Ferric peroxide

How does nature prevent the release of hydrogen peroxide during the cytochrome oxidase-mediated four-electron reduction of dioxygen It would appear that cytochrome oxidase behaves in the same manner as other heme proteins which utilize hydrogen peroxide, such as catalase and peroxidase (vide infra), in that once a ferric peroxide complex is formed the oxygen-oxygen bond is broken with the release of water and the formation of an oxo iron(IV) complex which is subsequently reduced to the ferrous aquo state (12). Indeed, this same sequence of events accounts for the means by which oxygen is activated by cytochromes P-450. 

The cytotoxicity of BLM is believed to result from its ability to bind iron, activate oxygen, and form an activated BLM (Fe-114) (556) which cleaves DNA and possibly RNA (557). The ability of the Fe(II)-BLM complex to bind to oxygen and produce oxygenated BLM species such as 02-Fe(III)-BLM or 02-Fe(II)-BLM may be due to the presence of delocalized 77-electrons around the iron and the strong iron-pyrimidine 77-back-bonding (558, 559). Oxygenated BLM accepts an additional electron to form activated low-spin ferric-peroxide-BLM (Oi -Fe(III)-BLM) (558, 559). The structural features of Fe-BLM responsible for DNA (or RNA) degradation remain unclear (560). Bleo-... 

Fig. 2. The Bonnichsen, Chance, and Theorell 34) mechanism for the dismutation of hydrogen peroxide by catalase. (A) The simple ping-pong mechanism (ferric-peroxide compound (ycle) involves only the successive formation and decomposition of the compound 1 intermediate by two successive molecules of H2O2. (B) Reversible ES(Fe -H202) and ternary (compound I-H2O2]) complexes are added to the mechanism in A.

The ferric peroxide intermediate could undergo either heterolytic or homolytic cleavage to give either a ferryl or diferryl oxene. In the path shown in Fig. 14, the organic radical is generated by reaction with the Fe center after heterolytic 0—0 bond cleavage. For RNRB2 this is the... 

Figure 16-11 (A) Stereo drawing showing folding pattern for beef liver catalase and the positions of the NADPH (upper left) and heme (center). From Fita and Rossmann.198 (B) Diagram of proposed structure of an Fe(III)-OOH ferric peroxide complex of human catalase (see also Fig. 16-14).

J. W. Sam, X. J. Tang, J. Peisach, Electrospray mass spectrometry of iron bleomycin Demonstration that activated Bleomycin is a ferric peroxide complex, J. Am. Chem. Soc. 116 (1994) 5250. 

The thiolate ligand was implicated as a crucial factor in the 0-0 bond cleavage process through its push effect that leads to Cpd I (ref [54]). Ogliaro et al. have addressed this issue by comparing the proton affinity of 6 to a reference complex without a thiolate ligand. The thiolate ligand was found to increase the proton affinity of ferric peroxide by 81 kcal mol . At the same time, the protonated reference complex devoid of thiolate loses water spontaneously as well. Thus, the push effect of the thiolate does not concern... 

Ortiz De Montellano, P.R. (1998). Heme oxygenase mechanism Evidence for an electrophilic, ferric peroxide species. Accounts Chem. Res. 31, 543-549. 

Figure 6.36. Proposed mechanism for the C17-C20 lyase reaction catalyzed by CYPI7A. The key steps involve addition of a P450 ferric peroxide species to the C20 carbonyl and subsequent free radical fragmentation of the peroxyhemiacetal,...

The role of the ferric peroxo moiety in the mechanism has been supported by mutagenesis studies in which Thr306 has been replaced by an alanine. This threonine is believed to be the active site residue that directs the delivery of protons required to cleave the 0-0 bond and form the ferry 1 species. As expected, its loss results in an approximately 20-fold decrease in the ferryl dependent C17 hydroxylation activity but a much smaller decrease in C17-C20 lyase activity mediated by the ferric peroxo moiety. Experiments involving analysis of the solvent deuterium isotope effect as a function of pH have suggested that the protonation of the ferric peroxide intermediate governs whether the reaction proceeds via a ferryl dependent (17a hydroxylation) or a peroxy adduct (C17-C20 lyase) pathway . 

Figure 6.39. The currently accepted mechanism for the final step in the aromatase catalyzed reaction. The timing of enolization of the carbonyl with respect to the addition of the ferric peroxide to the aldehyde and to C-C and 0-0 bond fission is still uncertain.

The study of model compounds or simply, models of heme proteins is very helpful in elucidating structure-function relationships. Models are compounds with low molecular weight that imitate struetural, spectroscopic, or functional details of the original enzymes. The latter are macromolecules and hence more difficult to study. Synthetic models for states 7-5 must be thiolate-ligated. Such models have been prepared and extensively characterized. The models from several laboratories have recently been reviewed [22]. A model system having a ferric-peroxide composition, as is present in 6, has also been described [40]. Relevant models are listed in the tables (see Sects. 3.1 and 4.1.1). Model chemistry has been extremely important in characterizing these intermediates. 

The second reaction, the 17,20-lyase reaction, is more complex and difficult to rationalize with a classic compound I mechanism. Work from Akhtar s laboratory led to the proposal that the reaction involves a nucleophihc attack of the ferric peroxide (anion Fe02, or Fe"02") on the C-20 carbonyl (of 17a-hydroxyprogesterone or pregnenolone) [2156, 2162]. One of the key pieces of evidence was the result of 02 labeling experiments [2156], The results of site-directed mutagenesis studies on Thr-306 are also consistent with the conclusions about Fe"02 involvement [2156, 2163]. sugar s group has also presented resonance Raman spectra [2164] and solvent deuterium kinetic isotope effect studies [2165] in support of the involvement of this entity. 

When either 19-deuterated 19-oxoandro-stenedione or testosterone was incubated with recombinant P450 19A1 and 02, 0 label was not incorporated into the recovered formic acid (Fig. 9.28). These results differ from those reported previously [2289, 2305] the major technical differences are the use of recombinant purified P450 19A1, a more sensitive probe for trapping and analyzing formic acid, and the use of UPLC-coupled high-resolution mass spectrometry, which avoided issues inherent in analysis of labeled formic acid [2295]. The results ate not consistent with the proposed ferric peroxide mechanism, in which an 0 atom is expected to be recovered in formic acid. 

An issue with the ferric peroxide mechanism is that the substrate is in the (hydrated) gew-diol form following the second reaction (Fig. 9.28). However, the proposed ferric peroxide mechanism involves a nucleophilic attack (Fe " 02 ) on the carbonyl (aldehyde). Thus, the gew-diol must be dehydrated before this step can run. The rate of dehydration has been estimated at > 0.5 s (in the absence of P450 19A1 using 0 exchange methods [2295], which is faster than the (8 min ) for going from 19-hydroxy androstenedione to estrone [220]. The reaction could occur with the aldehyde or the ge/w-diol, the latter of... 

The mechanism of the last oxygenation and the A-ring aromatization remains controversial. If loss of formic acid from C-19 and the ip-hydrogen occurs early in this process, a second double bond would be introduced in the A-ring, and tautomerization of the 3-ketone would complete the aromatization reaction. Other proposals include a mechanism in which a ferric peroxide attacks the C-19 oxo-intermediate... 

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