Glycoside Hydrolase Family 145

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

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Glycoside Hydrolase Family GH145
Clan	GH-x
Mechanism	retaining
Active site residues	known
CAZy DB link
http://www.cazy.org/GH145.html

Substrate specificities

Two members of this family have been shown to be α-L-rhamnosidases, targeting rhamnose linked α-1,4 to glucuronic acid in the complex arabinogalactan protein gum Arabic. The active site of these a-L-rhamnosidases is located on the opposite side of CAZymes with similar b-propeller folds. Surprisingly, the "normal" side of the b-propeller bears the highest residue conservation in the family and may well have another function. The latter is unknown but a strong ressemblance to family PL25 suggests a polysaccharide lyase activity could be possible.

Kinetics and Mechanism

NMR, using the arabinogalactan protein (AGP) gum arabic as the substrate, revealed the family operates via a retaining mechanism. Rather than using a standard double displacement mechanism the enzyme is predicted to perform catalysis via an epoxide intermediate, similar to GH99 enzymes. GH145, however, is proposed to perform catalysis via a substrate assisted mechanism, requiring the carboxyl group of the glucuronic acid and a single catalytic histidine; both acting as an acid/base. This histidine is predicted to deprotonate the O2 of rhamnose, allowing O2 to attack C1 and form an epoxide. Simultaneously the carboxyl group of the glucuronic acid may deprotonate a water molecule generating a hydroxyl group to attach the C1 of rhamnose and allowing protonation of its own O4 thus, leading to glycosidic bond cleavage.

Catalytic Residues

A single catalytic histidine has been shown to be critical for activity. The introduction of the cataytic histidine into related enzymes, which lack the histidine and rhamnosidase activity, is sufficient to introduce rhamnosidase activity into these enzymes.

Three-dimensional structures

Content is to be added here.

Family Firsts

First stereochemistry determination

Determined for the bacteroides thetaiotaomicron enzyme BT3686

First catalytic acid/base residue identification

Predicted to a histidine

Second general acid/base residue identification

Predicted to be provided by the substrate.

First 3-D structure

BT3686, BACINT_00347 and BACCELL_00856 were the first enzymes to have their structures solved.

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.