Localization of a Sodium Binding Site in the Sodium Translocating NADH: Ubiquinone Oxidoreductase

Abstract

Many marine, soil and pathogenic bacteria, including Vibrio cholerae, Pseudomonas aeruginosa, Yersinia pestis and Chlamydia trachomatis, possess a unique respiratory enzyme called Na+-translocating NADH:quinone oxidoreductase (NQR). This enzyme is capable of pumping Na+ across the cell membrane, thus creating an electrochemical gradient that the cell can use for metabolic functions. NQR pumps Na+ by using the energy released by the oxidation-reduction (redox) reaction between NADH and ubiquinone. While the electron transfer reactions have been well characterized in this enzyme, the Na+ translocation mechanism is not yet well known. Several acid groups oriented towards the cytoplasmic surface participate in Na+ uptake, while another group of acidic residues are required for Na+ release; however, there is no clear pathway for Na+ to bind through NQR. A further investigation into one of the residues required for Na+ uptake, NqrE-E95, has led to the discovery of one of the two predicted Na+ binding sites. After determining that this residue is essential for enzyme function, a closer look into the crystal structure revealed that NqrE-E95 is in the proper orientation to form a binding site with NqrE-D99 and NqrE-Y106. It was then found that these residues are also important for the utilization of Na+ by NQR. Further studies are being conducted to elucidate the exact role of NqrE-D99 and NqrE-Y106 in binding and transport of Na+. Locating binding sites in NQR is the first step in understanding how Na+ is translocated, and ultimately how the translocation is coupled to the redox reactions.