Substrate binding triggers a switch in the iron coordination in dehaloperoxidase from Amphitrite Ornate

Abstract

We have explored the effect of substrate binding on the heme iron conformation in the enzyme dehaloperoxidase (DHP). DHP is a dimeric hemoglobin that also has significant peroxidase activity under physiological conditions and has been shown to oxidize trihalophenols to dihaloquinones in a dehalogenation reaction that uses hydrogen peroxide as a cosubstrate. Hyperfine sublevel correlation spectroscopic (HYSCORE) analysis of the ferric form of DHP was carried out to characterize effects of the substrate 2,4,6-trifluorophenol (TFP) binding on the iron coordination in order to elucidate molecular mechanisms responsible for switching the protein function from a globin to a peroxidase. The CW EPR spectrum shows that at pH 6.0 DHP heme iron exists in a highly axial high spin (HS) state that could be interpreted as arising from two different populations of the HS iron centers. Substrate binding does not change the spin state at pH 6.0, however, affects the magnetic parameters of the signal. HYSCORE spectra recorded at magnetic field corresponding to g = 2 revealed the presence of exchangeable protons with hyperfine coupling of ca. 6 MHz, consistent with a water molecule being the sixth ligand in the iron coordination. These protons' spectral features disappeared upon substrate binding. At pH 9.6 the EPR spectrum from heme iron of DHP shows the presence of both high- and low-spin states with the low spin signal characteristic of hydroxyl form. Upon TFP binding the low spin signal disappears. HYSCORE spectra at pH 9.6 also show the presence of exchangeable protons that disappear upon substrate binding. This observation highlights the proposed role of molecules in the distal pocket to control the peroxidase function of DHP.Supported by the NSF Grant MCB-0451510 to T.I.S. and ARO grant 52278-LS to S.F.