Glycoside Hydrolase Family 137

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

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Glycoside Hydrolase Family GH137
Clan	None
Mechanism	Unknown
Active site residues	known
CAZy DB link
https://www.cazy.org/GH137.html

Substrate specificities

The only activity identified to date is β-L-arabinofuranosidase activity displayed by the enzyme BT0996. This enzyme removes β linked L-arabinose from the terminus of side chain B in the complex glycan rhamnogalacturonan ii (RGII) [1].

Kinetics and Mechanism

BT0996 was active both on intact RGII and on Chain B released from RGII [1]. A thorough mutagenic strategy was performed on BT0996 and identified two glutamates to be essential for activity, Glu159 and Glu240 (see below) [1]. The mechanism for arabinofuranosidedases are challenging to elucidate due to rapid tautomerisation of free arbinofuranose in solution to α and β anomers of both arabinopyranose (major confomer) and arabinofuranose.

Catalytic Residues

The catalytic residues were shown to be a pair of glutamates. They sit around 6 Angstroms apart. Typically retaining enzymes are expected to have their catalytic residues ~5.5 Angstroms apart, whilst in inverting enzymes they are expected to be ~10 Angstroms apart [2]. Glu159 sits beneath the alpha face of the -1 β-linked arabinose, 3.2 Angstroms from the anomeric carbon, placing it in a an ideal position to act as a catalytic nucleophile. Glu240 resides 5 Angstroms from the glycosidic oxygen which somewhat far to act as catalytic acid/base in a retaining mechanism. Alternatively, one could also speculate that at a distance of 4.6 Angstroms from the anomeric carbon Glu240 could potentially act as a catalytic base in an inverting mechanism, with Glu159 acting as the catalytic acid being at a distance of 3.7 Angstroms from the glycosidic oxygen [1].

Three-dimensional structures

The structure of BT0996 comprises a single domain which is a five bladed β-propeller fold. Each blade is composed of three to four anti parallel β-strands that extend out radially from the central core. The final blade is formed by strands from both the N- and C-terminus of the protein which is termed as 'molecular velcro' and is believed to add considerable stability to the fold. It should be noted that BT0996 is appended to a GH2 from clan GHA. This GH2 targets β-D-GlcA linkage in Chain A of RGII [1].

Family Firsts

First stereochemistry determination

Not Known.

First catalytic nucleophile/base identification

Inferred to be Glu159 in BT0996 [1].

First general acid/base residue identification

Inferred to be Glu240 in BT0996 [1].

First 3-D structure

The first structure determination for GH137 was of BT0996 from the organism Bacteroides thetaiotaomicon [1].

References

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