Glycoside Hydrolase Family 140

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• Author: ^^^Alan Cartmell^^^
• Responsible Curator: ^^^Harry Gilbert^^^

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Glycoside Hydrolase Family GH140
Clan	GH-x
Mechanism	retaining
Active site residues	known
CAZy DB link
https://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. 2009;37(Database issue):D233-8. DOI:10.1093/nar/gkn663 |

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.