title: Displacement of the tyrosyl radical cofactor in ribonucleotide reductase obtained by single-crystal high-field EPR and 1.4-A x-ray data.
abstract: The R2 protein of class I ribonucleotide reductase generates and stores a tyrosyl radical essential for ribonucleotide reduction and, thus, DNA synthesis. X-ray structures of the protein have enabled detailed mechanistic suggestions, but no structural information has been available for the active radical-containing state of the protein. Here we report on methods to generate the functional tyrosyl radical in single crystals of R2 from Escherichia coli (Y122(*)). We further report on subsequent high-field EPR experiments on the radical-containing crystals. A full rotational pattern of the spectra was collected and the orientation of the g-tensor axes were determined, which directly reflect the orientation of the radical in the crystal frame. The EPR data are discussed in comparison with a 1.42-A x-ray structure of the met (oxidized) form of the protein, also presented in this paper. Comparison of the orientation of the radical Y122(*) obtained from high-field EPR with that of the reduced tyrosine Y122-OH reveals a significant rotation of the tyrosyl side chain, away from the diiron center, in the active radical state. Implications for the radical transfer connecting the diiron site in R2 with the substrate-binding site in R1 are discussed. In addition, the present study demonstrates that structural and functional information about active radical states can be obtained by combined x-ray and high-field EPR crystallography.
authors.affiliation: Department of Biochemistry and Biophysics, Stockholm University, Roslagstullsbacken 15, Albanova University Center, SE-10691 Stockholm, Sweden.
authors.forename: Martin
authors.initials: M
authors.lastname: Högbom
authors.forename: Marcus
authors.initials: M
authors.lastname: Galander
authors.forename: Martin
authors.initials: M
authors.lastname: Andersson
authors.forename: Matthias
authors.initials: M
authors.lastname: Kolberg
authors.forename: Wulf
authors.initials: W
authors.lastname: Hofbauer
authors.forename: Günter
authors.initials: G
authors.lastname: Lassmann
authors.forename: Pär
authors.initials: P
authors.lastname: Nordlund
authors.forename: Friedhelm
authors.initials: F
authors.lastname: Lendzian
mesh_terms: D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D004926:Escherichia coli / Q000201:enzymology; D005609:Free Radicals / Q000737:chemistry; D008958:Models, Molecular; D015394:Molecular Structure; D011487:Protein Conformation; D012264:Ribonucleotide Reductases / Q000737:chemistry*; D055672:Static Electricity; D014443:Tyrosine / Q000737:chemistry
chemical_list: D005609:Free Radicals; D014443:Tyrosine; D012264:Ribonucleotide Reductases; C108427:ribonucleotide reductase R2 subunit