Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA.
Interdiscip Sci. 2010 Jun;2(2):133-9. doi: 10.1007/s12539-010-0070-5. Epub 2010 May 1.
Xyloglucan endotransglycosylase/hydrolases (XTH) are believed to play an important role in modifying the cell wall structure through two different, but related actions: the cleavage of a cross-linking xyloglucan polymer (xyloglucan endohydrolase (XEH) activity) and transfer of a newly generated end to another sugar polymer (xyloglucan endotransglycosylase (XET) activity). These enzymes normally show predominantly either XET or XEH. Question remains concerning the origin of the XET or XEH activity of poplar (Populus trichocarpa) XTH proteins as well as the 3-dimensional structural features that might contribute to the different activities for different phylogenetic groups. Previous investigations have demonstrated that the key structural features controlling the activity may involve the loop 1 and 2 regions of the XTH enzymes. Here we report homology models for 40 poplar (Populus trichocarpa) XTH proteins and analyze their loop 1 and 2 regions. These analyses allow us to generate some hypotheses concerning the possible activities of these enzymes. The results show that the protein models for subfamilies (SFs) I/II/IV match well with that of the known XET enzyme (Ptt-XET16-34 from a Populus hybrid), suggesting that they might mainly function as the XET. It is found that the SF III-A models match well with that of the known XEH TmXTH1. Therefore, they may function as XEH as well. Although the SF III-B members are found to have a truncated loop 2, comparison of the homology models with the existing TmXTH1 structure seems to suggest that they might have relatively high possibility to function as XEH instead of XET.
木葡聚糖内转糖基酶/水解酶（XTH）被认为通过两种不同但相关的作用来修饰细胞壁结构：交联木葡聚糖聚合物的裂解（木葡聚糖内切水解酶（XEH）活性）和新生成末端转移到另一种糖聚合物（木葡聚糖内转糖基酶（XET）活性）。这些酶通常主要表现出 XET 或 XEH 活性。目前尚不清楚杨树（Populus trichocarpa）XTH 蛋白的 XET 或 XEH 活性的起源，以及可能导致不同进化群具有不同活性的三维结构特征。先前的研究表明，控制活性的关键结构特征可能涉及 XTH 酶的环 1 和环 2 区域。在这里，我们报告了 40 种杨树（Populus trichocarpa）XTH 蛋白的同源模型，并分析了它们的环 1 和环 2 区域。这些分析使我们能够生成关于这些酶可能活性的一些假设。结果表明，亚家族（SFs）I/II/IV 的蛋白模型与已知的 XET 酶（来自杂种杨树的 Ptt-XET16-34）非常匹配，表明它们可能主要作为 XET 发挥作用。发现 SF III-A 模型与已知的 XEH TmXTH1 非常匹配。因此，它们也可能作为 XEH 发挥作用。尽管 SF III-B 成员的环 2 被截断，但同源模型与现有 TmXTH1 结构的比较似乎表明它们可能具有相对较高的可能性作为 XEH 而不是 XET 发挥作用。
