Structural Genomics Consortium, Old Road Research Campus Building, Oxford, United Kingdom OX3 7DQ.
Proc Natl Acad Sci U S A. 2011 Dec 27;108(52):21028-33. doi: 10.1073/pnas.1113921108. Epub 2011 Dec 12.
Glycogenin initiates the synthesis of a maltosaccharide chain covalently attached to itself on Tyr195 via a stepwise glucosylation reaction, priming glycogen synthesis. We have captured crystallographic snapshots of human glycogenin during its reaction cycle, revealing a dynamic conformational switch between ground and active states mediated by the sugar donor UDP-glucose. This switch includes the ordering of a polypeptide stretch containing Tyr195, and major movement of an approximately 30-residue "lid" segment covering the active site. The rearranged lid guides the nascent maltosaccharide chain into the active site in either an intra- or intersubunit mode dependent upon chain length and steric factors and positions the donor and acceptor sugar groups for catalysis. The Thr83Met mutation, which causes glycogen storage disease XV, is conformationally locked in the ground state and catalytically inactive. Our data highlight the conformational plasticity of glycogenin and coexistence of two modes of glucosylation as integral to its catalytic mechanism.
糖元蛋白通过逐步的葡萄糖基化反应，在 Tyr195 上启动糖元合成，将一个与自身共价连接的麦芽寡糖链添加到其上。我们在糖元蛋白的反应循环中捕获了晶体学快照，揭示了糖基供体 UDP-葡萄糖介导的、在基础状态和活性状态之间的动态构象转换。这种转换包括包含 Tyr195 的多肽链的有序化，以及覆盖活性位点的大约 30 个残基“盖子”片段的主要运动。重新排列的盖子以依赖于链长和空间因素的方式，将新生的麦芽寡糖链引导到活性位点中，以单体或亚基间模式，并将供体和受体糖基置于催化位置。导致糖原贮积病 XV 的 Thr83Met 突变，在构象上被锁定在基础状态，且无催化活性。我们的数据突出了糖元蛋白的构象可塑性，以及两种葡萄糖基化模式的共存，这是其催化机制的重要组成部分。
