Heinemann U, Aÿ J, Gaiser O, Müller J J, Ponnuswamy M N
Max-Delbrück-Centrum für Molekulare Medizin, Berlin, Germany.
Biol Chem. 1996 Jul-Aug;377(7-8):447-54.
1,3-1,4-beta-D-glucan 4-glucanohydrolases (beta-glucanases) are synthesized in both plants and bacteria. The enzymes specifically hydrolyze beta-1,4 glycosyl bonds that are adjacent to beta-1,3 linkages in beta-glucan, a linear polysaccharide containing these bonds in an approximate ratio of 2.5:1. Here we review structural studies by X-ray crystallography of natural Bacillus beta-glucanases and engineered variants characterized by hybrid sequences, single-site mutations and circular permutations. In combination with biochemical data and site-directed mutagenesis, the crystallographic evidence permits the formulation of a likely reaction mechanism for the retaining Bacillus beta-glucanases. In addition, the shape of the active site channel, the known binding mode of a cellobioside epoxyalkyl inhibitor and the energy profile of the beta-glucan substrate explain the specificity of the enzymes for beta-glucan and the requirement for a beta-1,3 glycosyl bond next to the scissile bond. beta-Glucanases with circularly permuted sequences retain conformational stability, enzymatic activity and the native fold. The jellyroll tertiary structure of Bacillus beta-glucanases is remarkably stable, resisting changes in amino acid sequence, chain topology, ligand binding and crystal packing.
1,3-1,4-β-D-葡聚糖4-葡聚糖水解酶（β-葡聚糖酶）在植物和细菌中均有合成。这些酶特异性水解β-葡聚糖中与β-1,3连接相邻的β-1,4糖苷键，β-葡聚糖是一种线性多糖，其中这些键的比例约为2.5:1。本文综述了天然芽孢杆菌β-葡聚糖酶以及通过杂交序列、单点突变和环形排列表征的工程变体的X射线晶体学结构研究。结合生化数据和定点诱变，晶体学证据有助于构建保留型芽孢杆菌β-葡聚糖酶可能的反应机制。此外，活性位点通道的形状、纤维二糖苷环氧烷基抑制剂的已知结合模式以及β-葡聚糖底物的能量分布解释了这些酶对β-葡聚糖的特异性以及对可裂解键旁β-1,3糖苷键的需求。具有环形排列序列的β-葡聚糖酶保留构象稳定性、酶活性和天然折叠。芽孢杆菌β-葡聚糖酶的果冻卷三级结构非常稳定，能抵抗氨基酸序列、链拓扑结构、配体结合和晶体堆积的变化。
