Glycoside Hydrolase Family 158

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• Author: ^^^Kazune Tamura^^^
• Responsible Curator: ^^^Harry Brumer^^^

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Glycoside Hydrolase Family GH158
Clan	GH-A
Mechanism	retaining
Active site residues	known
CAZy DB link
http://www.cazy.org/GH158.html

Substrate specificities

Members of family 158 have been shown to display activity towards β(1,3)-glucans, making it the fourth clan GH-A family known to contain β(1,3)-glucanase activity, alongside GH17, GH128, and GH148. The founding member of this family, Vvad_PD1638 from Victivallis vadensis, was shown to be active on carboxymethyl-curdlan in a high-throughput screen [1].

BuGH158 from the prominent human gut symbiont Bacteroides uniformis was the first GH158 member to receive detailed characterization [2]. BuGH158 is an endo β(1,3)-glucanase with high specificity towards laminarin from Laminaria digitata, a β(1,3)-glucan with single β(1,6)-glucose branches. BuGH158 is unable to tolerate more extensive branching as evidenced by poor activity towards other β(1,3)-glucans with longer, more frequent branches like laminarin from Eisenia bicyclis and yeast β-glucan [2]. The unbranched, linear β(1,3)-glucan curdlan was also not effectively hydrolyzed by BuGH158, due the glucans poor solubility in water (Vvad_PD1638 described above was active on a curdlan proxy that was chemically modified to increase water-solubility [1]).

Kinetics and Mechanism

As a family within clan GH-A, GH158 members were inferred to be retaining enzymes. Retention of anomeric stereochemistry was experimentally confirmed by ¹H NMR on the product of hydrolysis of 2-chloro-4-nitrophenyl laminaribioside by BuGH158 from Bacteroides uniformis [2]. As such, these enzymes follow the classical Koshland double-displacement mechanism, which proceed via a covalent glycosyl-enzyme intermediate.

Catalytic Residues

The catalytic nucleophile and catalytic acid/base residues of BuGH158 were determined to be E220 and E137 [2]. This glutamate pair is located on loops immediately following beta-strands 7 (nucleophile) and 4 (acid/base), consistent with all other clan GH-A enzymes.

Three-dimensional structures

The X-ray crystal structure of BuGH158 from Bacteroides uniformis determined by multi-wavelength anomalous dispersion represents the founding structural representative of this family [2]. The 1.8 A structure reveal in addition to an N-terminal (a/b)8 TIM barrel domain, which is a hallmark of clan GH-A structures, a C-terminal eight-stranded immunoglobulin (Ig)-like domain that makes extensive contacts with the TIM barrel. A loop from the Ig-like domain extends over the TIM barrel to shape the active site cleft.

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

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References

1. Helbert W, Poulet L, Drouillard S, Mathieu S, Loiodice M, Couturier M, Lombard V, Terrapon N, Turchetto J, Vincentelli R, and Henrissat B. (2019). Discovery of novel carbohydrate-active enzymes through the rational exploration of the protein sequences space. Proc Natl Acad Sci U S A. 2019;116(13):6063-6068. DOI:10.1073/pnas.1815791116 |

3. 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.