Mononuclear iron containing

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

N-Heterocycles as ligands in dioxygen activation by enzymes with mononuclear nonheme iron-containing active sites 96CRV2607. 

There are hundreds of iron-containing enzymes. In general, the iron can exist as (a) a mononuclear site, in which it is coordinated by a tetrapyrrole structure (hemes) or strictly by amino acid residues that donate oxo, nitrogen, or sulfur ligands (b) a dinuclear site in which the irons are bridged by oxo, nitrogen, or sulfur coordination (c) a trinuclear site as in the 3Fe-4S clusters or (d) a tetranuclear site as in the [4Fe-4S] clusters. 

Two types of SORs have been firstly described by Lombard et al. [44] and Jenney et al. [45]. The first one is a small protein called desulfoferrodoxin (Dfx) found in anaerobic sulfate-reducing bacteria Desulfoarculus baarsii containing two protein domains iron center I and iron center II [44]. Iron center II is supposed to be responsible for the superoxide reducing activity. Another SOR has been isolated from anerobic archaea, Pyrococcus furiosus, which has a unique mononuclear iron center [45], Lombard et al. [46] and Jovanovic et al. [47] also demonstrated that the Treponema pallidum protein of T. pallidum belongs to a new class of SORs. 

There are many other proteins that contain iron in a form that is neither in haem nor in iron-sulfur clusters. We have already encountered the iron storage and transport proteins, ferritin and transferrin (see Chapter 8). We propose to discuss here two other classes of iron-containing proteins, those with mononuclear non-haem iron centres and those with dinuclear non-haem iron centres. 

The enzyme catalyzing the formation of retinal 2 by means of central cleavage of P-carotene 1 has been known to exist in many tissues for quite some time. Only recently, however, the active protein was identified in chicken intestinal mucosa (3) following an improvement of a novel isolation and purification protocol and was cloned in Escherichia coli and BHK cells (4,5). Iron was identified as the only metal ion associated with the (overexpressed) protein in a 1 1 stoichiometry and since a chromophore is absent in the protein heme coordination and/or iron complexation by tyrosine can be excluded. The structure of the catalytic center remains to be elucidated by X-ray crystallography but from the information available it was predicted that the active site contains a mononuclear iron complex presumably consisting of histidine residues. This suggestion has been confirmed by... 

Pierik AJ, Wolbert RBG, Portier GL, et al. 1993. Nigerythrin and rubrerythrin from Desulfovibrio vulgaris each contain two mononuclear iron centers and two dinu-clear iron clusters. Eur J Biochem 212 237 5. 

The preparation of a novel mononuclear iron(III)-t-butylperoxo-complex, end-on coordinated, with bis(6-pivalamido-2-pyridylmethyl)(2-pyridylmethyl)amine, bppa = (206), to Fe, " was followed by the isolation of a similar complex containing hydroperoxide. The latter reacts by O—O bond cleavage and a [(bppaH2)Fe —O ] intermediate." ... 

Since the first purifications of Fe hydrogenases in the early 1970s a range of different models for the H-cluster active site have been proposed including mononuclear iron and clusters of 2, 3, 4, and 6Fe [56,57,72-77], At least in part this changing stoichiometry reflects improvements in purification, elemental analysis, and spectroscopy. The more recent models propose the H cluster to contain approximately 6 Fe on the basis of elemental analysis [56,57] and a putative 3.3 A Fe-Fe distance indicated by EXAFS spectroscopy [58], The data are indicative at best, because counting Fe in proteins has an uncertainty typically of the order of 10% (i.e., l-2Fe), and because no EXAFS on 6Fe models has been published. 

The main difference between mononuclear complexes containing either a M—H—C or a M—H—Si three-center bond is that most tj2-CH complexes correspond to an earlier stage of the addition reaction than do the 7j2-SiH complexes 7(CMH) coupling constants are usually closer to the values for /(OH), while /(SiMH) values are closer to 2/(SiMH), and the relative lengthening of the C—H distance on 172 coordination is usually smaller than that of coordinated Si—H bonds. For example, in the representative iron complex 21 [the structure of which was determined by neutron diffraction analysis (74)], the coordinated C—H bond... 

The class of mononuclear dioxygenases [30] can e.g. perform hydroperoxidation of lipids, the cleavage of catechol and dihydroxylation of aromatics. A prominent example is naphthalene dioxygenase, which was the first identified by its crystal structure. It contains iron and a Rieske (2Fe-2S) cluster and is commonly referred to as a Rieske-type dioxygenase [33]. The iron in this case is flanked by two histidines and one aspartic acid residue. Among the mononuclear iron enzymes, the 2-His-l-carboxylate is a common motif, which flanks one-side of the iron in a triangle and plays an important role in dioxygen activation [34] (Fig. 4.17). 

Iron-containing proteins are classified as heme, mononuclear non-heme, and binuclear non-heme enzymes. The Fe center... 

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