Glycoside Hydrolase Family 134

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• Author: ^^^Spencer Williams^^^
• Responsible Curator: ^^^Spencer Williams^^^

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Glycoside Hydrolase Family GH134
Clan	none
Mechanism	inverting
Active site residues	known
CAZy DB link
http://www.cazy.org/GH134.html

Substrate specificities

Glycoside hydrolase family 134 exclusively contains β-1,4-mannanases, and was created based on the discovery of Man134A from Aspergillus nidulans [1]. Man134A exhibited weak activity on galactomannan but robust activity on glucomannan, and activity on β-1,4-linked mannopentaose and hexaose [1]. SsGH134 from Streptomyces sp. NRRL B-24484 possessed a strong preference for unsubstituted linear β-mannans over gluco- and galactomannans [2]. SsGH134 possessed activity on β-1,4-linked mannotetraose, pentaose and hexaose.

Kinetics and Mechanism

¹H NMR spectroscopic analysis of SsGH134 catalyzed cleavage of a benzoylhydrazine derivative of β-mannohexaose provided evidence for the formation of the α-anomer, consistent with an inverting mechanism [2]. The enzyme is believed to follow a classical Koshland inverting mechanism. Crystallographic evidence from binary complexes of SsGH134 with substrate and product, complemented by quantum mechanics/molecular mechanics calculations, supports a southern hemisphere ¹C₄→³H₄^‡→³S₁ conformational reaction coordinate [2].

Catalytic Residues

Structural analysis of SsGH134 from Streptomyces sp. (see below) suggested roles for Glu45 as a general acid catalyst and for Asp57 as a general base catalyst [2]. These roles were supported by mutagenesis with the corresponding Glu45Ala and Asp57Ala single mutants inactive on β-pentaose and hexaose.

Three-dimensional structures

The three dimensional structure was first reported for SsGH134, and exists as a mixed α-helix/β-sheet fold with resemblance to HEWL (GH22), as well as GH19 chitinases, GH23 G-type lysozyme, and GH124 cellulases [2]. A binary Michaelis complex of the E45A variant of SsGH134 with mannopentaose bound in the active site defined the -3 to +2 subsites, and showed the sugar binding in the -1 subsite in a ¹C₄ conformation; a similar conformation was observed for the reducing-end mannose of a complex of mannotriose bound to the -3 to -1 subsites of wild-type SsGH134 [2]. Within the Michaelis complex, the carboxylate of the highly conserved C-terminal residue Ile173 is hydrogen bonded to O6 of the –2 mannosyl unit, and also forms a salt-bridge with the highly conserved K59. Within only the Michaelis complex, this interaction results in the formation of an active cleft tunnel reminiscent of processive glycoside hydrolases; the C-terminal segment is disordered in both apo and mannotriose product-bound forms [2].

Family Firsts

First stereochemistry determination

SsGH134 was shown to be inverting by ¹H NMR spectroscopy [2].

First general acid residue identification

Glu45 in SsGH134 by structural study supported by kinetic analysis of mutants [2].

First general base residue identification

Asp57 in SsGH134 by structural study supported by kinetic analysis of mutants [2].

First 3-D structure

SsGH134 from Streptomyces sp., PDB code 5JTS [2].

References

1. pmid= 26385921

2. pmid=???