Mechanisms of sodium transport in bacteria

In some bacteria, an Na + circuit is an important link between exergonic and endergonic membrane reactions. The physiological importance of Na + ion cycling is described in detail for three different bacteria. Klebsiella pneumoniae fermenting citrate pumps Na + outwards by oxaloacetate decarboxylase and uses the Na + ion gradient thus established for citrate uptake. Another possible function of the Na + gradient may be to drive the endergonic reduction of NAD + with ubiquinol as electron donor. In Vibrio alginolyticus , an Na + gradient is established by the NADH: ubiquinone oxidoreductase segment of the respiratory chain; the Na + gradient drives solute uptake, flagellar motion and possibly ATP synthesis. In Propionigenium modestum , ATP biosynthesis is entirely dependent on the Na + ion gradient established upon decarboxylation of methylmalonyl-CoA. The three Na + -translocating enzymes, oxaloacetate decarboxylase of Klebsiella pneumoniae , NADH: ubiquinone oxidoreductase of Vibrio alginolyticus and ATPase ( F 1 F 0 ) of Propionigenium modestum have been isolated and studied with respect to structure and function. Oxaloacetate decarboxylase consists of a peripheral subunit (α), that catalyses the carboxyl transfer from oxaloacetate to enzyme-bound biotin. The subunits β and γ are firmly embedded in the membrane and catalyse the decarboxylation of the carboxybiotin enzyme, coupled to Na + transport. A two-step mechanism has also been demonstrated for the respiratory Na + pump. Semiquinone radicals are first formed with the electrons from NADH; subsequently, these radicals dismutate in an Na + -dependent reaction to quinone and quinol. The ATPase of P. modestum is closely related in its structure to the F 1 F 0 ATPase of E. coli , but uses Na + as the coupling ion. A specific role of protons in the ATP synthesis mechanism is therefore excluded.