Dimroth P
Institut für Physiologische Chemie, Technischen Universität, München, FRG.
Res Microbiol. 1990 Mar-Apr;141(3):332-6. doi: 10.1016/0923-2508(90)90007-d.
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.
在适度丙酸杆菌中，一个钠离子循环将甲基丙二酰辅酶A的放能脱羧反应与吸能的ATP合成偶联起来。该ATP酶是一种F1F0型酶，与大肠杆菌的F1F0 ATP酶密切相关。适度丙酸杆菌ATP酶对钠离子的特异性并非绝对，因为它在低钠离子浓度下催化质子运输。钠离子结合位点位于F0部分。因此，由适度丙酸杆菌的F0和大肠杆菌的F1组成的杂合体是一种功能性钠离子泵，而大肠杆菌的F1F0则不是。在肺炎克雷伯菌中，由草酰乙酸脱羧酶泵出细胞的钠离子会与生长底物柠檬酸盐协同转运再次被吸收。草酰乙酸脱羧酶的反应机制包括通过外周α亚基催化的草酰乙酸羧基转移作用使辅基生物素基团羧化。牢固结合在膜上的β和γ亚基通过羧基生物素中间体的脱羧作用完成循环，该脱羧作用与钠离子通过膜的转运相偶联。
