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Catalytic Site Atlas

CSA LITERATURE entry for 1a8s

E.C. namechloride peroxidase
SpeciesPseudomonas fluorescens (Bacteria)
E.C. Number (IntEnz)
CSA Homologues of 1a8s
CSA Entries With UniProtID O31158
CSA Entries With EC Number
PDBe Entry 1a8s
PDBSum Entry 1a8s
MACiE Entry 1a8s

Literature Report

IntroductionHaloperoxidases catalyze the halogenation of organic compounds in the presence of halide ions and peroxides such as H2O2. They are classifed according to their cofactor dependence as heme-type, vanadium dependent, metal-free and the more recent NADH/FAD dependent haloperoxidases. Co-factor free haloperoxidases possess a catalytic triad and require an organic acid for activity.
MechansimThe substrate is initially bound with one oxygen bound in the oxyanion hole. The hydrogen atom at Ser94 O is transferred to His253 N. A tetrahedral intermediate then is formed by nucleophilic attack of the Ser94 O on the carboxyl carbon of the organic acid. The tetrahedral intermediate is stabilised in the oxyanion hole and forms a covalent acyl-enzyme complex by elimination of a water molecule. the acyl-enzyme is subsequently hydrolyzed by a hydrogen peroxide molecule, forming the peroxoacid.
The peroxoacid is protected from hydrolysis by the hydrophobic environment of the active site as chloride, bromide or iodide enter the active site via a tunnel. The halogen then attacks the distil oxygen of the peroxoacid reforming the organic acid and hypohalous acid.
Hypohalous acid may leave the active site and react with a range of substrates in solution. There is some evidence to support binding of some small hydrophobic substrates to the active site to aid specific halogenation.

Catalytic Sites for 1a8s

Annotated By Reference To The Literature - Site 3 (Perform Site Search)
ResidueChainNumberUniProtKB NumberFunctional PartFunctionTargetDescription
TrpA2829macie:mainChainAmideForms the oxyanion hole which stabilises the tetrahedral transition state.
AspA224225macie:sideChainElectrostatic stabilisation of His253 allowing it to accept a proton from Ser94.
ThrA9596macie:mainChainAmideForms the oxyanion hole which stabilises the tetrahedral transition state.
SerA9495macie:sideChainAfter abstraction of a proton by His253 the oxyanion attacks the substrate carbonyl group. Productively this leads to the formation of the acyl-enzyme intermediate. Attack of the intermediate by H2O2 reforms the oxyanion which accepts a proton from the intermediate to form the product.
HisA253254macie:sideChainAccepts a proton from Ser94 increasing its nucleophilicity. Later donates this proton to the transition state as it collapses, allowing the elimination of water.

Literature References

Notes:The reaction catalyzed by the bacterial non-heme, metal-ion- and cofactor- independent haloperoxidases is not a true peroxidase reaction, but the perhydrolysis of an ester formed between the serine residue of the catalytic triad and the organic acid. In the presence of hydrogen peroxide this ester is perhydrolyzed leading to the formation of the organic peracid which can then oxidize halide ions resulting in the formation of hypohalous acid which acts as the halogenating agent.