target Type-II NADH Dehydrogenase.

Type-II NADH:quinone oxidoreductases (NDH-2s) are an important element of microbial pathogen electron transport chains and an attractive drug target. Despite being widely studied, its mechanism and catalysis are still poorly understood in a hydrophobic membrane environment. A recent report for the Escherichia coli NDH-2 showed NADH oxidation in a solution-based assay but apparently showed the reverse reaction in electrochemical studies, calling into question the validity of the electrochemical approach. Here we report electrochemical catalysis in the well-studied NDH-2 from Caldalkalibacillus thermarum (CthNDH-2). In agreement with previous reports, we demonstrated CthNDH-2 NADH oxidation in a solution assay and electrochemical assays revealed a system artifact in the absence of quinone that was absent in a membrane system. However, in the presence of either immobilized quinone or mobile quinone in a membrane, NADH oxidation was observed as in solution-phase assays. This conclusively establishes surface-based electrochemistry as a viable approach for interrogating electron transfer chain drug targets.