Enzymes iron oxygenase

Soluble methane monooxygenase (sMMO) is the best studied binuclear non-heme iron oxygenase enzyme, largely due... 

Nothing is known about the identity of the iron species responsible for dehydrogenation of the substrate. Iron-oxo species such as FeIV=0 or Fem-OOH are postulated as the oxidants in most heme or non-heme iron oxygenases. It has to be considered that any mechanistic model proposed must account not only for the observed stereochemistry but also for the lack of hydroxylation activity and its inability to convert the olefinic substrate. Furthermore, no HppE sequence homo-logue is to be found in protein databases. Further studies should shed more light on the mechanism with which this unique enzyme operates. 

These enzymes catalyze a variety of oxidative reactions in natural product biosynthesis with two C—Hhydroxylation examples shown in Figure 13.24 [72,73]. It should be noted thatC—H activation by nonheme iron oxygenases, such as aromatic dioxygenases, is an important pathway in degradation of aromatics into m-dibydrodiols, which are important chiral building blocks for chemical synthesis [74,75]. 

The number of known or presumed mononuclear, non-heme iron oxygenases and related enzymes continues to grow. This is due to intensive biochemical research and especially based on sequence data derived from genome research projects i.14). For several of these enzymes structural data are available by now from protein crystallography (12-14). In many of the iron oxygenases the iron is facially bound by two histidines and one carboxylate donor, either glutamic acid or aspartic acid. Thus, the term 2-His-l-carboxylate facial triad has been introduced by L. Que Jr. for this motif (19). 

This iron-dependent enzyme [EC 1.13.11.3], also known as protocatechuate oxygenase, catalyzes the reaction of 3,4-dihydroxybenzoate with dioxygen to produce 3-carboxy-d5, CK-muconate. 

The common motif shared by non-heme iron oxygenases contains an active site, where two histidines and one carboxylate occupy one face of the Fe(ll) coordination sphere. These enzymes catalyze a variety of oxidative modification of natural products. For example, in the biosynthesis of clavulanic acid, clavaminic acid synthase demonstrates remarkable versatility by catalyzing hydroxylation, oxidative ring formation and desaturation in the presence of a-ketoglutarate (eq. 1 in Scheme 7.22) [80]. The same theme was seen in the biosynthesis of isopenicillin, the key precursor to penicillin G and cephalosporin, from a linear tripeptide proceeded from a NRPS, where non-heme iron oxygenases catalyze radical cyclization and ring expansion (eq. 2 in Scheme 7.22) [81, 82]. 

Tryptophan oxygenase is another iron-containing enzyme which, like catalase, is inhibited by pyrazole in vivo but not in vitro. Again, however, the metabolite 4-hydroxypyrazole is active in vitro and shows, in contrast to catalase, competitive inhibition with tryptophan for the enzyme (79MI10505). Adrenaline and other phenols are also inhibitors of this enzyme and in this case, therefore, the heterocyclic ring appears not to be essential for activity. 

Figure 3 Illustration of possible partial reaction cycles of some oxygenase enzymes. Water molecules and protein ligands have sometimes been omitted for clarity, (a) P450 (18) (b) intradiol dioxygenase (7) (c) lipoxygenase (7) (d) a-KG-dependent non-heme Iron enzymes (14) (e) soluble methane monooxygenase (15) (f) uncoupled blnuclear copper (16) (g) coupled blnuclear copper (h) flavin monooxygenases (17).

As mentioned, many non-heme iron enzymes also catalyze oxidase-type reactions, such as desaturation, in biological systems (7). Similar to the non-heme iron oxygenases, the reactions are thought to proceed through an Fe =0 intermediate. Two examples of enzymes that catalyze biologically interesting oxidase reactions are isopenicillin N-synthase (IPNS) and 1-aminocyclopropane-l-carboxylate oxidase (ACCO). 

Many mononuclear nonheme iron oxygenases require a reducing cofactor (pterin or alpha-keto acid) for dioxygen activation.23,128 These enzymes utilize Fe(I V) > Fe (II) reduction by two-electron substrates. A simplified catalytic cycle for 2-keto-glutarate-dependent enzymes is shown in Figure 4.34. Keto-glutarate cofactors assist... 

Our understanding of mononuclear nonheme iron oxygenases has been greatly enhanced in the ten years prior to 2003 due to the explosion of protein crystallographic information on these enzymes and the development of functional model systems. As this manuscript was being completed, two papers on synthetic mononuclear oxoiron(IV) complexes were published, one of which reports the... 

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