Jockel P, Schmid M, Steuber J, Dimroth P
Mikrobiologisches Institut, Eidgenössische Technische Hochschule, ETH-Zentrum, Schmelzbergstrasse 7, CH-8092 Zürich, Switzerland.
Biochemistry. 2000 Mar 7;39(9):2307-15. doi: 10.1021/bi992261v.
The oxaloacetate decarboxylase Na+ pump consists of subunits alpha, beta, and gamma, and contains biotin as the prosthetic group. Membrane-bound subunit beta catalyzes the decarboxylation of carboxybiotin coupled to Na+ translocation, and consumes a periplasmically derived proton. Site-directed mutagenesis of conserved amino acids of transmembrane helix VIII indicated that residues N373, G377, S382, and R389 are functionally important. The polar side groups of these amino acids may constitute together with D203 a network of ionizable groups which promotes the translocation of Na+ and the oppositely oriented H+ across the membrane. Evidence is presented that two Na+ ions are bound simultaneously to subunit beta during transport with D203 and S382 acting as binding sites. Sodium ion binding from the cytoplasm to both sites elicits decarboxylation of carboxybiotin, and a conformational switch exposes the bound Na+ ions toward the periplasm. After dissociation of Na+ and binding of H+, the cytoplasmically exposed conformation is regained.
草酰乙酸脱羧酶钠泵由α、β和γ亚基组成，并含有生物素作为辅基。膜结合亚基β催化羧基生物素的脱羧反应，该反应与钠离子转运偶联，并消耗来自周质的一个质子。对跨膜螺旋VIII保守氨基酸进行的定点诱变表明，N373、G377、S382和R389残基在功能上很重要。这些氨基酸的极性侧基可能与D203一起构成一个可电离基团网络，该网络促进钠离子和相反方向的氢离子跨膜转运。有证据表明，在转运过程中，两个钠离子同时与亚基β结合，D203和S382作为结合位点。钠离子从细胞质与两个位点结合引发羧基生物素的脱羧反应，并且构象转换使结合的钠离子朝向周质暴露。钠离子解离并结合氢离子后，恢复细胞质暴露的构象。
