ИСТИНА

The Na+-translocating NADH:ubiquinone oxidoreductase (Na+-NQR) is a component of the respiratory chain of various bacteria. This enzyme is an analogous but not homologous counterpart of mitochondrial Complex I. Na+-NQR drives the same chemistry and also uses released energy to translocate ions across the membrane, but it pumps Na+ instead of H+. Most likely the mechanism of sodium pumping is quite different from that of proton pumping (for example, it could not accommodate the Grotthuss mechanism of ion movement); this is why the enzyme structure, subunits and prosthetic groups are completely special. The main mechanism of energy transduction by heterotrophic organisms is based on conversion of chemical energy of NADH oxidation by molecular oxygen into transmembrane electrochemical potential of hydrogen ions ( H+). This process is carried out by enzyme complexes of the respiratory chain, which function as molecular generators of electric current. Understanding of the mechanism of proton translocation by these generators is the central task of modern bioenergetics. However, this study is hampered by the complexity of separation between protons taking part in the catalytic redox reactions and the protons involved in the transmembrane translocation process (pumping). From that point of view, the investigation of a redox-driven sodium pump gives a unique possibility to separate these ions, because in this case the redox chemistry still require protons, but the translocation activity depends only on the presence of Na+. For example, for sodium-translocating enzymes it is possible to resolve the events of the catalytic cycle, which are slowed down at low concentrations of the coupling ion. This approach reveals the specific transitions of the catalytic cycle responsible for the sodium translocation. On the other hand, study of the dependence of the thermodynamic properties of a sodium pump on Na+ concentration allows determination of the mechanism of conversion of the redox energy into the sodium-motive force. This review summarizes modern knowledge on the structural and catalytic properties of bacterial Na+-translocating NADH:quinone oxidoreductases. The sequence of electron transfer through the enzyme cofactors and thermodynamic properties of those cofactors are discussed. The resolution of the intermediates of the catalytic cycle and localization of sodium-dependent steps are combined in a possible molecular mechanism of sodium transfer by the enzyme.