@article{776f8065e4474ac8834060b26ce92e2f,
title = "Probing axial ligands in ferric haemoproteins: An ESR study of myoglobin and horseradish peroxidase in H2 17O",
abstract = "Substitution of H2 16O by H2 17O induces a substantial broadening of the high-field line in the electron-spin resonance spectrum of ferric myoglobin due to the presence of H2 17O at the axial ligand-site. Computer simulations of the experimental spectra yielded the values of the reciprocal relaxation time T2 -1 = 7.8 G and the 17O-hyperfine coupling constant A = 18 ± 1 G. Under identical experimental conditions no effect of H2 17O was observed in horseradish peroxidase. The latter finding excludes the possibility that a water molecule is liganded to the peroxidase haem-iron and supports either the idea that both axial ligands are amino acid residues or that the haem in ferric horseradish peroxidase is pentacoordinate.",
author = "S. Vuk-Pavlovi{\'c} and Y. Siderer",
note = "Funding Information: DISCUSSION Judging from the profound effect in the parallel region of the spectrum in acquo complex of ferric myoglobin, isotopic substitution appears to be a sensitive method for the investigation of identity and/or reactions of oxygen-containing ligands in ferric haemoproteins. The lack of any similar effect in horseradish peroxidase can be interpreted in two different ways. The first possibility is the “closed-crevice” structure (2) where sixth-ligand position would also be occupied by an amino acid residue. A tyrosine has been considered as a likely candidate (3,11), but in the recently distal sequences of five plant peroxidases there is no tyrosine (1). case of “closed crevice” structure our data would be consistent also with a carboxylate involved in the distal contact with the iron, a possibility discussed by Ricard et al (3). The second possibility, raised recently basis of magnetic circular dichroism data (S), is the pentacoordinated However, the present experiments cannot discriminate between the two ities. Obviously, more data are needed in order to reach a definite about the actual sixthligand of the iron, but a water molecule as the ligand can be ruled out in favor of either of described alternatives. Electron-spin resonance experiments using “0 as a probe in the study of metalloproteins, similar to the present one, have been introduced only recently (12,13). Using higher enrichments of {"}0 and even lower temperatures than in the present study, the method may be sensitive enough to monitor changes of the spin density at “O-nucleus. Such changes may result from direct or protein-mediated effects on the haem due, for instance, to the altered properties of the medium, the presence of allosteric effecters and/or substrates and inhibitors. We thank Dr. R. Poupko for his generous and friendly help in all stages of the work, especially in the computer simulations of the spectra, Dr. S. Wherland for reading and Mrs. E. Majerowich for typing the manuscript. S.V.-P. gratefully acknowledges a long-term fellowship by European Molecular Biology organization. Copyright: Copyright 2018 Elsevier B.V., All rights reserved.",
year = "1977",
month = dec,
day = "7",
doi = "10.1016/0006-291X(77)91193-7",
language = "English (US)",
volume = "79",
pages = "885--889",
journal = "Biochemical and Biophysical Research Communications",
issn = "0006-291X",
publisher = "Academic Press Inc.",
number = "3",
}