Difference between revisions of "Carbohydrate Esterase Family 1"

Revision as of 12:09, 23 March 2020

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Author: ^^^Casper Wilkens^^^
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Carbohydrate Esterase Family 1
Clan	GH-x
Mechanism	retaining/inverting
Active site residues	known/not known
CAZy DB link
https://www.cazy.org/CE1.html

Substrate specificities

Carbohydrate esterase family 1 (CE1) is one of the biggest and most diverse CE families including acetylxylan esterases (EC 3.1.1.72), feruloyl esterases (EC 3.1.1.73), cinnamoyl esterases (EC 3.1.1.-), carboxylesterases (EC 3.1.1.1), S-formylglutathione hydrolases (EC 3.1.2.12), diacylglycerol O-acyltransferases (EC 2.3.1.20), and thehalose 6-O-mycolyltransferases (EC 2.3.1.122) and others [1].

Kinetics and Mechanism

CE1 enzymes target a large variety of substrates, however, all appear to utilize the canonical serine hydrolase mechanism, involving a catalytic triad comprising a nucleophilic serine, a histidine, and an acidic amino acid [2, 3]. Both aspartic and glutamic acid are commonly observed in the position [4]. After substrate binding, the serine is activated by the proton relay consisting of the histidine and the acid residue, which facilitates nucleophilic attack of the carbonyl carbon atom of the substrate. This results in the formation of a covalent acyl-enzyme intermediate via a tetrahedral transition state sometimes known as the "tetrahedral intermediate," which is stabilized through interactions with two main chain NH groups in the "oxyanion hole." Following release of the corresponding alcohol as the first product, the acyl-enzyme intermediate is hydrolyzed by the near-microscopic reverse of the first step, with water, activated by the proton relay, acting as the nucleophile [2, 3].

Catalytic Residues

The serine general base is located at the center of a universally conserved pentapeptide with the consensus sequence G-X-S-X-G. This pentapeptide segment constitutes the so-called “nucleophilic elbow”, which has become the fingerprint feature commonly used to identify proteins of this enzyme family based on their primary structure alone [2]. The histidine acting as general acid-base catalyst is also conserved [2, 3], while the general acid commonly is observed as both a aspartic or glutamic acid [4].

Three-dimensional structures

CE1's are members of the α/β-hydrolase superfamily [5], which are comprised of a central β-sheet with 8 or 9 strands connected by α-helices [6]. A number of CE1 enzymes have a CBM48 appended, which proved to be essential for these feruloyl esterases acticity on polymeric xylan [4].

Family Firsts

First characterized: Content is to be added here.
First mechanistic insight: The crystal structure of Mycobacterium tuberculosis H37Rv mycolyltransferase in complex with the covalently bound inhibitor, diethyl phosphate gave the first insight into the mechanism, which involved the highly conserved catalytic Ser-Glu-His triad [5].
First 3-D structure: Mycobacterium tuberculosis H37Rv mycolyltransferase crystal structure in 2000 [5].

References

All Medline abstracts: PubMed

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 == Kinetics and Mechanism ==
-CE1 enzymes target a large variety of substrates, however, the general mechanism of hydrolysis, involving the serine [[general base]], a histidine acting as general acid-base catalyst, and a [[general acid]], appears to be conserved <cite>Schubot2001 Prates2001</cite>. The [[general acid]] is structurally conserved, but both aspartic and glutamic acid are commonly observed in the position <cite>Holck2019</cite>  After substrate binding, the serine is activated by the histidine, which allows the nucleophilic attack of the substrate’s carbonyl carbon atom leading to the formation of a covalent acyl-enzyme intermediate via tetrahedral transition states sometimes known as the “tetrahedral intermediates.” Simultaneously, a proton is transferred from the serine to the histidine. The resulting tetrahydral intermediate, negatively charged carbonyl oxygen atom (“oxyanion”) is stabilized through interactions with two main chain NH groups in the “oxyanion hole”, while the positively charged histidine is stabilized by a hydrogen bond to the catalytic acid. In the next step, the formed alcohol is released from the substrate and the acid part forms an ester bond with the serine oxygen. This bond, in turn, is hydrolyzed in a two- step mechanism involving a water molecule, and the enzyme is returned to the starting point <cite>Schubot2001 Prates2001</cite>.
+CE1 enzymes target a large variety of substrates, however, all appear to utilize the canonical serine hydrolase mechanism, involving a catalytic triad comprising a nucleophilic serine, a histidine, and an acidic amino acid  <cite>Schubot2001 Prates2001</cite>. Both aspartic and glutamic acid are commonly observed in the position <cite>Holck2019</cite>.  After substrate binding, the serine is activated by the proton relay consisting of the histidine and the acid residue, which facilitates nucleophilic attack of the carbonyl carbon atom of the substrate.  This results in the formation of a covalent acyl-enzyme intermediate via a tetrahedral transition state sometimes known as the "tetrahedral intermediate," which is stabilized through interactions with two main chain NH groups in the "oxyanion hole." Following release of the corresponding alcohol as the first product, the acyl-enzyme intermediate is hydrolyzed by the near-microscopic reverse of the first step, with water, activated by the proton relay, acting as the nucleophile <cite>Schubot2001 Prates2001</cite>.
 == Catalytic Residues ==