Heme-thiolate ferryl of aromatic peroxygenase is basic and reactive

Abstract

A kinetic and spectroscopic characterization of the ferryl intermediate (APO-II) from APO, the heme-thiolate peroxygenase from Agrocybe aegerita, is described. APO-II was generated by reaction of the ferric enzyme with metachloroperoxybenzoic acid in the presence of nitroxyl radicals and detected with the use of rapid-mixing stopped-flow UV-visible (UV-vis) spectroscopy. The nitroxyl radicals served as selective reductants of APO-I, reacting only slowly with APO-II. APO-II displayed a split Soret UV-vis spectrum (370 nm and 428 nm) characteristic of thiolate ligation. Rapid-mixing, pH-jump spectrophotometry revealed a basic pKa of 10.0 for the FeIV−O−H of APO-II, indicating that APO-II is protonated under typical turnover conditions. Kinetic characterization showed that APO-II is unusually reactive toward a panel of benzylic C−H and phenolic substrates, with second-order rate constants for C−H and O−H bond scission in the range of 10–107 M−1⋅s−1. Our results demonstrate the important role of the axial cysteine ligand in increasing the proton affinity of the ferryl oxygen of APO intermediates, thus providing additional driving force for C−H and O−H bond scission.