Open AccessMechanism of nitrite‐stimulated catalysis by lactoperoxidase
Author(s) -
Brück T. B.,
Fielding R. J.,
Symons M. C. R.,
Harvey P. J.
Publication year - 2001
Publication title -
european journal of biochemistry
Language(s) - English
Resource type - Journals
eISSN - 1432-1033
pISSN - 0014-2956
DOI - 10.1046/j.1432-1327.2001.02213.x
Subject(s) - chemistry , reaction rate constant , nitrite , catalysis , reaction mechanism , ferric , inorganic chemistry , reaction intermediate , lactoperoxidase , medicinal chemistry , kinetics , organic chemistry , peroxidase , nitrate , enzyme , quantum mechanics , physics
The reactions of lactoperoxidase (LPO) intermediates compound I, compound II and compound III, with nitrite ( ) were investigated. Reduction of compound I by was rapid ( k 2 = 2.3 × 10 7 m −1 ·s −1 ; pH = 7.2) and compound II was not an intermediate, indicating that radicals are not produced when reacts with compound I. The second‐order rate constant for the reaction of compound II with at pH = 7.2 was 3.5 × 10 5 m −1 ·s −1 . The reaction of compound III with exhibited saturation behaviour when the observed pseudo first‐order rate constants were plotted against concentrations and could be quantitatively explained by the formation of a 1 : 1 ratio compound III/NO 2 − complex. The K m of compound III for was 1.7 × 10 −4 m and the first‐order decay constant of the compound III/ complex was 12.5 ± 0.6 s −1 . The second‐order rate constant for the reaction of the complex with was 3.3 × 10 3 m −1 ·s −1 . Rate enhancement by does not require as a redox intermediate. accelerates the overall rate of catalysis by reducing compound II to the ferric state. With increasing levels of H 2 O 2 , there is an increased tendency for the catalytically dead‐end intermediate compound III to form. Under these conditions, the ‘rescue’ reaction of with compound III to form compound II will maintain the peroxidatic cycle of the enzyme.