CHITINASES, CHITOSANASES, AND LYSOZYMES CAN BE DIVIDED INTO PROKARYOTIC AND EUKARYOTIC FAMILIES SHARING A CONSERVED CORE

Barley chitinase, bacterial chitosanase, and lysozymes from goose (GEW L), phage (T4L) and hen (HEWL) all hydrolyse related polysaccharides. The proteins share no significant aminoacid similarities, but have a s tructurally invariant core consisting of two helices and a three-stran ded beta-sheet which form the substrate-binding and catalytic cleft. T hese enzymes rep resent a superfamily of hydrolases which are likely t o have arisen by divergent evolution. Based on structural criteria, we divide the hydrolase superfamily into a bacterial family (chitosanase and T4L) and a eucaryotic family represented by chitinase and GEWL. B oth families contain the core but have differing N- and C-terminal dom ains. Inclusion of chitinase and chitosanase in the superfamily sugges ts the archetypal catalytic mechanism of the group is an inverting mec hanism. The retaining mechanism of HEWL is unusual.