title: Identification of a hydroxide ligand at the iron center of ribonucleotide reductase by resonance Raman spectroscopy.
abstract: The resonance Raman spectrum of protein B2 of ribonucleotide reductase from Escherichia coli shows several features to its oxo-bridged binuclear iron center. A peak at 492 cm-1 is assigned to the symmetric stretch of the Fe-O-Fe moiety on the basis of its 13-cm-1 shift to lower energy upon 18O substitution. The 18O species shows an additional peak at 731 cm-1, which is a good candidate for the asymmetric stretch of the Fe-O-Fe moiety. Its exact location in the 16O species is obscured by the presence of a protein tryptophan vibration at 758 cm-1. A third resonance-enhanced peak at 598 cm-1 is identified as an Fe-OH vibration on the basis of its 24-cm-1 shift to lower energy in H2 18O, its 2-cm-1 shift to lower energy in D2O, and its pH-dependent intensity. A hydrogen-bonded mu-oxo bridge similar to that in hemerythrin is suggested by the unusually low frequency for the Fe-O-Fe symmetric stretch and the 3-cm-1 shift to higher energy of vs(Fe-O-Fe) in D2O. From the oxygen isotope dependence of vs(Fe-O-Fe), an Fe-O-Fe angle of 138 degrees can be calculated. This small angle suggests that the iron center consists of a tribridged core as in hemerythrin. A model for the binuclear iron center of ribonucleotide reductase is presented in which the hydroxide ligand sites provide an explanation for the half-of-sites reactivity of the enzyme.
authors.affiliation: Department of Molecular Biology, Swedish University of Agricultural Sciences, Uppsala Biomedical Center.
authors.forename: B M
authors.initials: BM
authors.lastname: Sjöberg
authors.forename: J
authors.initials: J
authors.lastname: Sanders-Loehr
authors.forename: T M
authors.initials: TM
authors.lastname: Loehr
mesh_terms: D004926:Escherichia coli / Q000201:enzymology*; D006878:Hydroxides; D007501:Iron; D008024:Ligands; D008958:Models, Molecular; D011487:Protein Conformation; D012264:Ribonucleotide Reductases / Q000378:metabolism*; D013059:Spectrum Analysis, Raman
chemical_list: D006878:Hydroxides; D008024:Ligands; D007501:Iron; D012264:Ribonucleotide Reductases