Varghese J N, Garrett T P, Colman P M, Chen L, Høj P B, Fincher G B
Biomolecular Research Institute, Parkville, Victoria, Australia.
Proc Natl Acad Sci U S A. 1994 Mar 29;91(7):2785-9. doi: 10.1073/pnas.91.7.2785.
The three-dimensional structures of (1-->3)-beta-glucanase (EC 3.2.1.39) isoenzyme GII and (1-->3,1-->4)-beta-glucanase (EC 3.2.1.73) isoenzyme EII from barley have been determined by x-ray crystallography at 2.2- to 2.3-A resolution. The two classes of polysaccharide endohydrolase differ in their substrate specificity and function. Thus, the (1-->3)-beta-glucanases, which are classified amongst the plant "pathogenesis-related proteins," can hydrolyze (1-->3)- and (1-->3,1-->6)-beta-glucans of fungal cell walls and may therefore contribute to plant defense strategies, while the (1-->3,1-->4)-beta-glucanases function in plant cell wall hydrolysis during mobilization of the endosperm in germinating grain or during the growth of vegetative tissues. Both enzymes are alpha/beta-barrel structures. The catalytic amino acid residues are located within deep grooves which extend across the enzymes and which probably bind the substrates. Because the polypeptide backbones of the two enzymes are structurally very similar, the differences in their substrate specificities, and hence their widely divergent functions, have been acquired primarily by amino acid substitutions within the groove.
通过X射线晶体学在2.2至2.3埃分辨率下测定了大麦中（1→3）-β-葡聚糖酶（EC 3.2.1.39）同工酶GII和（1→3,1→4）-β-葡聚糖酶（EC 3.2.1.73）同工酶EII的三维结构。这两类多糖内切酶在底物特异性和功能上有所不同。因此，被归类为植物“病程相关蛋白”的（1→3）-β-葡聚糖酶能够水解真菌细胞壁的（1→3）-和（1→3,1→6）-β-葡聚糖，因此可能有助于植物的防御策略，而（1→3,1→4）-β-葡聚糖酶在发芽谷物胚乳动员期间或营养组织生长期间的植物细胞壁水解中发挥作用。这两种酶均为α/β桶状结构。催化氨基酸残基位于贯穿酶的深沟内，这些深沟可能与底物结合。由于这两种酶的多肽主链在结构上非常相似，它们底物特异性的差异以及因此而产生的广泛不同的功能，主要是由沟内的氨基酸取代所导致的。
