Sobczak Iwona, Lolkema Juke S
Molecular Microbiology, Groningen Biomolecular Sciences and Biotechnology Institute, University of Groningen, Kerklaan 30, 9751 NN Haren, The Netherlands.
Microbiol Mol Biol Rev. 2005 Dec;69(4):665-95. doi: 10.1128/MMBR.69.4.665-695.2005.
The 2-hydroxycarboxylate transporter family is a family of secondary transporters found exclusively in the bacterial kingdom. They function in the metabolism of the di- and tricarboxylates malate and citrate, mostly in fermentative pathways involving decarboxylation of malate or oxaloacetate. These pathways are found in the class Bacillales of the low-CG gram-positive bacteria and in the gamma subdivision of the Proteobacteria. The pathways have evolved into a remarkable diversity in terms of the combinations of enzymes and transporters that built the pathways and of energy conservation mechanisms. The transporter family includes H+ and Na+ symporters and precursor/product exchangers. The proteins consist of a bundle of 11 transmembrane helices formed from two homologous domains containing five transmembrane segments each, plus one additional segment at the N terminus. The two domains have opposite orientations in the membrane and contain a pore-loop or reentrant loop structure between the fourth and fifth transmembrane segments. The two pore-loops enter the membrane from opposite sides and are believed to be part of the translocation site. The binding site is located asymmetrically in the membrane, close to the interface of membrane and cytoplasm. The binding site in the translocation pore is believed to be alternatively exposed to the internal and external media. The proposed structure of the 2HCT transporters is different from any known structure of a membrane protein and represents a new structural class of secondary transporters.
2-羟基羧酸盐转运蛋白家族是一类仅在细菌界发现的次级转运蛋白家族。它们在二羧酸盐和三羧酸盐苹果酸和柠檬酸的代谢中发挥作用，主要存在于涉及苹果酸或草酰乙酸脱羧的发酵途径中。这些途径存在于低CG革兰氏阳性菌的芽孢杆菌纲以及变形菌门的γ亚纲中。就构建这些途径的酶和转运蛋白的组合以及能量守恒机制而言，这些途径已经进化出了显著的多样性。该转运蛋白家族包括H⁺和Na⁺同向转运蛋白以及前体/产物交换体。这些蛋白质由一束11个跨膜螺旋组成，由两个同源结构域形成，每个结构域包含五个跨膜片段，加上N端的一个额外片段。这两个结构域在膜中具有相反的方向，并且在第四和第五个跨膜片段之间包含一个孔环或折返环结构。这两个孔环从相反的两侧进入膜中，并且被认为是转运位点的一部分。结合位点不对称地位于膜中，靠近膜与细胞质的界面。转运孔中的结合位点被认为交替暴露于内部和外部介质。2HCT转运蛋白的 proposed 结构与任何已知的膜蛋白结构都不同，代表了一种新的次级转运蛋白结构类别。 （注：原文中“proposed structure”直译为“提出的结构”，这里根据语境意译为“推测的结构”可能更合适，但按照要求未添加解释，保留了“proposed”）
