Coupling mechanism of the oxaloacetate decarboxylase Na(+) pump.

The oxaloacetate decarboxylase Na(+) pump consists of subunits alpha, beta and gamma, and contains biotin as the prosthetic group. The peripheral alpha subunit catalyzes the carboxyltransfer from oxaloacetate to the prosthetic biotin group to yield the carboxybiotin enzyme. Subsequently, this is decarboxylated in a Na(+)-dependent reaction by the membrane-bound beta subunit. The decarboxylation is coupled to Na(+) translocation from the cytoplasm into the periplasm, and consumes a periplasmically derived proton. The gamma subunit contains a Zn(2+) metal ion which may be involved in the carboxyltransfer reaction. It is proposed to insert with its N-terminal alpha-helix into the membrane and to form a complex with the alpha subunit with its water-soluble C-terminal domain. The beta subunit consists of nine transmembrane alpha-helices, a segment (IIIa) which inserts from the periplasm into the membrane but does not penetrate it, and connecting hydrophilic loops. The most highly conserved regions of the molecule are segment IIIa and transmembrane helix VIII. Functionally important residues are D203 (segment IIIa), Y229 (helix IV) and N373, G377, S382 and R389 (helix VIII). The polar of these amino acids may constitute a network of ionizable groups which promotes the translocation of Na(+) and the oppositely oriented translocation of H(+) across the membrane. Evidence indicates that two Na(+) ions are bound simultaneously to subunit beta with D203 and S382 acting as binding sites. Sodium ion binding from the cytoplasm to both sites elicits decarboxylation of carboxybiotin possibly with the consumption of the proton extracted from S382 and delivered via Y229 to the carboxylated prosthetic group. A conformational change exposes the bound Na(+) ions toward the periplasm. With H(+) entering from the periplasm, the hydroxyl group of S382 is regenerated, and as a consequence, the Na(+) ions are released into this compartment. After switching back to the original conformation, Na(+) pumping continues.