The Role of Glycine Residues 140 and 141 of Subunit B in the Functional Ubiquinone Binding Site of the Na+-pumping NADH:quinone Oxidoreductase from Vibrio cholerae

Background: Na+-NQR is a bacterial respiratory enzyme that catalyzes the oxidation of NADH, the reduction of ubiquinone, and the translocation of Na+ across the membrane. Results: Mutations at NqrB-G140 and NqrB-G141 impair the reaction of Na+-NQR with ubiquinone. Conclusion: Residues NqrB-G140 and -G141 are critical for binding and reaction with ubiquinone. Significance: This work identifies the functional ubiquinone binding site in Na+-NQR. The Na+-pumping NADH:quinone oxidoreductase (Na+-NQR) is the main entrance for electrons into the respiratory chain of many marine and pathogenic bacteria. The enzyme accepts electrons from NADH and donates them to ubiquinone, and the free energy released by this redox reaction is used to create an electrochemical gradient of sodium across the cell membrane. Here we report the role of glycine 140 and glycine 141 of the NqrB subunit in the functional binding of ubiquinone. Mutations at these residues altered the affinity of the enzyme for ubiquinol. Moreover, mutations in residue NqrB-G140 almost completely abolished the electron transfer to ubiquinone. Thus, NqrB-G140 and -G141 are critical for the binding and reaction of Na+-NQR with its electron acceptor, ubiquinone.