Bacterial energy transductions coupled to sodium ions.

In Propionigenium modestum, an Na+ cycle couples the exergonic decarboxylation of methylmalonyl-CoA to endergonic ATP synthesis. The ATPase is an F1F0-type enzyme, closely related to the F1F0 ATPase of Escherichia coli. The specificity of the P. modestum ATPase for Na+ is not absolute, as it catalyses proton transport at low Na+ concentrations. The Na(+)-binding site is located on the F0 sector. Therefore, a hybrid composed of F0 from P. modestum and F1 from E. coli, but not F1F0 from E. coli, was a functional Na+ pump. In Klebsiella pneumoniae, the Na+ ions pumped out of the cell by oxaloacetate decarboxylase are taken up again in symport with the growth substrate citrate. The reaction mechanism of oxaloacetate decarboxylase involves carboxylation of the prosthetic biotin group by carboxyltransfer from oxaloacetate, catalysed by the peripheral alpha-subunit. The firmly membrane-bound subunits beta and gamma complete the cycle by decarboxylation of the carboxybiotin intermediate which is coupled to Na+ translocation through the membrane.