van der Rest M E, Molenaar D, Konings W N
Department of Microbiology, University of Groningen, Haren, The Netherlands.
J Bacteriol. 1992 Aug;174(15):4893-8. doi: 10.1128/jb.174.15.4893-4898.1992.
Citrate transport via CitS of Klebsiella pneumoniae has been shown to depend on the presence of Na+. This transport system has been expressed in Escherichia coli, and uptake of citrate in E. coli membrane vesicles via this uptake system was found to be an electrogenic process, although the pH gradient is the main driving force for citrate uptake (M. E. van der Rest, R. M. Siewe, T. Abee, E. Schwartz, D. Oesterhelt, and W. N. Konings, J. Biol. Chem. 267:8971-8976, 1992). Analysis of the affinity constants for the different citrate species at different pH values of the medium indicates that H-citrate2- is the transported species. Since the electrical potential across the membrane is a driving force for citrate transport, this indicates that transport occurs in symport with at least three monovalent cations. Citrate efflux is stimulated by Na+ concentrations of up to 5 mM but inhibited by higher Na+ concentrations. Citrate exchange, however, is stimulated by all Na+ concentrations, indicating sequential events in which Na+ binds before citrate for translocation followed by a release of Na+ after release of citrate. CitS has, at pH 6.0 and in the presence of 5 mM citrate on both sides of the membrane, an apparent affinity (K(app)) for Na+ of 200 microM. The Na+/citrate stoichiometry was found to be 1. It is postulated that H-citrate2- is transported via CitS in symport with one Na+ and at least two H+ ions.
已证明肺炎克雷伯菌通过CitS进行的柠檬酸盐转运依赖于Na⁺的存在。该转运系统已在大肠杆菌中表达，并且发现通过该摄取系统在大肠杆菌膜囊泡中摄取柠檬酸盐是一个生电过程，尽管pH梯度是柠檬酸盐摄取的主要驱动力（M.E.范德雷斯特、R.M.西韦、T.阿贝、E.施瓦茨、D.奥斯特黑尔特和W.N.康宁斯，《生物化学杂志》267:8971 - 8976，1992年）。对不同pH值培养基中不同柠檬酸盐种类的亲和常数分析表明，H - 柠檬酸盐²⁻是被转运的种类。由于跨膜电位是柠檬酸盐转运的驱动力，这表明转运是与至少三个单价阳离子同向转运发生的。高达5 mM的Na⁺浓度刺激柠檬酸盐外流，但更高的Na⁺浓度则抑制外流。然而，所有Na⁺浓度都刺激柠檬酸盐交换，这表明存在一系列事件，其中Na⁺在柠檬酸盐之前结合以便转运，随后在柠檬酸盐释放后释放Na⁺。在pH 6.0且膜两侧均存在5 mM柠檬酸盐的情况下，CitS对Na⁺的表观亲和力（K(app)）为200 μM。发现Na⁺/柠檬酸盐化学计量比为1。据推测，H - 柠檬酸盐²⁻通过CitS与一个Na⁺和至少两个H⁺离子同向转运。
