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Catalytic Site Atlas

CSA LITERATURE entry for 7atj

E.C. nameperoxidase
SpeciesArmoracia rusticana (Horseradish)
E.C. Number (IntEnz) 1.11.1.7
CSA Homologues of 7atjThere are 218 Homologs
CSA Entries With UniProtID P00433
CSA Entries With EC Number 1.11.1.7
PDBe Entry 7atj
PDBSum Entry 7atj
MACiE Entry M0239

Literature Report

IntroductionHorseradish peroxidase C (HRPC) is the most studied member of the class III peroxidases. These enzymes occur in higher plants and catalyse the oxidation of phenolic compounds to phenol radicals using H2O2 which is reduced to H2O. Since plant peroxidases generally have broad substrate specificity and produce highly reactive radical products which can participate in non-enzymatic reactions, the biological functions of these enzymes have been difficult to verify. They have been proposed to have roles in lignin production and cell wall formation, for example by catalysing the formation of monolignol radicals which subsequently polymerise to lignin, and by catalysing cell wall cross-linking reactions involving ferulic acid.
MechansimThe reaction occurs in three steps: the enzyme is first oxidised by H2O2 and is then reduced in two sequential one-electron transfer steps from reducing substrates (typically small molecule phenol derivatives). The mechanism of the first step is believed to be common to all haem peroxidases. HOOH is deprotonated by His 42 to produce singly ionised Fe(III)-coordinated peroxide. The O-O bond of the coordinated peroxide is now heterolytically cleaved to generate an Fe(IV)=O species and a hydroxide ion, which is protonated by His 42. Arg 38 is though to promote the reaction by stabilising negative charge in the transient Fe(III)-OOH intermediate and associated transition states.
In the second step, a reducing phenol substrate binds to the active site and transfers an electron to haem and a proton to His 42. The proton transfer is thought to occur via an active site water molecule, and is proposed to be assisted by Arg 38 which forms a hydrogen bond to the substrate phenolic oxygen. The product radical now dissociates from the active site and a second reducing phenol substrate is bound. This second reducing substrate transfers an electron to the haem, and a proton (via an active site water) to the Fe-coordinated O atom. This is accompanied by proton transfer from His 42 to the Fe-coordinated oxygen, so that the overall result is formation of a water molecule and regeneration of the resting state haem Fe(III).
Reaction

Catalytic Sites for 7atj

Annotated By Reference To The Literature - Site 1 (Perform Site Search)
ResidueChainNumberUniProtKB NumberFunctional PartFunctionTargetDescription
ArgA3868macie:sideChainStabilises negative charge in the transient Fe(III)-OOH intermediate and associated transition states. Assists proton transfer from the phenol substrates to either His 42 or the ferryl oxygen.
HisA4272macie:sideChainDeprotonates hydrogen peroxide to give singly ionised Fe(III)-coordinated peroxide. Protonates the departing hydroxide ion during heterolytic cleavage of the O-O bond in the coordinated peroxide. Accepts a proton from the phenol substrate (via an active site water molecule). Protonates the Fe-coordinated oxygen in the final step of the reaction.
AsnA70100macie:sideChainModifies the pKa of His 42.

Literature References

Notes:
Gajhede M.
Plant peroxidases: substrate complexes with mechanistic implications.
Biochem Soc Trans 2001 29 91-98
PubMed: 11356134
Meno K
Structural analysis of the two horseradish peroxidase catalytic residue variants H42E and R38S/H42E: implications for the catalytic cycle.
Acta Crystallogr D Biol Crystallogr 2002 58 1803-1812
PubMed: 12351824
Henriksen A
The structures of the horseradish peroxidase C-ferulic acid complex and the ternary complex with cyanide suggest how peroxidases oxidize small phenolic substrates.
J Biol Chem 1999 274 35005-35011
PubMed: 10574977
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