Iron enzymes

Three enzymes in Azotobacter vinelandii (Riittimann-Johnson et al. 2003) and Rho-dopseudomonas palustris (Oda et al. 2005) are capable of reducing dinitrogen the nif-encoded enzyme containing molybdenum and iron, the vn/that encodes a vanadium and iron enzyme, and the anf that is an iron-only enzyme. 

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

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... 

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. 

Vaillancourt, F.H., Yeh, E., Vosburg, D.A. et al. (2005) Cryptic chlorination by a non-haem iron enzyme during cyclopropyl amino acid biosynthesis. Nature, 436, 1191-1194. 

Table 2.3 Mononuclear non-haem iron enzymes classified according both to enzyme type and reaction type. Reprinted with permission from Holm et al., 1996. Copyright (1996) American Chemical Society. 

Dioxygen reduction (oxidase activity) and activation for incorporation into organic substrates are catalysed by a number of mononuclear non-haem iron enzymes. We will first consider the intramolecular dioxygenases, in which both atoms of oxygen are introduced into the substrate, then the monoxygenases (in which we choose to include the pterin-dependent hydroxylases), the large family of a-hetoacid-dependent enzymes, and finally isopenicillin N-synthase. 

Among the mononuclear non-haem iron enzymes catalysing hydroxylation reactions (Table 2.3) we can distinguish between intramolecular dioxygenases and external mononoxygenases. The former can be divided into those which are pterin-dependent and those which use a-ketoacids such as a-keto glutarate as obligatory... 

The a-ketoacid-dependent enzymes are distinguished from other non-haem iron enzymes by their absolute requirement for an a-ketoacid cofactor as well as Fe(II) and O2 for activity. They catalyse two types of reaction (Table 2.3), hydroxyla-tion and oxidation. In both, the a-ketoglutarate is decarboxylated and one oxygen atom introduced into the succinate formed in the hydroxylases, the other oxygen atom is introduced into the substrate, while in the oxidases it is found in water, together with the cyclized product. In general these enzymes require one equivalent of Fe(II) an a-ketoacid, usually a-ketoglutarate and ascorbate. Examples of these enzymes include proline 4-hydroxylase, prolyl and lysyl hydroxylase, which... 

One last class of mononuclear non-haem iron enzyme that we have not yet considered, consists of the microbial superoxide dismutases with Fe(III) at their active site. The crystal structure of the E. coli enzyme shows a coordination geometry reminiscent of protocatechuate 3,4-dioxygenase, with four endogenous protein ligands, three His and one Asp residue, and one bound water molecule (Carlioz et ah, 1988). 

Fig. 6.9 The catalysts for denitrification. Nitrate is reduced by a molybdenum enzyme while nitrite and oxides of nitrogen are reduced today mainly by copper enzymes. However, there are alternatives, probably earlier iron enzymes. The electron transfer bct complex is common to that in oxidative phosphorylation and similar to the bf complex of photosynthesis, while cytochrome c2 is to be compared with cytochrome c of oxidative phosphorylation. These four processes are linked in energy capture via proton (H+) gradients see Figure 6.8(a) and (b) and the lower parts of Fig. 6.9 which show separately the active site of the all iron NO-reductase, and the active site of cytochrome oxidase (02 reductase).

Abscisic acid Growth inhibitor largely linear unsaturated aliphatic compounds. Hydroxylated by iron enzyme... 

Figure 13.19 Reactions catalysed by each of the five families of mononuclear non-haem iron enzymes with a 2-His-l-carboxylate facial triad. Dioxygen is labelled to indicate the fate of each oxygen atom. (From Koehntop et al., 2005. With kind permission of Springer Science and Business Media.)...

Most of the 2-OG dependent iron enzymes are hydroxylases. The hydroxylation of a C-H bond is coupled to an oxidative decarboxylation of the 2-OG to succinate and CO2 (8). One oxygen atom of O2 forms the hydroxyl group of the product (Scheme 3). The second oxygen atom ends up in the succinate (8,18). 

Scheme 3. Reaction of 2-oxoglutarate dependent iron enzymes (8).

As already mentioned earlier, the ruthenium complex [Ru(bdmpza) Cl(PPh3)2l (24) easily releases one of the two phosphine ligands and allows the substitution not only of a chlorido but also of a triphenylphosphine ligand for K -coordinating carboxylato or 2-oxocarboxylato ligands (58). The purpose of these studies was to find structural ruthenium models for the active site of 2-OG dependent iron enzymes, since ruthenium(II) complexes are low spin and thus suitable for NMR characterization, whereas ferrous iron complexes with NJV,0-ligands are often difficult to investigate, due to their... 

Our future work will focus on the coordination of typical inhibitors of 2-OG dependent iron enzymes. First ruthenium complexes bearing N-oxalylglycine or triketone ligands have already been synthesized. 

The A, AT, 0-binding motif is found in many non-heme iron enzymes as well as in some zinc enzymes as metal-binding motif Thus, to mimic this motif is the purpose of small organic 0-ligands such as... 

Huynh BH, Czechowski MH, Kruger HJ, et al. 1984. Desulfovibrio vulgaris hydro-genase a nonheme iron enzyme lacking nickel that exhibits anomalous EPR and Mossbauer spectra. Proc Natl Acad Sci USA 81 3728-32. 

The iron enzymes catalase and peroxidasepromote these reactions very effectively, but it is unlikely that both reactants are coordinated to the metal during the reaction. It is generally true that the metal site is restricted in enzymes, allowing coordination of only one reactant. The other reactant is however often held close by the protein structure. 

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