The structure of an archaeal β-glucosaminidase provides insight into glycoside hydrolase evolution

The archaeal exo-β-D-glucosaminidase (GlmA) is a dimeric enzyme that hydrolyzes chitosan oligosaccharides into monomer glucosamines. GlmA is a member of the glycosidase hydrolase (GH)-A superfamily-subfamily 35 and is a novel enzyme in terms of its primary structure. Here, we present the crystal structure of GlmA in complex with glucosamine at 1.27 Å resolution. The structure reveals that a monomeric form of GlmA shares structural homology with GH42 β-galactosidases, whereas most of the spatial positions of the active site residues are identical to those of GH35 β-galactosidases. We found that upon dimerization, the active site of GlmA changes shape, enhancing its ability to hydrolyze the smaller substrate in a manner similar to that of homotrimeric GH42 β-galactosidase. However, GlmA can differentiate glucosamine from galactose based on one charged residue while using the "evolutionary heritage residue" it shares with GH35 β-galactosidase. Our study suggests that GH35 and GH42 β-galactosidases evolved by exploiting the structural features of GlmA.