Manjasetty Babu A, Chance Mark R
New York Structural Genomix Research Consortium, Center for Synchrotron Biosciences, National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY 11973, USA.
J Mol Biol. 2006 Jul 7;360(2):297-309. doi: 10.1016/j.jmb.2006.04.040. Epub 2006 May 5.
Escherichia coli L-arabinose isomerase (ECAI; EC 5.3.1.4) catalyzes the isomerization of L-arabinose to L-ribulose in vivo. This enzyme is also of commercial interest as it catalyzes the conversion of D-galactose to D-tagatose in vitro. The crystal structure of ECAI was solved and refined at 2.6 A resolution. The subunit structure of ECAI is organised into three domains: an N-terminal, a central and a C-terminal domain. It forms a crystallographic trimeric architecture in the asymmetric unit. Packing within the crystal suggests the idea that ECAI can form a hexameric assembly. Previous electron microscopic and biochemical studies supports that ECAI is hexameric in solution. A comparison with other known structures reveals that ECAI adopts a protein fold most similar to E. coli fucose isomerase (ECFI) despite very low sequence identity 9.7%. The structural similarity between ECAI and ECFI with regard to number of domains, overall fold, biological assembly, and active site architecture strongly suggests that the enzymes have functional similarities. Further, the crystal structure of ECAI forms a basis for identifying molecular determinants responsible for isomerization of arabinose to ribulose in vivo and galactose to tagatose in vitro.
大肠杆菌L-阿拉伯糖异构酶（ECAI；EC 5.3.1.4）在体内催化L-阿拉伯糖异构化为L-核酮糖。该酶在体外催化D-半乳糖转化为D-塔格糖，因此也具有商业价值。已解析出ECAI的晶体结构，并将其精修至2.6埃分辨率。ECAI的亚基结构由三个结构域组成：一个N端结构域、一个中央结构域和一个C端结构域。它在不对称单元中形成晶体学三聚体结构。晶体中的堆积情况表明ECAI可以形成六聚体组装体。先前的电子显微镜和生化研究支持ECAI在溶液中是六聚体。与其他已知结构的比较表明，尽管序列同一性非常低（9.7%），但ECAI采用的蛋白质折叠结构与大肠杆菌岩藻糖异构酶（ECFI）最相似。ECAI和ECFI在结构域数量、整体折叠、生物组装和活性位点结构方面的相似性强烈表明这两种酶具有功能相似性。此外，ECAI的晶体结构为确定负责体内阿拉伯糖异构化为核酮糖以及体外半乳糖异构化为塔格糖的分子决定因素奠定了基础。
