Structural Insights into the Molecular Evolution of the Archaeal Exo-β-d-Glucosaminidase

Abstract

The archaeal exo-β-d-glucosaminidase (GlmA), a thermostable enzyme belonging to the glycosidase hydrolase (GH) 35 family, hydrolyzes chitosan oligosaccharides into monomer glucosamines. GlmA is a novel enzyme in terms of its primary structure, as it is homologous to both GH35 and GH42 β-galactosidases. The catalytic mechanism of GlmA is not known. Here, we summarize the recent reports on the crystallographic analysis of GlmA. GlmA is a homodimer, with each subunit comprising three distinct domains: a catalytic TIM-barrel domain, an α/β domain, and a β1 domain. Surprisingly, the structure of GlmA presents features common to GH35 and GH42 β-galactosidases, with the domain organization resembling that of GH42 β-galactosidases and the active-site architecture resembling that of GH35 β-galactosidases. Additionally, the GlmA structure also provides critical information about its catalytic mechanism, in particular, on how the enzyme can recognize glucosamine. Finally, we postulate an evolutionary pathway based on the structure of an ancestor GlmA to extant GH35 and GH42 β-galactosidases.

Characterization and structural analyses of a novel glycosyltransferase acting on the β-1,2-glucosidic linkages

“…In the case of TkGlmA, Cys101 is located at the position corresponding to Leu100 in IaSGT as well as SpBgaC ( Fig. S11 middle), implying that Cys101 is unlikely to be involved in the distinction of O4 epimers ( 39 ). Instead, Trp308 at the bottom of the pyranose ring is likely to make TkGlmA unpreferable for the tg rotamer of a 6-hydroxy group.…”

“…While GH35 enzymes, which are distributed in a wide range of microorganisms, plants, and animals, are mainly β-galactosidases (β-galactosidase, exo-β-1,4-galactanase, and β-1,3-galactosidase) according to the CAZy database ( 34 , 35 , 36 ), several GH35 enzymes from Archaea have been found to be β-glucosaminindases (GlmAs), and GlmA from Thermococcus kodakaraensis (TkGlmA) hydrolyzes chitosan and chitooligosaccharides ( 37 , 38 ). The binding modes of natural substrates are not sufficiently understood, since only the glucosamine (GlcN) complex is available among GlmAs ( 39 ). Though many GH35 enzymes have been reported, as described above, no glucoside-acting enzyme has been reported in this family.…”

A β-Galactosidase acting on unique galactosides: the structure and function of a β-1,2-galactosidase fromBacteroides xylanisolvens, an intestinal bacterium

“…β-1,3-Galactosidases (EC 3.2.1.-) and exo-β-1,4-galactanases (EC 3.2.1.-) with narrow substrate specificity are also found in the GH35 family. In addition to βgalactosidases, exo-β-1,4-glucosaminidase (EC 3.2.1.165) and SGT (EC 2.4.1.391) have been reported 25,[30][31][32] . However, these enzymes belong to small groups far from Bxy_22780 in the GH35 family.…”