Family 18 chitinase-oligosaccharide substrate interaction:: subsite preference and anomer selectivity of Serratia marcescens chitinase A

The sizes and anomers of the products formed during the hydrolysis of chitin oligosaccharides by the Family 18 chitinase A (ChiA) from Serratia marcescens were analysed by hydrophilic interaction chromatography using a novel approach in which reactions were performed at 0 degreesC to stabilize the anomer conformations of the initial products. Crystallographic studies of the enzyme, having the structure of the complex of the ChiA E315L (Glu(315) --> Leu) mutant with a hexasaccharide, show that the oligosaccharide occupies subsites - 4 to + 2 in the substrate-binding cleft, consistent with the processing of beta-chitin by the release of disaccharide at the reducing end. Products of the hydrolysis of hexa- and penta-saccharides by wild-type ChiA, as well as by two mutants of the residues Trp(275) and Phe(396) important in binding the substrate at the + 1 and + 2 sites, show that the substrates only occupy sites - 2 to + 2 and that additional N-acetyl-D-glucosamines extend beyond the substrate-binding cleft at the reducing end. The subsites - 3 and - 4 are not used in this four-site binding mode. The explanation for these results is found in the high importance of individual binding sites for the processing of short oligosaccharides compared with the cumulative recognition and processive hydrolysis mechanism used to digest natural beta-chitin.