Graduate School of Life Science, Hokkaido University, Kita-ku, Sapporo, Japan.
FEBS Lett. 2013 Apr 2;587(7):840-6. doi: 10.1016/j.febslet.2013.02.007. Epub 2013 Feb 24.
Enzymatic epimerization is an important modification for carbohydrates to acquire diverse functions attributable to their stereoisomers. Cellobiose 2-epimerase (CE) catalyzes interconversion between d-glucose and d-mannose residues at the reducing end of β-1,4-linked oligosaccharides. Here, we solved the structure of Ruminococcus albus CE (RaCE). The structure of RaCE showed strong similarity to those of N-acetyl-D-glucosamine 2-epimerase and aldose-ketose isomerase YihS with a high degree of conservation of residues around the catalytic center, although sequence identity between them is low. Based on structural comparison, we found that His184 is required for RaCE activity as the third histidine added to two essential histidines in other sugar epimerases/isomerases. This finding was confirmed by mutagenesis, suggesting a new catalytic mechanism for CE involving three histidines.
酶促差向异构化是一种重要的碳水化合物修饰方法，可赋予其立体异构体不同的功能。纤维二糖 2-差向异构酶（CE）催化β-1,4 连接寡糖还原端的 d-葡萄糖和 d-甘露糖残基之间的相互转化。本文解析了白瘤胃球菌 CE（RaCE）的结构。RaCE 的结构与 N-乙酰-d-葡萄糖胺 2-差向异构酶和醛糖酮糖异构酶 YihS 的结构具有很强的相似性，催化中心周围的残基高度保守，尽管它们之间的序列同一性较低。基于结构比较，我们发现 His184 是 RaCE 活性所必需的，因为它是其他糖差向异构酶/异构酶中两个必需组氨酸的第三个组氨酸。通过突变分析证实了这一发现，表明 CE 涉及三个组氨酸的新催化机制。
