Glycoside Hydrolase Family 92

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Author: ^^^Harry Gilbert^^^
Responsible Curator: ^^^Harry Gilbert^^^

Glycoside Hydrolase Family GHnn
Clan	GH-x
Mechanism	retaining/inverting
Active site residues	known/not known
CAZy DB link
http://www.cazy.org/fam/GHnn.html

Substrate specificities

GH92 enzymes are exo-acting alpha-mannosidases. The first reported enzyme activity from this family was an alpha1,2-mannosidase from Microbacterium sp. M-90. [1] Recently the characterization of 22 GH92 enzymes from Bacteroides thetaiotaomicron confirmed an exo-mode of action but alpha1,2-mannosidase, alpha1,3-mannosidase, alpha1,4-mannosidase and alpha1,6-mannosidase activities were detected [2, 3, 4].

Kinetics and Mechanism

1H-NMR studies on three GH92s that displayed alpha1,2-, alpha1,3- and alpha1,4-mannosidase activities all generated beta-mannose indicating that these enzymes catalyse glycosidic bond hydrolysis through a single displacement mechanism leading to inversion of anomeric configuration [2, 3, 4]. GH92 enzymes are calcium-dependent alpha-mannosidases. The requirement for the metal ion is restricted to only three GH92 families all of which are exo-alpha mannosidases. Mechanistically this may indicated that the lack of distorting binding energy provided by the -2 or +1 subsites imposes a requirement for conformational flexibility at the -1 subsite (recognition of the ground state and the transition state conformations), which is best achieved by a metal ion interaction with O2 and O3. Three inhibitors bound to the alpha1,2-mannosidase Bt3990 in approximate 1S5/B2,5 and 1,4B/1S5 conformations indicating that catalysis is mediated by a Boat2,5 transition state.

Catalytic Residues

Based on 3D structural data on the alpha1,2-mannosidase Bt3990, Glu533 is the predicted catalytic acid. This view is supported by an inactive mutant of this residue, and the conservation of the glutamate throughout the GH92 family. The catalytic base, in common with many inverting glycoside hydrolases, is more difficult to identify. Asp644 and Asp642 both lie in the canonical position one would expect for a general base in the inverting mechanism. Mutants of both resisdues inactivtes the enzyme, however, while Asp644 is invariant, Asp642 can be an Asn or Asp in GH92 members. It appears that Asp644 is the likely catalytic base.

Three-dimensional structures

GH92 enzymes display a two domain structure. The small N-terminal domain is a beta-sandwich and the large C-terminal domain adopts a adorned alpha/alpha)6 barrel fold. Amino acids in the active site of the enzyme, a shallow pocket, are contributed by both the N- and C-terminal domains.

Family Firsts

First sterochemistry determination: 1H-NMR showed three GH92s generate beta-mannose and thus these alpha-mannosidases are inverting enzymes. First catalytic acid identification: Based on mutagenesis and 3D structural information the conserved catalytic acid has been identified. First general base residue identification: Based on mutagenesis and 3D structural information a pair of likely catalytic bases were identified. As one of these residues is invariant this is the proposed catalytic base. First 3-D structure: The 3D structure revelas two domains; an N-terminal beta-sandwich domain and a C-terminal adorned (alha/alpha)6 barrele. Both domains contribute residues to the active site.

References

1 pmid=8149382 Comfort2007 pmid=17323919
2 Zhu et al