GtfC-type 4,6-α-glucanotransferase (α-GT) enzymes from Glycoside Hydrolase Family 70 (GH70) are of interest for the modification of starch into low-glycemic index food ingredients. Compared to the related GH70 GtfB-type α-GTs, found exclusively in lactic acid bacteria (LAB), GtfCs occur in non-LAB, share low sequence identity, lack circular permutation of the catalytic domain, and feature a single-segment auxiliary domain IV and auxiliary C-terminal domains. Despite these differences, the first crystal structure of a GtfC, GbGtfC-ΔC from Geobacillus 12AMOR1, and the first one representing a non-permuted GH70 enzyme, reveals high structural similarity in the core domains with most GtfBs, featuring a similar tunneled active site. We propose that GtfC (and related GtfD) enzymes evolved from starch-degrading α-amylases from GH13 by acquiring α-1,6 transglycosylation capabilities, before the events that resulted in circular permutation of the catalytic domain observed in other GH70 enzymes (glucansucrases, GtfB-type α-GTs). AlphaFold modeling and sequence alignments suggest that the GbGtfC structure represents the GtfC subfamily, although it has a so far unique alternating α-1,4/α-1,6 product specificity, likely determined by residues near acceptor binding subsites +1/+2.

中文翻译：

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嗜热地芽孢杆菌 12AMOR1 的 4,6-α-葡聚糖转移酶 GtfC-ΔC 的晶体结构：非置换 GH70 酶中的淀粉转糖基作用

来自糖苷水解酶家族 70 (GH70) 的 GtfC 型 4,6-α-葡聚糖转移酶 (α-GT) 酶对将淀粉改性为低升糖指数食品成分具有重要意义。与仅在乳酸菌 (LAB) 中发现的相关 GH70 GtfB 型 α-GT 相比，GtfC 出现在非 LAB 中，具有低序列同一性，缺乏催化结构域的循环排列，并具有单段辅助结构域IV 和辅助 C 端域。尽管存在这些差异，第一个GtfC的晶体结构，来自 Geobacillus 的 GbGtfC-ΔC12AMOR1 和第一个代表非置换的 GH70 酶，揭示了与大多数 GtfB 的核心结构域的高度结构相似性，具有相似的隧道活性位点。我们提出 GtfC（和相关的 GtfD）酶通过获得 α-1,6 转糖基化能力，从 GH13 的淀粉降解 α-淀粉酶进化而来，在导致其他 GH70 酶（葡聚糖蔗糖酶， GtfB 型 α-GT）。AlphaFold 建模和序列比对表明 GbGtfC 结构代表 GtfC 亚家族，尽管它具有迄今为止独特的交替 α-1,4/α-1,6 产物特异性，可能由受体结合​​亚位点附近的残基 +1/+2 决定.