Ribonucleotide reductase oxygen intermediates

Fig. 14. Intermediates in the activation of oxygen by binuclear iron centers. Hr, Heme-rythrin RNR, ribonucleotide reductase MMO, methane monooxygenase.

During the ferroxidation reaction, a blue color with an absorption maximum of 650 run appears. This persists in oxygen-limited conditions and decays as iron oxidation proceeds. " In frog H-chain ferritin, resonance Raman studies indicate a similar absorption is associated with an Fe(III)-tyrosinate. Harrison and Treffty have considered these and other studies and attribute the transient color to formation of a /x-l,2-peroxodiferric intermediate, which decays to a more stable /x-oxodiferric species as occurs in methane monooxygenase, ribonucleotide reductase, and model compounds. Protein radicals distinct from reactive oxygen species have been observed that have been attributed to damage caused by Fenton chemistry. ... 

A specific example is the combined multifrequency EPR and 35 GHz ENDOR study of the dLiron center in the R2 subunit of Escherichia coli ribonucleotide reductase (RNR) Samples in natural isotopic abundance (0.038% 0) and using H2 0 (34.9% 0) or 02 (85.5% 0) were prepared. ENDOR provided 0 (/ = 5/2) hyperfine coupling constants and these values were used to quantify the line broadening seen in 0-enriched versus natural abundance samples, which showed that the intermediate X contained two oxygen atoms, initially derived from dioxygen, as opposed to an alternate model with only one such oxygen atom. 

Lippard et al. proposed several possible mechanisms of the alkane hydroxylation [9]. One possible mechanism is shown in Scheme 3, in which an T 2,T 2-peroxo-bridged diiron(III) acts as an active intermediate which directly transfers oxygen to an alkane substrate [9]. This mechanism suggests the participation of a mercapto radical of Cys-151. This amino acid occupies the equivalent region of space to the tyrosyl radical of ribonucleotide reductase, as indicated by sequence homology and the X-ray crystallographic results [53, 55]. 

Figure 23. Proposed catalytic cycle for ribonucleotide reductase from Chlamydia trachomatis. High-valent iron oxygen intermediate X is shown in step III. Reprinted with permission from [306]. Copyright 2006, National Academy of Sciences, USA.

WiUems J-P, Lee HI, Burdi D, Doan PE, Stubbe J, Hoffman BM. 1997. Identification of the protonated oxygenic ligands of ribonucleotide reductase intermediate X by Q-band H CW and pulsed ENDOR. JAm Chem Soc 119 9816-9824. 

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