The mechanism for crystalline chitin hydrolysis by chitinases

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An ability to hydrolyze insoluble and crystalline chitin is the most intrinsic and interesting feature of chitinases. In this study, the mechanisms for crystalline chitin hydrolysis by chitinase A1 (ChiAl) from Bacillus circulans WL-12 and chitinase A (ChiA) from Serratia marcescens 2170 were studied on the basis of their 3D-structures.(1) Roles of the aromatic amino acid residues exposed on the surface of ChiA1 from B. circulans WL-12. ChiA1 comprises a catalytic domain, two FnIII domains and a chitin-binding domain. On the surface of the catalytic domain, two exposed aromatic residues were identified. Site-directed mutagenesis of these residues revealed that they play major role in introducing a chitin chain from the surface of crystalline chitin into the catalytic cleft.(2) Solution structure of the FnIII domain. The FnIII domain of ChiA1 was produced in E. coli by using PET expression system and purified. Then, solution structure of the FnIII domain was determined by NMR. The struc … More ture appeared to be very similar to those of the FnIII domains in animal proteins, supporting the idea that bacterial FnIII domains were acquired from animal through horizontal gene transfer.(3) The mechanism for crystalline chitin hydrolysis by ChiA from S. marcescens 2170. Four exposed aromatic residues, two in the N-terminal domain and two in the catalytic domain of ChiA from S. marcescens 2170 were identified. Importance of these residues in crystalline chitin hydrolysis was studied by site-directed mutagenesis of these residues. The results obtained lead us to propose following model for crystalline chitin hydrolysis by ChiA. "ChiA binds to crystalline chitin through interaction between three aromatic residues and the GlcNAc residues in a single chitin chain on the crystalline chitin surface. The chitin chain is introduced into the catalytic cleft from the reducing end side of the chain. The introduced chitin chain slide through the cleft to the catalytic site and progressively cleaved releasing (GlcNAc)2 units continuously." Less