Parker C, Barnell W O, Snoep J L, Ingram L O, Conway T
School of Biological Sciences, University of Nebraska, Lincoln 68588-0118, USA.
Mol Microbiol. 1995 Mar;15(5):795-802. doi: 10.1111/j.1365-2958.1995.tb02350.x.
Zymomonas mobilis is known to transport glucose by a facilitated diffusion process. A putative glucose facilitator gene (glf), closely related to a large family of glucose transporters, is located in a cluster of genes that code for enzymes of glucose metabolism. The Z. mobilis glf gene is able to complement glucose transport in an Escherichia coli strain that is defective in native glucose transport and glucokinase. In this study, the recombinant E. coli was shown to be capable of influx counterflow when preloaded with glucose and had an apparent Km for glucose of approximately 1.1-2.9 mM, consistent with the function of Glf as a low-affinity glucose facilitator. The ability of glucokinase mutants expressing glf to transport glucose made it clear that glucokinase activity was not required for Glf-dependent glucose transport. The possibility that glucokinase can interact with Glf to improve the affinity for glucose was not supported since expression of the Z. mobilis glucokinase gene, in addition to glf, did not affect the Km of Glf for glucose in recombinant E. coli. The inability of various sugars to compete with glucose during glucose transport by recombinant E. coli expressing glf indicated that Glf is specific for glucose. While the results of fructose transport assays did not completely rule out the possibility of very low affinity for fructose, the apparent specificity of Glf for glucose makes it possible that Z. mobilis utilizes a different transporter(s) for fructose.
运动发酵单胞菌已知通过易化扩散过程转运葡萄糖。一个假定的葡萄糖易化转运蛋白基因（glf），与一大类葡萄糖转运蛋白密切相关，位于编码葡萄糖代谢酶的基因簇中。运动发酵单胞菌的glf基因能够在天然葡萄糖转运和葡萄糖激酶有缺陷的大肠杆菌菌株中补充葡萄糖转运功能。在本研究中，重组大肠杆菌在预先加载葡萄糖时显示出能够进行流入逆流，其对葡萄糖的表观Km约为1.1 - 2.9 mM，这与Glf作为低亲和力葡萄糖易化转运蛋白的功能一致。表达glf的葡萄糖激酶突变体转运葡萄糖的能力表明，依赖Glf的葡萄糖转运不需要葡萄糖激酶活性。葡萄糖激酶与Glf相互作用以提高对葡萄糖亲和力的可能性未得到支持，因为除了glf之外，运动发酵单胞菌葡萄糖激酶基因的表达并未影响重组大肠杆菌中Glf对葡萄糖的Km。在表达glf的重组大肠杆菌进行葡萄糖转运过程中，各种糖类无法与葡萄糖竞争，这表明Glf对葡萄糖具有特异性。虽然果糖转运试验的结果并未完全排除对果糖亲和力极低的可能性，但Glf对葡萄糖的明显特异性使得运动发酵单胞菌可能利用不同的转运蛋白来转运果糖。
