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Glycoside Hydrolase Family 134
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|Glycoside Hydrolase Family GH134|
|Active site residues||known|
|CAZy DB link|
Glycoside hydrolase family 134 exclusively contains β-1,4-mannanases, and was created based on the discovery of Man134A from Aspergillus nidulans . Man134A exhibits weak activity on galactomannan but robust activity on glucomannan, and activity on β-1,4-linked mannopentaose and hexaose . SsGH134 from Streptomyces sp. NRRL B-24484 possesses a strong preference for unsubstituted linear β-mannans over gluco- and galactomannans . SsGH134 possesses 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 . 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 itinerary along the reaction coordinate .
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 . These roles were supported by mutagenesis with the corresponding Glu45Ala and Asp57Ala single mutants inactive on β-mannopentaose and hexaose.
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 . 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 . 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 . SsGH134 can be classified as a syn protonator, with the acid residue situated syn to the C1-O5 bond.
- First stereochemistry determination
- SsGH134 was shown to be inverting by 1H NMR spectroscopy .
- First general acid residue identification
- Glu45 in SsGH134 by structural study supported by kinetic analysis of mutants .
- First general base residue identification
- Asp57 in SsGH134 by structural study supported by kinetic analysis of mutants .
- First 3-D structure
- SsGH134 from Streptomyces sp. (PDB ID 5JTS) .
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