Related Experiment Video
Updated: Jul 6, 2025
Structural Biology and Analytical Chemistry Approaches for Characterizing C-Glycoside Metabolic Enzymes in Human Gut Microbiota
Published on: May 23, 2025
Crystal structure of the GH-46 subclass III chitosanase from Bacillus circulans MH-K1 in complex with chitotetraose
Michihiko Suzuki1, Akihiro Saito2, Mariko Kobayashi3
1Molecular Analysis Center, Research Unit, R&D Division, Kyowa Kirin, Sunto-gun, Shizuoka 411-8731, Japan.
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.
