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Glycoside Hydrolase Family 86
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|Glycoside Hydrolase Family GH86|
|Active site residues||inferred from clan GH-A as two Glu|
|CAZy DB link|
Glycoside hydrolases of family 86 have first been identified to be β-agarases (EC 188.8.131.52) that cleave β-1,4 glycosidic bonds of agarose. This polysaccharide chain is ideally made of alternating 1,3-linked β-D-galactose (G) and 1,4-linked 3,6-anhydro-α-L-galactose (LA) residues forming a parallel double helix. Four agarolytic enzymes have been characterized: AgrA from Pseudoalteromonas atlantica, AgaO from Microbulbifer thermotolerans JAMB-A94, Aga86E from Saccharophagus degradans 2-40 [1, 2, 3] and more recently a GH86 β-agarase from a non marine Vibrio species (sp. OA-2007). AgaO from M. thermotolerans was reported to be an endo-hydrolytic enzyme, releasing neoagaro-hexaose as main product , while the recombinant Aga86E from S. degradans released only neoagarobiose in an exo-acting manner . In june 2012, a first GH86 enzyme was identified in the human gut Bacteroidetes B. plebeius that was active on porphyran, an agarocolloïd in which the 3,6-anhydro-L-galactose unit (LA) of neutral agarose is replaced by L-galactose-6-sulfate (L6S). The endo-acting enzyme, BpGH86A, is inactive on agarose and the main products released by cleavage of β-1,4 glycosidic bonds of porphyran, are tetrasaccharides having the sequence L6S-G-L6S-G∼ .
Kinetics and Mechanism
A potential retaining mechanism of this glycoside hydrolase family can be inferred from analogy to clan GH-A enzymes, and is confirmed by structural analyses of a product complex showing an arrangement of the putative catalytic residues that is in agreement with the double displacement mechanism . However, no mechanistic or kinetic analysis demonstrating the stereochemical outcome of the reaction have been reported for this family to date.
The catalytic residues can be inferred from the crystal structure and by analogy to clan GH-A enzymes as two glutamate residues, which are Glu152 (acid/base) and Glu279 (nucleophile) in BpGH86A from B. plebeius .
The first 3D structure was determined in 2012 by  et al. In agreement with distant sequence homology to clan GH-A enzymes, the overall topology contains an N-terminal (β/α)8 barrel. However the enzyme architecture is more complex and contains two additional Cterminal β-sandwich domains, which align via their convex faces to the exterior of the (β/α)8 barrel and connect with two N-terminal β-strands that become part of these C-terminal β-sandwich domains.
- Identification of first family member
- The first member of this family, AgrA, was identified in Pseudoalteromonas atlantica .
- First stereochemistry determination
- the retaining mechanism is inferred from the arrangement of the most probable catalytic residues in the crystal structure of a product complex .
- First catalytic nucleophile identification
- Glu279 in BpGH86A from B.plebeius.
- First general acid/base residue identification
- Glu152 in BpGH86A from B.plebeius.
- First 3-D structure
- The crystal structure of BpGH86A from B.plebeius, (PDB 4aw7).
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- Hehemann JH, Kelly AG, Pudlo NA, Martens EC, and Boraston AB. (2012) Bacteria of the human gut microbiome catabolize red seaweed glycans with carbohydrate-active enzyme updates from extrinsic microbes. Proc Natl Acad Sci U S A. 109, 19786-91. DOI:10.1073/pnas.1211002109 |