Carbohydrate Esterase Family 15

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• Author: ^^^Jenny Arnling Bååth^^^ and ^^^Scott Mazurkewich^^^
• Responsible Curator: ^^^Johan Larsbrink^^^

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Carbohydrate Esterase Family CE15
Clan	GH-x
Mechanism	retaining/inverting
Active site residues	known/not known
CAZy DB link
https://www.cazy.org/CE15.html

Substrate specificity

All CE15 enzymes characterized to-date are glucuronoyl esterases, cleaving esters of D-glucuronic acid. The first reported glucuronoyl esterase was ScGE1 from the white-rot fungus Schizophyllum commune, and the activity was demonstrated by TLC on a methyl ester of 4-O-methyl-D-glucuronic acid [1]. While CE15 members are found in both fungal and bacterial species, several bacterial CE15 enzymes are more promiscuous than their fungal counterparts and are active also on esters of galacturonoate [2]. Feruloyl- and acetyl esterase activities have been reported for certain CE15 enzymes as side activities [3, 4]. The proposed physiological role of CE15 enzymes is to hydrolyze lignin-carbohydrate ester linkages between lignin and glucuronoxylan in plant cell walls, and a few studies have demonstrated their activity on lignocellulose-derived materials and plant biomass [4, 5, 6].

Three-dimensional structures

As of January 2019, five structures of CE15 enzymes have been determined: TrGE (Cip2) from T. reesei (Hypocrea jecorina) (PDB: 3pic) [7], StGE2 from Thermothelomyces thermophila (previously Sporotrichum thermophile (PDB: 4g4g, 4g4i and 4g4j) [8], MZ0003 (PDB: 6ehn; cloned from a marine metagenome) [9], OtCE15A (PDB: 6grw and 6gs0) and SuCE15C (PDB: 6gry and 6gu8) [2]. All structurally determined CE15 enzymes share an alpha/beta hydrolase fold, consisting of a three-layer alpha-beta-alpha sandwich with the active site in a solvent-exposed cleft. The structures of the bacterial enzymes determined thus far exhibit sizeable inserts which result in much deeper active site pockets compared to the shallow active sites seen in fungal glucuronoyl esterase structures [2, 9].

Catalytic Residues and Mechanism

All CE15 enzymes are serine-type hydrolases, containing a catalytic triad of Glu/Asp-His-Ser [2, 7, 8, 9]. The position of the acidic residue of the triad is not similarly positioned in all CE15 members as the residue can be found on different loops of the conserved fold [9]. A conserved arginine found in all of the CE15 structures, proximal to the catalytic triad, has been proposed to stabilize the formation of the oxyanion during catalysis [2].

Family Firsts

First 3-D structure

The first solved structure of a CE15 enzyme was the Cip2 catalytic domain from Trichoderma reesei (TrGE) [7].

First mechanistic insight

The crystal structure of StGE2 (from Sporotrichum thermophile) in complex with the ligand 4-O-methyl-beta-D-glucopyranuronate gave the first direct insight into substrate binding [8].

References

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2. Arnling Bååth J, Mazurkewich S, Knudsen RM, Poulsen JN, Olsson L, Lo Leggio L, and Larsbrink J. (2018). Biochemical and structural features of diverse bacterial glucuronoyl esterases facilitating recalcitrant biomass conversion. Biotechnol Biofuels. 2018;11:213. DOI:10.1186/s13068-018-1213-x |