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Carbohydrate Esterase Family 2

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Carbohydrate Esterase Family CE2
Clan
Mechanism
Active site residues
CAZy DB link
http://www.cazy.org/CE3.html

Substrate specificities

All of the well characterized carbohydrate esterase family 2 enzymes have been shown to remove acetate groups from the synthetic molecule, 4-nitrophenyl acetate (4p-NP-AcC) (Montanier et al. 2009; Till et al. 2013). Contrary to typical acetyl xylan esterases, CE2 family members are shown to have a strong preference for the deacetylation of xylopyranosides at positions 3 and 4 instead of the typical deacetylation at position 2. CE2 family members were also shown to have significant preference for deacetylation of glucopyranosyl and mannopyranosyl residues at the 6-O position and significantly preferred deacetylation of glucopyranosyl and mannopyranosyl residues relative to the deacetylation of xylopyranosides. For these reasons CE2 family members are considered to be 6-de-O-acetylases (Topakas et al. 2010).

Catalytic Residues

Most CE2 family members contain a catalytic dyad (Ser-His) as opposed to a catalytic triad that is typically found in esterases. CjCE2A is an exception and contains a Ser-His-Asp catalytic triad (Montanier et al. 2009). The structurally characterized, CtCE2, CjCE2B, and Est2A are examples of CE2 enzymes that contain catalytic dyads of conserved serine and histidine residues and lack the aspartate residue found in the triad. Without the support of an aspartate residue, the histidine residue of the catalytic dyads are supported by main-chain carbonyl groups provided by a backbone amino acid. The catalytic aspartate residue that would commonly complete the catalytic triad simply does not exist in many CE2 members. When CE2 enzymes contain a potential catalytic aspartate residue, often enough, there also exists a tryptophan residue that sits between the catalytic histidine and aspartate residues thereby prevent the aspartate residue from completing the triad. CE2 enzymes also possess an aromatic residue (either a tyrosine or a tryptophan residue) above their binding clefts that promote greater substrate specificity (Montanier et al. 2009; Till et al. 2013). Lastly, the oxyanion hole is comprised of the catalytic serine residue, a glycine, and an asparagine residue that appears to be invariant across the CE2 family and that of other related acetyl-esterases (Montanier et al. 2009; Till et al. 2013).

Kinetics and Mechanism

Three-dimensional structures

Family Firsts

First characterized
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First mechanistic insight
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First 3-D structure
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References

  1. Faik A (2010). Xylan biosynthesis: news from the grass. Plant Physiol. 2010;153(2):396-402. DOI:10.1104/pp.110.154237 | PubMed ID:20375115 [Faik2010]