Background

Chitosanases (EC 3.2.1.132) hydrolyze chitosan which is a polymer of glucosamine (GlcN) linked by β − 1,4 bonds, and show cleavage specificity against partially acetylated chitosan containing N-acetylglucosamine (GlcNAc) residues. Chitosanases' structural underpinnings for cleavage specificity and the conformational switch from open to closed structures are still a mystery.

Methods

The GH-46 subclass III chitosanase from Bacillus circulans MH-K1 (MH-K1 chitosanase), which also catalyzes the hydrolysis of GlcN-GlcNAc bonds in addition to GlcN-GlcN, has had its chitotetraose [(GlcN)₄]-complexed crystal structure solved at 1.35 Å resolution.

Results

The MH-K1 chitosanase's (GlcN)₄-bound structure has numerous structural similarities to other GH-46 chitosanases in terms of substrate binding and catalytic processes. However, subsite −1, which is absolutely specific for GlcN, seems to characterize the structure of a subclass III chitosanase due to its distinctive length and angle of a flexible loop. According to a comparison of the (GlcN)₄-bound and apo-form structures, the particular binding of a GlcN residue at subsite −2 through Asp77 causes the backbone helix to kink, which causes the upper- and lower-domains to approach closely when binding a substrate.

Conclusions

Although GH-46 chitosanases vary in the finer details of the subsites defining cleavage specificity, they share similar structural characteristics in substrate-binding, catalytic processes, and potentially in conformational change.

General significance

The precise binding of a GlcN residue to the −2 subsite is essential for the conformational shift that occurs in all GH-46 chitosanases, as shown by the crystal structures of the apo- and substrate-bound forms of MH-K1 chitosanase.

中文翻译：

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环状芽孢杆菌 MH-K1 的 GH-46 亚类 III 壳聚糖酶与壳四糖复合物的晶体结构

背景

壳聚糖酶 (EC 3.2.1.132) 水解壳聚糖，壳聚糖是通过 β - 1,4 键连接的葡糖胺 (GlcN)聚合物，并且对含有N - 乙酰葡糖胺 (GlcNAc) 残基的部分乙酰化壳聚糖表现出切割特异性。壳聚糖酶的切割特异性和从开放结构到封闭结构的构象转换的结构基础仍然是一个谜。

方法

来自环状芽孢杆菌MH-K1的 GH-46 亚类 III 壳聚糖酶（MH-K1 壳聚糖酶）除了催化 GlcN-GlcN 之外，还催化 GlcN-GlcNAc 键的水解，具有壳四糖 [(GlcN) ₄ ] 复合晶体结构解析度为 1.35 Å。

结果

MH-K1 壳聚糖酶的 (GlcN) ₄结合结构在底物结合和催化过程方面与其他 GH-46 壳聚糖酶具有许多结构相似性。然而，GlcN 绝对特异的亚位点 -1 似乎表征了 III 类壳聚糖酶的结构，因为它具有独特的柔性环长度和角度。根据 (GlcN) ₄结合和 apo 形式结构的比较，GlcN 残基通过 Asp77 在子位点 -2 处的特定结合导致主链螺旋扭结，从而导致上部结构域和下部结构域紧密接近当结合底物时。

结论

尽管 GH-46 壳聚糖酶在定义切割特异性的子位点的细节上有所不同，但它们在底物结合、催化过程以及潜在的构象变化方面具有相似的结构特征。

一般意义

GlcN 残基与 -2 亚位点的精确结合对于所有 GH-46 壳聚糖酶中发生的构象转变至关重要，如 MH-K1 壳聚糖酶的脱辅基和底物结合形式的晶体结构所示。