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Search The CSA
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Catalytic Site Atlas

CSA LITERATURE entry for 1cwy

E.C. name4-alpha-glucanotransferase
SpeciesThermus aquaticus (Bacteria)
E.C. Number (IntEnz) 2.4.1.25
CSA Homologues of 1cwy1esw,1fp8,1fp9,1tz7,1x1n,2owc,2oww,2owx,
CSA Entries With UniProtID O87172
CSA Entries With EC Number 2.4.1.25
PDBe Entry 1cwy
PDBSum Entry 1cwy
MACiE Entry 1cwy

Literature Report

IntroductionAmylomaltase enzymes are structurally and mechanistically related to alpha amylases, although they almost exclusively catalyse trans-glycosylation reactions whereas alpha amylase enzymes catalyse hydrolysis reactions. The amylomaltase enzyme catalyses amylose disproportionation and the synthesis of large cyclic glucans, making these enzymes of interest to chemical and pharmaceutical industries.
MechansimThree conserved carboxylic acid residues are central to the catalytic mechanism. The first acid residue acts as a general acid towards the oxygen of the scissile glycosidic bond. Simultaneously, the oxygen of a second carboxylic residue acts as a nucleophile towards the C1 of the substrate. This forms a covalent glycosyl-enzyme intermediate though a planar oxocarbenium intermediate. This covalent intermediate can be broken down by either a water molecule, resulting in hydrolysis or by a hydroxyl group of another sugar molecule. The third catalytic carboxylic residue binds the sugar in the -1 subsite, distorting it towards a partially planar conformation and contributing to the transition state stabilisation through hydrogen bonding.
Reaction

Catalytic Sites for 1cwy

Annotated By Reference To The Literature - Site 1 (Perform Site Search)
ResidueChainNumberUniProtKB NumberFunctional PartFunctionTargetDescription
AspA293293macie:sideChainThe residue acts as a nucleophile to the C1 of the scissile glycosidic bond in a SN2 displacement mechanism, resulting in the formation of covalent glycosyl-enzyme intermediate. The residue is released from the intermediate by SN2 displacement at the C1 from an activated second sugar moiety.
GluA340340macie:sideChainThe residue acts as a general acid to the oxygen of the glycosidic bond, enhancing nucleophilic attack at the substrate's C1 position. It then acts as a general base towards the attacking sugar molecule in the collapse of the glycosyl-enzyme intermediate and formation of a new glycosidic link. In hydrolysis, this residue activates a water molecule towards hydrolysis at the glycosidic bond.
It is interesting to note that the orientation of Glu340 is optimised as to reduce water activation, protecting the intermediate from hydrolysis. This fits with the experimental observation that the Thermus enzyme is 5000 times less likely to utilise water in hydrolysis than maltotriose in glucan formation.
AspA395395macie:sideChainThe residue binds to the sugar substrate in the -1 subsite by hydrogen boding to the 2- and 3-OH groups, distorting its structure towards a planar conformation and stabilising the reaction transition state.

Literature References

Notes:
Barends TR
Three-way stabilization of the covalent intermediate in amylomaltase, an alpha-amylase-like transglycosylase.
J Biol Chem 2007 282 17242-17249
PubMed: 17420245
Przylas I
Crystal structure of amylomaltase from thermus aquaticus, a glycosyltransferase catalysing the production of large cyclic glucans.
J Mol Biol 2000 296 873-886
PubMed: 10677288
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