GH3 GlcNAcases catalyse the hydrolytic reaction through a standard ‘retaining’ mechanism involving a covalent glycosyl-enzyme intermediate [14 ,15 ], while GH20 and GH84 GlcNAcases hydrolyse chitooligosaccharides through a ‘substrate-assisted’ mechanism involving the transient formation of an oxazolinium ion intermediate [16 –18 ].