McCoy Jason G, Levin Elena J, Zhou Ming
Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, TX 77030, USA.
Biochim Biophys Acta. 2015 Mar;1850(3):577-85. doi: 10.1016/j.bbagen.2014.03.013. Epub 2014 Mar 20.
The enzyme IIC (EIIC) component of the phosphotransferase system (PTS) is responsible for selectively transporting sugar molecules across the inner bacterial membrane. This is accomplished in parallel with phosphorylation of the sugar, which prevents efflux of the sugar back across the membrane. This process is a key part of an extensive signaling network that allows bacteria to efficiently utilize preferred carbohydrate sources.
The goal of this review is to examine the current understanding of the structural features of the EIIC and how it mediates concentrative, selective sugar transport. The crystal structure of an N,N'-diacetylchitobiose transporter is used as a structural template for the glucose superfamily of PTS transporters.
Comparison of protein sequences in context with the known EIIC structure suggests that members of the glucose superfamily of PTS transporters may exhibit variations in topology. Despite these differences, a conserved histidine and glutamate appear to have roles shared across the superfamily in sugar binding and phosphorylation. In the proposed transport model, a rigid body motion between two structural domains and movement of an intracellular loop provide the substrate binding site with alternating access, and reveal a surface required for interaction with the phosphotransfer protein responsible for catalysis.
The structural and functional data discussed here give a preliminary understanding of how transport in EIIC is achieved. However, given the great sequence diversity between varying glucose-superfamily PTS transporters and lack of data on conformational changes needed for transport, additional structures of other members and conformations are still required. This article is part of a Special Issue entitled: Structural biochemistry and biophysics of membrane proteins.
磷酸转移酶系统（PTS）的酶IIC（EIIC）组分负责选择性地将糖分子转运穿过细菌内膜。这一过程与糖的磷酸化同时完成，从而防止糖重新穿过膜外流。该过程是一个广泛信号网络的关键部分，使细菌能够有效利用首选碳水化合物来源。
本综述的目的是研究目前对EIIC结构特征的理解以及它如何介导浓缩性、选择性糖转运。N，N'-二乙酰壳二糖转运蛋白的晶体结构用作PTS转运蛋白葡萄糖超家族的结构模板。
将蛋白质序列与已知的EIIC结构进行比较表明，PTS转运蛋白葡萄糖超家族的成员可能在拓扑结构上存在差异。尽管存在这些差异，但一个保守的组氨酸和谷氨酸似乎在整个超家族中在糖结合和磷酸化方面发挥着共同作用。在提出的转运模型中，两个结构域之间的刚体运动和一个细胞内环的移动为底物结合位点提供交替访问，并揭示了与负责催化的磷酸转移蛋白相互作用所需的表面。
这里讨论的结构和功能数据对EIIC中的转运如何实现给出了初步理解。然而，鉴于不同的葡萄糖超家族PTS转运蛋白之间存在巨大的序列多样性，并且缺乏转运所需构象变化的数据，仍然需要其他成员的额外结构和构象。本文是名为：膜蛋白的结构生物化学和生物物理学的特刊的一部分。
