Crystal structure of the GH-46 subclass III chitosanase from Bacillus circulans MH-K1 in complex with chitotetraose

• Published in: Biochimica et biophysica acta. General subjects Vol. 1868; no. 3; p. 130549
• Main Authors: Suzuki, Michihiko, Saito, Akihiro, Kobayashi, Mariko, Yokoyama, Tomofumi, Omiya, Shoko, Li, Jian, Sugita, Kei, Miki, Kunio, Saito, Jun-ichi, Ando, Akikazu
• Format: Journal Article
• Language: English
• Published: Netherlands 01.03.2024
• Subjects: Bacillus; Chitosan; Glucosamine - metabolism; Glycoside Hydrolases - metabolism; Oligosaccharides; Bacillus circulans; Cleavage specificity; Glycoside hydrolase; Chitosanase; GH-46
• ISSN: 0304-4165; 1872-8006; 1872-8006
• DOI: 10.1016/j.bbagen.2023.130549

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. 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) ]-complexed crystal structure solved at 1.35 Å resolution. The MH-K1 chitosanase's (GlcN) -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) -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. 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. 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.