Glycoside Hydrolase Family 92

This page has been approved by the Responsible Curator as essentially complete. CAZypedia is a living document, so further improvement of this page is still possible. If you would like to suggest an addition or correction, please contact the page's Responsible Curator directly by e-mail.

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

Glycoside Hydrolase Family GH92
Clan	none
Mechanism	inverting
Active site residues	known/not known
CAZy DB link
http://www.cazy.org/fam/GH92.html

Substrate specificities

Glycoside hydrolases of family 92 are exo-acting alpha-mannosidases. The first reported enzyme activity from this family was an alpha-1,2-mannosidase from Microbacterium sp. M-90. [1] Recently the characterization of 22 GH92 enzymes from Bacteroides thetaiotaomicron confirmed an exo-mode of action with alpha-1,2-mannosidase, alpha-1,3-mannosidase, alpha-1,4-mannosidase and alpha-1,6-mannosidase activities detected [2].

Kinetics and Mechanism

¹H-NMR studies on three GH92s that displayed alpha-1,2-, alpha-1,3- and alpha-1,4-mannosidase activities all generated beta-D-mannose indicating that these enzymes catalyse glycosidic bond hydrolysis through a single displacement mechanism leading to inversion of anomeric configuration [2]. GH92 enzymes are calcium-dependent alpha-mannosidases. The requirement for the metal ion is currently restricted to only three GH families all of which are exo-alpha mannosidases. Mechanistically, this may indicate that the lack of distorting binding energy provided by the -2 or +1 subsites impose a requirement for conformational flexibility at the -1 subsite (recognition of both the ground state and the transition state conformations), which is achieved by a metal ion interaction with O2 and O3. Three inhibitors bound to the alpha-1,2-mannosidase Bt3990 in approximate ¹S₅/B_2,5 and ^1,4B/¹S₅ conformations indicating that catalysis is mediated by a B_2,5 transition state.

Catalytic Residues

Based on 3D structural data on the alpha-1,2-mannosidase Bt3990, Glu533 is the predicted catalytic general acid residue. This view is supported by an inactive mutant of this residue, and the conservation of the glutamate throughout the GH92 family [2]. The catalytic general 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 an inverting enzyme. Mutants of both residues inactivte the enzyme, however, while Asp644 is invariant, Asp642 can be an Asn or Asp in GH92 members [2]. It appears that Asp644 is the likely catalytic general 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 [2].

Family Firsts

First sterochemistry determination: ¹H-NMR showed three GH92s generate beta-mannose and thus these alpha-mannosidases are inverting enzymes [2].
First general acid residue identification: Based on mutagenesis and 3D structural information the conserved catalytic acid has been identified [2].
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 [2].
First 3-D structure: The 3D structure reveals two domains; an N-terminal beta-sandwich domain and a C-terminal adorned (alpha/alpha)₆ barrel. Both domains contribute residues to the active site [2].

References

All Medline abstracts: PubMed