Pre-steady-state Kinetics Reveal the Substrate Specificity and Mechanism of Halide Oxidation of Truncated Human Peroxidasin 1

Human peroxidasin 1 is a homotrimeric multidomain peroxidase that is secreted to the extracellular matrix. The heme enzyme was shown to release hypobromous acid which mediates the formation of specific covalent sulfilimine bonds to reinforce collagen IV in basement membranes. Maturation by proteolytic cleavage is known to activate the enzyme. Here we present the first multi-mixing stopped-flow study on a fully functional truncated variant of human peroxidasin 1 comprising four immune-globulin-like domains and the catalytically active peroxidase domain. The kinetic data unravel the so far unknown substrate specificity and mechanism of halide oxidation of human peroxidasin 1. The heme enzyme is shown to follow the halogenation cycle which is induced by the rapid H2O2-mediated oxidation of the ferric enzyme to the redox intermediate Compound I. We demonstrate that chloride cannot act as two-electron donor of Compound I, whereas thiocyanate, iodide and bromide efficiently restore the ferric resting state. We present all relevant apparent bimolecular rate constants, the spectral signatures of the redox intermediates and the standard reduction potential of the Fe(III)/Fe(II) couple and we demonstrate that the prosthetic heme group is posttranslationally modified and cross-linked with the protein. These structural features provide the basis of human peroxidasin 1 to act as an effective generator of hypobromous acid which mediates the formation of covalent crosslinks in collagen IV.