Glycoside hydrolysis by retaining family 18 chitinases involves a catalytic acid (Glu) which is part of a conserved DXDXEsequence motif that spans strand four of a (ba)8 barrel (TIMbarrel) are unusual in that thepositive charge emergingon theanomeric carbon after departure of the leaving group is stabilized by the substrate itself (the N-acetyl group of the distorted )1 sugar), rather than by a carboxylate group on the enzyme.