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Glycoside Hydrolase Family 140

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Glycoside Hydrolase Family GH140
Clan GH-x
Mechanism retaining
Active site residues known
CAZy DB link
http://www.cazy.org/GH140.html


Substrate specificities

Thus far only one member of the family has been characterised, BT1012 from bacteroides thetaiotaomicron. BT1012 displays apiosidase activity targeting apiose in the complex glycan rhamnogalacturonan ii (RGII). The apiose is found at the base of Chains A and B in RGII and linked α1,2 to the galacturonic acid backbone. Cleavage of the backbone must occur for BT1012 to then act.

Kinetics and Mechanism

GH140 likely uses a, retaining, double displacement mechanism. This is strongly supported by methanolysis experiments using the trisaccharide L-rhamnose-β1,3-D-apiose-α1,2-D-galacturonic acid-O-methyl in the presence of 10 % methanol. This experiment generates the product L-rhamnose-β1,3-D-apiose-O-methyl which is only possible via a retaining mechanism.

Catalytic Residues

The catalytic residues are an aspartate and glutamate located on beta strands 4 and 7, respectively.

Three-dimensional structures

GH140 adopts a (β/α)8 , TIM barrel, where a central barrel of eight β strands are encircled by eight α helices. BT1012, the only GH140 strcuture available, also has a Ig like domain that stacks against the TIM barrel likely providing structural stability, similar to the role of Ig like domains in GH43 enzymes.

Family Firsts

First stereochemistry determination
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First catalytic nucleophile identification
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First general acid/base residue identification
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First 3-D structure
BT1012 from bacteroides thetaiotaomicron.

References

  1. Cantarel BL, Coutinho PM, Rancurel C, Bernard T, Lombard V, and Henrissat B. (2009) The Carbohydrate-Active EnZymes database (CAZy): an expert resource for Glycogenomics. Nucleic Acids Res. 37, D233-8. DOI:10.1093/nar/gkn663 | PubMed ID:18838391 | HubMed [Cantarel2009]
  2. Davies, G.J. and Sinnott, M.L. (2008) Sorting the diverse: the sequence-based classifications of carbohydrate-active enzymes. The Biochemist, vol. 30, no. 4., pp. 26-32. Download PDF version.
    [DaviesSinnott2008]