Identification of the coupling step in Na+-translocating NADH:quinone oxidoreductase from real-time kinetics of electron transfer

• Vibrio harveyi NADH:quinone oxidoreductase (Na+-NQR) is a primary Na+ pump.
• Six steps of Na+-NQR reduction by NADH were resolved in time in a stopped-flow study.
• Na+-NQR apparently couples tight Na+ binding with Cys4[Fe] center reduction.
• Electron transfer in Na+-NQR may proceed via an alternating access mechanism.

Bacterial Na+-translocating NADH:quinone oxidoreductase (Na+-NQR) uses a unique set of prosthetic redox groups—two covalently bound FMN residues, a [2Fe–2S] cluster, FAD, riboflavin and a Cys4[Fe] center—to catalyze electron transfer from NADH to ubiquinone in a reaction coupled with Na+ translocation across the membrane. Here we used an ultra-fast microfluidic stopped-flow instrument to determine rate constants and the difference spectra for the six consecutive reaction steps of Vibrio harveyi Na+-NQR reduction by NADH. The instrument, with a dead time of 0.25 ms and optical path length of 1 cm allowed collection of visible spectra in 50-μs intervals. By comparing the spectra of reaction steps with the spectra of known redox transitions of individual enzyme cofactors, we were able to identify the chemical nature of most intermediates and the sequence of electron transfer events. A previously unknown spectral transition was detected and assigned to the Cys4[Fe] center reduction. Electron transfer from the [2Fe–2S] cluster to the Cys4[Fe] center and all subsequent steps were markedly accelerated when Na+ concentration was increased from 20 μM to 25 mM, suggesting coupling of the former step with tight Na+ binding to or occlusion by the enzyme. An alternating access mechanism was proposed to explain electron transfer between subunits NqrF and NqrC. According to the proposed mechanism, the Cys4[Fe] center is alternatively exposed to either side of the membrane, allowing the [2Fe–2S] cluster of NqrF and the FMN residue of NqrC to alternatively approach the Cys4[Fe] center from different sides of the membrane.