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Báo cáo khoa học: Unraveling the catalytic mechanism of lactoperoxidase and myeloperoxidase A reflection on some controversial features Elena Ghibaudi and Enzo Laurenti

Although belonging to the widely investigated peroxidase superfamily, lactoperoxidase (LPO) and myeloperoxidase (MPO) share structural and functional features that make them peculiar with respect to other enzymes of the same group. A survey of the available literature on their catalytic intermediates enabled us to ask some questions that remained unanswered. These questions concern controver-sial features of theLPOandMPOcatalytic cycle, suchas the existence of Compound I and Compound II isomers and the identification of their spectroscopic properties | Eur. J. Biochem. 270 4403-4412 2003 FEBS 2003 doi 10.1046 j.1432-1033.2003.03849.x REVIEW ARTICLE Unraveling the catalytic mechanism of lactoperoxidase and myeloperoxidase A reflection on some controversial features Elena Ghibaudi and Enzo Laurenti Dipartimento di Chimica I.F.M. Universitci di Torino Italy Although belonging to the widely investigated peroxidase superfamily lactoperoxidase LPO and myeloperoxidase MPO share structural and functional features that make them peculiar with respect to other enzymes of the same group. A survey of the available literature on their catalytic intermediates enabled us to ask some questions that remained unanswered. These questions concern controversial features of the LPO and MPO catalytic cycle such as the existence of Compound I and Compound II isomers and the identification of their spectroscopic properties. After addressing each of these questions we formulated a hypo thesis that describes an integrated vision of the catalytic mechanism of both enzymes. The main points are a a re-evaluation of the role of superoxide as a reductant in the catalytic cycle b the existence of Cpd I isomers c reciprocal interactions between catalytic intermediates and d a mechanistic explanation for catalase activity in both enzymes. Keywords lactoperoxidase myeloperoxidase aminoacid radical Compound I Compound II Compound III catalytic intermediates. Introduction The catalytic cycle of peroxidases including lactoperoxidase LPO and myeloperoxidase MPO is described usually as a sequence of three consecutive reactions according to Scheme 1. Compound I Cpd I which arises from the reaction of the native enzyme with hydrogen peroxide H2O2 is two oxidizing equivalents above the resting state. It reacts with a substrate molecule and is converted into a secondary compound that has lost one equivalent generally indicated as Compound II Cpd II . A second substrate molecule recycles Cpd II into the resting enzyme. A large excess of H2O2 converts Cpd I .