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Carbohydrate Esterase Family 20

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Carbohydrate Esterase Family CE20
Fold β-sandwich/(α-β-α) sandwich/β-sandwich
Mechanism serine hydrolase
Active site residues known
CAZy DB link

Substrate specificities

Carbohydrate esterase family 20 (CE20) currently comprises xyloglucan acetylesterases (E.C. [1] and acetylesterases putatively related to arabinoxylan deacetylation [2].

Catalytic residues

Figure 1: The tridimensional structure and architecture of XacXaeA, the founding member of CE20 family. PDB ID 7KMM (a) Domain organization showing the position of catalytic residues (red triangles). (b) Structural topology, delimited by domains and (c) crystal structure, color-coded according to (a), showed in cartoon and surface representations. Catalytic residues are circled, in brown sticks. X448 domain is absent in the tridimensional model due to proteolysis removal in the crystallization steps, represented in (c) as pink surface. Domains in (b) and (c) are annotated according to (a). Adapted from [1] originally published under Creative Commons Attribution 4.0 International License.

XacXaeA, the founding member of family CE20 [1], harbors the canonical catalytic triad Ser-Asp-His and the conserved electropositive oxyanion hole. Thus, it is supposed to have the same mechanism of action described for other carbohydrate esterases, such as the members of family CE3 and CE6. However, this assumption still requires experimental validation. The catalytic-relevant residues are present and well conserved in Xanthomonas proteins, denoted as Block I (GQSNME) and Block V (DIH) (Infered catalytic residues in bold). Oxyanion hole residues are present in Blocks I (GQSNME) and III (YQGET). Although consensus Block II is present and participating in the oxyanion hole through Glycine backbone, Block IV is absent [1], as is expected for other related carbohydrate esterase (CE) families CE2, CE3, CE6, CE12 and CE17, that harbour the SGNH-hydrolase fold.

Kinetics and Mechanism

XacXaeA is specific to O-acetylation since it is not capable of cleaving N-acetylated carbohydrates. It shows activity on a broad range of O-acetylated mono- and disaccharides and did not show a positional preference for acetylated oxygens. XacXaeA was active towards cell wall extracted xyloglucan oligosaccharides, deacetylating distinct types of structures such as XXLG/XLXG, XXFG, and XLFG. XacXaeA showed lower activity towards metanoate compared to the prefered substrate acetate, but catalysis was not observed for longer chains [1]. Kinetic data for the second characterized member of family CE20, XuaJ, is available for the substrate 1-Naphthyl acetate [2].

Three-dimensional structures

The CE20 structure is composed of a central catalytic core, which displays the SGNH hydrolase fold, flanked by two antiparallel seven-stranded β-sandwiches intimately linked to the central core, forming a monolithic structure [1]. Such structural architecture diverges from CE families described in the CAZy database so far. In XacXaeA, the founding member of family CE20 [1], the catalytic core is composed of two halves (residues 104-216 and 397-541) due to the insertion of a domain (residues 217-396, named X448 in the CAZy database [3, 4]) in the α5-η3 loop.

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 biochemical characterization
Xanthomonas citri subsp. citri 306 xyloglucan acetylesterase in 2021 [1].
First 3-D structure
Xanthomonas citri subsp. citri 306 xyloglucan acetylesterase crystal structure in 2021 PDB ID 7KMM [1].


  1. Vieira PS, Bonfim IM, Araujo EA, Melo RR, Lima AR, Fessel MR, Paixão DAA, Persinoti GF, Rocco SA, Lima TB, Pirolla RAS, Morais MAB, Correa JBL, Zanphorlin LM, Diogo JA, Lima EA, Grandis A, Buckeridge MS, Gozzo FC, Benedetti CE, Polikarpov I, Giuseppe PO, and Murakami MT. (2021). Xyloglucan processing machinery in Xanthomonas pathogens and its role in the transcriptional activation of virulence factors. Nat Commun. 2021;12(1):4049. DOI:10.1038/s41467-021-24277-4 | PubMed ID:34193873 [Vieira2021]
  2. Liu N, Gagnot S, Denis Y, Byrne D, Faulds C, Fierobe HP, and Perret S. (2022). Selfish uptake versus extracellular arabinoxylan degradation in the primary degrader Ruminiclostridium cellulolyticum, a new string to its bow. Biotechnol Biofuels Bioprod. 2022;15(1):127. DOI:10.1186/s13068-022-02225-8 | PubMed ID:36403068 [Liu2022]
  3. 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 | PubMed ID:18838391 [Cantarel2009]
  4. 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.


All Medline abstracts: PubMed