O2 activation by non-heme diiron proteins: identification of a symmetric mu-1,2-peroxide in a mutant of ribonucleotide reductase.
Non-heme diiron clusters occur in a number of enzymes (e.g., ribonucleotide reductase, methane monooxygenase, and Delta9-stearoyl-ACP desaturase) that activate O2 for chemically difficult oxidation reactions. In each case, a kinetically labile peroxo intermediate is believed to form when O2 reacts with the diferrous enzyme, followed by O-O bond cleavage and the formation of high-valent iron intermediates [formally Fe(IV)] that are thought to be the reactive oxidants. Greater kinetic stability of a peroxodiiron(III) intermediate in protein R2 of ribonucleotide reductase was achieved by the iron-ligand mutation Asp84 --> Glu and the surface mutation Trp48 --> Phe. Here, we present the first definitive evidence for a bridging, symmetrical peroxo adduct from vibrational spectroscopic studies of the freeze-trapped intermediate of this mutant R2. Isotope-sensitive bands are observed at 870, 499, and 458 cm-1 that are assigned to the intraligand peroxo stretching frequency and the asymmetric and symmetric Fe-O2-Fe stretching frequencies, respectively. Similar results have been obtained in the resonance Raman spectroscopic study of a peroxodiferric species of Delta9-stearoyl-ACP desaturase [Broadwater, J. A., Ai, J., Loehr, T. M., Sanders-Loehr, J., and Fox, B. G. (1998) Biochemistry 37, 14664-14671]. Similarities among these adducts and transient species detected during O2 activation by methane monooxygenase hydroxylase, ferritin, and wild-type protein R2 suggest the symmetrical peroxo adduct as a common intermediate in the diverse oxidation reactions mediated by members of this class.
Moënne-Loccoz
P
P
Department of Biochemistry and Molecular Biology, Oregon Graduate Institute of Science and Technology, Portland, Oregon 97291-1000.
Baldwin
J
J
Ley
B A
BA
Loehr
T M
TM
Bollinger
J M
JM
D007501:Iron / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D010103:Oxygen Isotopes; D010545:Peroxides / Q000378:metabolism*; D010649:Phenylalanine / Q000235:genetics; D012264:Ribonucleotide Reductases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D013059:Spectrum Analysis, Raman; D014364:Tryptophan / Q000235:genetics
D010103:Oxygen Isotopes; D010545:Peroxides; D010649:Phenylalanine; D014364:Tryptophan; D007501:Iron; D012264:Ribonucleotide Reductases; C108427:ribonucleotide reductase R2 subunit; D010100:Oxygen