Mononuclear iron containing enzymes

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. 

The reactions of iron-containing enzymes with O2 often involve high oxidation states of the metal. Generally, the initial reaction of dioxygen with both heme and mononuclear non-heme ferrous enzymes results in the formation of Fe -superoxide intermediates. Highly reactive Fe =0 intermediates often are employed often for C-H activation. The mechanism of substrate oxidation by binuclear non-heme enzymes involves high valent, oxo-bridged species, with Fe in the -i-3 or +4 oxidation state. 

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. 

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... 

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). 

A wide range of soluble redox enzymes contain one or more intrinsic [2Fe-2S]2+ +, [3Fe-4S]+ , or [4Fe S]2+ + clusters that function in electron transport chains to transfer electrons to or from nonheme Fe, Moco/Wco, corrinoid, flavin, thiamine pyrophosphate (TPP), Fe S cluster containing, or NiFe active sites. Many have been structurally and spectroscopically characterized and only a few of the most recent examples of each type are summarized here. Dioxygenases that function in the dihydroxylation of aromatics such as benzene, toluene, benzoate, naphthalene, and phthalate contain a Rieske-type [2Fe-2S] + + cluster that serves as the immediate electron donor to the monomeric nonheme Fe active site see Iron Proteins with Mononuclear Active Sites). The xanthine oxidase family of molybdoenzymes see Molybdenum MPT-containing Enzymes) contain two [2Fe-2S] + + clusters that mediate electron transfer between the Moco active site and the Other soluble molybdoen-... 

Cobalt B Enzymes Coenzymes Cytochrome Oxidase Iron Heme Proteins Electron Transport Iron Proteins with Dinuclear Active Sites Iron Proteins with Mononuclear Active Sites Iron-Sulfur Models of Protein Active Sites Metallocenter Biosynthesis Assembly. Metalloregulation Molybdenum MPT-containing Enzymes Nickel Enzymes Cofactors Nitrogenase Catalysis Assembly Photosynthesis Tungsten Proteins Vanadium in Biology Zinc DNA-binding Proteins. 

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

The copper-containing enzymes can be classified as mononuclear, binuclear, or multicopper proteins (9). Unlike iron, copper caimot readily access high oxidation states and the formation of Cu in biological systems remains controversial. Generally, Cu centers undergo one electron oxidations to activate dioxygen. Thus, the Cu-containing enzymes tend to employ multiple copper atoms or an additional cofactor for the final two or four-electron oxidation of their substrates. 

As mentioned in Sect. 5.3.3, a number of enzymes contain metal ions that participate in the catalytic reaction. Two specialized databases store information on metal ions and other bioinorganic motifs in enzymes. PROMISE (prosthetic centers and metal ions in protein active sites) is maintained by the University of Leeds and focuses on six major groups of metal containing proteins diiron-carboxylate proteins, haem proteins, iron-sulfur proteins, molybdopterin proteins, mononuclear iron proteins, and chlorophyll containing proteins156. ... 

Naphthalene dioxygenase consists of three components, which form an electron transfer chain an NADH-dependent flavoprotein reductase, a ferredoxin containing two [2Fe2S] Rieske iron-sulfur clusters, and a Rieske oxygenase containing both a [2Fe2S] Rieske iron-sulfiir cluster and a mononuclear iron(II) center in the enzyme active site. ° ... 

All known molybdenum- and tungsten-containing enzymes catalyse reduction-oxidation reactions. The oxidation state of the metal centre can vary between iv, v and vi, hence one- and two-electron transfer reaction steps are possible. In Nature two different ways exist to control the catalytic power and the oxidation state of the metal centre of molybdenum enzymes. One is a mononuclear metal centre, which consists of sulfur and oxygen atoms as coordination sphere around molybdenum and the other is the multinuclear metal centre in which the molybdenum is part of an iron-sulfur cluster, which is only known for bacterial nitroge-nase enzymes. ... 

BZDO enzyme also contains an iron-sulfur center. But it consists of two components an oxygenase of an (aP)3 subunit structure, in which the a contains a Rieske [2Fe-2S] cluster and a mononuclear iron site, and a reductase with one FAD and one [2Fe-2S] cluster (Yamaguchi et al. 1975 Yamaguchi and Fujisawa 1978,1980,1982). In the process of dioxidation of benzoate by BZDO, the iron ion plays an important role (Fig. 4). 

In the past 15-20 years, several studies have been performed on mononuclear non-heme iron(II) enzymes. Among them, Rieske oxygenases constitute a relevant and paradigmatic example of mononuclear non-heme iron(II) proteins involved in O2 activation reactions. Their efficiency and versatility are even greater than those of the related heme-containing Cyt P450, being able to catalyze stereoselective... 

There are two distinct forms of MMOs one is soluble MMO (sMMO), which contains a nonheme dinuclear iron center, and the other is particulate MMO (pMMO), which is a copper-containing enzyme. Although sMMO has been well characterized and studied (78,79), many aspects of pMMO chemistry and biochemistry remain unclear. Because of the difficulty in the isolation and purification of pMMO, four different research groups have reported various ranges of copper stoichiometries for purified pMMO 2 (80), 2-3 (81) 8-10 (82), and 15-20 (83) copper ions per 100 kDa. The first crystal structure of pMMO firom M. capsulatus Bath (28) shows a heterotrimer of three subunits, and each subunit contains three polypeptides pmoB (a, 47 kDa), pmoA (, 24 kDa), and pmoC (y, 22 kDa). Three types of metal centers were found in pMMO a mononuclear copper, a dinuclear copper center in pmoB, and a zinc center in pmoC. 

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