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. 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
is 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.
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