Non-heme iron

The enzymes of this type that have been characterized contain some type of redox-active cofactor, such as a flavin (3), or a metal ion (heme iron, non-heme iron, or copper), or both (4-6). Our understanding of the mechanism of these enzymes is most advanced in the case of the heme-containing enzyme cytochrome P450. But in spite of the availability of a crystal structure of an enzyme-substrate complex (7) and extensive information about related reactions of low molecular weight synthetic analogues of cytochrome P450 (8), a detailed picture of the molecular events that are referred to as "dioxygen activation" continues to elude us. 

Iron, Fe a bioelement found in all living cells. The human body contains 4-5 g Fe, of which 75 % is in hemoglobin. In living organisms Fe occurs in the II and III oxidation states in higher animals it is stored bound to protein. It is transported in the blood as a complex with transferrin (see Siderophilins), from which it is transferred enzymatically to metal-ffee porphyrin molecules (see Heme iron). Non-heme iron (see) is also found in a number of compounds, e.g. Iron-sulfur proteins (see). The Fe metabolism of microorganisms is mediated by a group of natural products called Siderochromes (see). 

W. A. Bulen, J. R. LeComte, R. C. Bums, and J. Hinkson, in A. San Pietro, ed., Non-Heme Iron Proteins Role in Lnerg Conversion Antioch Press, Yellow Springs, Ohio, 1965, p. 261. 

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. 

Tyrosine hydroxylase (TH) is an enzyme that catalyzes the hydroxylation of tyrosine to 3,4-dihydroxypheny-lalanine in the brain and adrenal glands. TH is the rate-limiting enzyme in the biosynthesis of dopamine. This non-heme iron-dependent monoxygenase requires the presence of the cofactor tetrahydrobiopterin to maintain the metal in its ferrous state. 

Bioinorganic applications of m.c.d. spectroscopy copper, rare earth ions, cobalt and non-heme iron systems. D. M. Dooley and J. H. Dawson, Coord. Chem. Rev., 1984, 60,1 (176). 

It has always been assumed that these simple proteins act as electron-transfer proteins. This is also a fair conclusion if we take in account that different proteins were isolated in which the Fe(RS)4 center is in association with other non-heme, non-iron-sulfur centers. In these proteins the Fe(RS)4 center may serve as electron donor/ac-ceptor to the catalytic site, as in other iron-sulfur proteins where [2Fe-2S], [3Fe-4S], and [4Fe-4S] clusters are proposed to be involved in the intramolecular electron transfer pathway (see the following examples). 

Ruettinger RT, GR Griffith, MJ Coon (1977) Characteristics of the u-hydroxylase of Pseudomonas oleovorans as a non-heme iron protein. Arch Biochem Biophys 183 528-537. 

Wills, E.D. (1969). Lipid peroxide formation in microsomes the role of non-heme iron. Biochem. J. 113, 325-332. 

Non-heme Di-Iron Enzymes Methane Monooxygenase and Ribonucleotide Reductase... 

Metalloenzymes with non-heme di-iron centers in which the two irons are bridged by an oxide (or a hydroxide) and carboxylate ligands (glutamate or aspartate) constitute an important class of enzymes. Two of these enzymes, methane monooxygenase (MMO) and ribonucleotide reductase (RNR) have very similar di-iron active sites, located in the subunits MMOH and R2 respectively. Despite their structural similarity, these metal centers catalyze very different chemical reactions. We have studied the enzymatic mechanisms of these enzymes to understand what determines their catalytic activity [24, 25, 39-41]. 

Oxygen activation is a central theme in biochemistry and is performed by a wide range of different iron and copper enzymes. In addition to our studies of the dinuclear non-heme iron enzymes MMO and RNR, we also studied oxygen activation in the mononuclear non-heme iron enzyme isopenicillin N synthase (IPNS). This enzyme uses O2 to transform its substrate ACV to the penicillin precursor isopenicillin N [53], a key step in the synthesis of the important P-lactam antibiotics penicillins and cephalosporins [54, 55],... 

The active-site model (and the ONIOM model system) includes Fe, one aspartate and two histidine ligands, a water ligand and selected parts of the substrate (see Figure 2-6). The 2-histidine-1-carboxylate ligand theme is shared by several other non-heme iron enzymes [59], For the protein system, we used two different... 

Hemoproteins which engage in electron transport — the cytochromes — are much more widely dispersed among living species and occur in microorganisms, plants and animals (13). Again there are two types of iron proteins which can perform the task of electron transport, the heme and the non-heme. The latter term has become practically synonymous... 

A component of the ribotide reductase complex of enzymes, protein Ba, has been shown to contain two non-heme iron atoms per mole (77). This enzyme plays a vital, albeit indirect, role in the synthesis of DNA. Curiously, the lactic acid bacteria do not employ iron for the reduction of the 2 hydroxyl group of ribonucleotides. In these organisms this role has been assumed by the cobalt-containing vitamin Bi2 coenzyme (18). The mechanism of the reaction has been studied and has been shown to procede with retention of configuration (19). 

In addition to these more-or-less well characterized proteins, iron is known to be bound to certain flavoproteins such as succinic dehydrogenase (20), aldehyde oxidase (27), xanthine oxidase (22) and dihydrooro-tate dehydrogenase (23). Iron is present and functional in non-heme segments of the electron transport chain but again no real structural information is at hand (24). 

Hydrogenase and other components of the N2 fixing apparatus of bacteria have been shown to be non-heme iron proteins (66). 

The hemerythrin of Golfingia gouldii consists of eight subunits, each of which contains two iron atoms, in a protein with molecular weight 108,000. Spectral and magnetic data point to an oxo-bridged structure around the non-heme iron atom (99). Protein B2 of ribotide reductase of E. coli has some properties in common with hemerythrin presumably a protein corresponding to that of E. coli reduces ribotides in animal tissues, a conclusion based on probes with inhibitors. 

Hegg, E. L. and L. Que (1997). The 2-His-l-carboxylate facial triad-An emerging structural motif in mononuclear non-heme iron(II) enzymes. Eur. J. Biochem. 250(3) 625-629. 

Que, L. and R. Y. N. Ho (1996). Dioxygen activation by enzymes with mononuclear non-heme iron active sites. Chem. Rev. 96(7) 2607-2624. 

Emerson, J.P., Farquhar, E.R. and Que, L. Jr. (2007) Structural snapshots along reaction pathways of non-heme iron enzymes. Angewandte Chemie, International Edition, 46, 8553-8556. 

Hewitson, K.S., Granatino, N., Welford, R.W.D. and Schofield, CJ. (2005). Oxidation by 2-oxog-lutarate oxygenases non heme iron synthesis. Phil. Trans Roy. Soc. A363, 807-829. 

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