Peroxo diiron intermediates

The first spectroscopically characterized intermediate in the reaction of the diferrous state (MMOred) with dioxygen is a peroxo species (Intermediate P). This intermediate subsequently converts to a high-valent bis(/x-oxo)diiron(IV) component (Intermediate Q). Intermediate Q reacts with methane releasing methanol and generating a water molecule. 

Peroxo-diiron(III) complexes can undergo not only redox but also ligand substitution reactions. Liberation of H202 was observed in the reactions with phenols and carboxylic acids leading also to the respective phenolate or carboxylate iron(III) complexes.86 Hydrolysis of a peroxo-diiron(III) complex results in an oxo-diiron(III) species and hydrogen peroxide. Such reaction is responsible for the autoxidation of hemerythrin, but is very slow for the native protein due to hydrophobic shielding of the active site (Section 4.2.3).20 The hydrolysis of iron(III) peroxides is reversible, and the reverse reaction, the formation of peroxo intermediates from H202 and the (di)iron(III), is often referred to as peroxide shunt and is much better studied for model complexes. 

The dinuclear systems, such as extensively studied methane monooxygenase (MMO) that utilizes a Fe Fej11 redox couple for a formally two-electron oxidation of methane into methanol with an intermediate Q of its hydroxylase component, take advantage of both iron centers in catalysis.76 Diiron(II) form of MMO reacts with 02 yielding a peroxo-diiron(III) species P as the first spectroscopically observable intermediate (Figure 4.35). Although intermediate P itself does not react with a... 

Fig. 2. Possible structures for a diiron(III) peroxide unit in the peroxo intermediate consistent with available Raman and Mossbauer spectroscopic data. The symbols N and 0 designate nitrogen and oxygen donor atoms of histidine and glutamate residues, respectively. Some of the latter must be bidentate to fill the coordination spheres.

Fig. 3. Proposed routes for conversion of the peroxo intermediate to intermediate Q, one involving loss of water (left-hand side) and one not (right-hand side). In the former case the resulting diiron(IV) oxo species could bind an oxygen atom with one iron, or the oxygen could be bound symmetrically by both iron atoms. Although written as an iron(IV) oxo species, Q can also be formulated as an iron(III) oxyl radical complex (35,51).

Fig. 4. Substrate first binds to the complete system containing all three protein components. Addition of NADH next effects two-electron reduction of the hydroxylase from the oxidized Fe(III)Fe(III) to the fully reduced Fe(II)Fe(II) form, bypassing the inactive Fe(II)Fe(III) state. The fully reduced hydroxylase then reacts with dioxygen in a two-electron step to form the first known intermediate, a diiron(III) peroxo complex. The possibility that this species itself is sufficiently activated to carry out the hydroxylation reaction for some substrates cannot be ruled out. The peroxo intermediate is then converted to Q as shown in Fig. 3. Substrate reacts with Q, and product is released with concomitant formation of the diiron(III) form of the hydroxylase, which enters another cycle in the catalysis.

First two complexes with a (p.-oxo)(p-hydroxo)diiron (III) core [Fe2(0)(0H)(6TLA)2(C104)3] (I) and [Fe2(0)2(6TLA)2(C104)2] (II), were isolated and characterized (Zang et al 1995). Structure of a (-l,2-peroxo)bis(-carboxylato)diiron(m)model for the peroxo intermediate in the methane monooxygenase hydroxylase reaction cycle is presented in Fig, 6.3. 

Figure 6..3.. Structure of a (-l,2-peroxo)bis(-carboxylato)diiron(III)model for the peroxo intermediate in the methane monooxygenase hydroxylase (Zang et al., 1995) Reproduced with permission.

Kim, K., and Lippard, S. J., 1996, Structure and M ssbauer spectrum of a (p-l,2-peroxo)-bis( r-carboxylato)diiron(III) model for the peroxo intermediate in the methane monooxygenase hydroxylase reaction cycle, J. Am. Chem. Soc. 118 4914n4915. 

The peroxo species has only been observed in the E. coli R2 mutant D84E (Bollinger et al., 1998), but is a well-known intermediate in the analogous catalytic cycle of methane monooxygenase (Liu et al., 1995). The peroxo species in D84E is a precursor of the tyrosyl radical-diiron centre, however it was not possible to determine whether the RTF mutant stabilised the intermediate enough to be observed, or if the mutation caused a new intermediate to follow a reaction sequence more similar to that of methane monooxygenase. 

Bollinger, J. M., Krehs, C., Vicol, A., Chen, S. X., Ley, B. A., Edmondson, D. E., and Huynh, B. H., 1998, Engineering the diiron site of Escherichia coli ribonucleotide reductase protein R2 to accumulate an intermediate similar to H-peroxo, the putative peroxodiiron(III) complex from the methane monooxygenase catalytic cycle. J. Am. Chem. Soc. 120 1094nl095. 

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