Mössbauer studies of the ferryl, ferrous and ferric states of dehaloperoxidase from A. ornata

Abstract

Dehaloperoxidase (DHP) is a multi-functional catalytic globin from the marine worm A. ornata, whose physiological functions include oxygen transport and oxidation of toxic substrates present in its habitat. In the Fe(III) state, DHPA has an isomer shift of 0.42 mm/s, characteristic for high-spin heme proteins. Changes in pH have subtle effects on the electronic structure of DHP in the Fe(III) state detectable in the high-field spectra, which show a pH-dependent mixture of species with different zero-field splittings between 5 and 18 cm−1. The short-lived intermediate obtained by direct reaction of the Fe(III) enzyme with H2O2 has an isomer shift of 0.10 mm/s, indicative of an Fe(IV)-oxo state and of an S = 1 electronic ground state confirmed by variable field studies. The O2-bound state of DHP has an isomer shift of 0.28 mm/s and a high-field spectrum characteristic for diamagnetic heme complexes, similarly to other haemoglobins.Overall, the isomer shift and quadrupole splitting of DHP in the four states studied are expectedly similar to both peroxidases and to myoglobin. The differences in electronic structure between DHP and other heme proteins and enzyme are observed in the high-field Mössbauer spectra of the ferric state, which show pH-dependent zero-field splittings suggesting a heme site in which the ligand field strength at the iron ion is tuned by pH. This tunability is correlated with variable electron-donating properties of the iron, which can perform multiple functions.