Nonheme Fe

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

Mechanistic Insight into the Nitric Oxide Dioxygenation Reaction of Nonheme Fe(III)-Superoxo and Mn(IV)-Peroxo Complexes... 

With the use of gene clusters of the natural products coronatine and kutznerides, the biosynthetic pathway of coronamic acid has also been elucidated by Walsh and coworkers. From the biosynthetic analyses, a nonheme Fe -dependent halogenase was identified as the chlorinating enzyme that converts L- //a-isoleucine to 7-chloroisoleucine. A second enzyme carries out a dehydrochlorination reaction to yield coronamic acid. The general biosynthetic pathway is shown below (Scheme 7). 

Thermosynechococcus (T.) vulcanus crystallizes as a dimer (Figure 3.4.2) that contains about 2,800 solvent water molecules [7], Each monomer consists of about 20 protein subunits that harbor 77 cofactors 35 chlorophylls (Chi a) 11 p-carotenes 2 plastoquinones (PQ) 2 pheophytines (Pheo a) 1 Mn4OsCa complex 2 heme Fe 1 nonheme Fe and 1 hydrogen carbonate, HCO3/CO3. The overall reaction of PSII is that of a light-driven water plastoquinone oxidoreductase [1] ... 

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-... 

Biochemical and biophysical studies have indicated that Nors contain a binuclear heme-Fe/nonheme Fe active site (/)3-FeB) where reduction of NO to N2O occurs (Figure 5a). 

Fig. 3, Ribbon drawings of the polypeptide chains in the M and L subunits of the Rp. viridis reaction center, redrawn from Deisenhofer et al. [102]. The drawings are oriented so that the normal to the chromatophore membrane is approximately vertical, with the periplasmic side of the membrane at the top and the cytoplasmic side at the bottom. The amino-terminal ends of the chains are on the cytoplasmic side of the membrane that of the L subunit is labeled 1. The five transmembrane helices are labeled A-E. In each subunit, the histidine residue that ligates one of the BChls of P is located near the top of helix D, on the periplasmic side of the hydrophobic region. The L and M subunits are closely appressed in the reaction center complex, so that the two BChls of P overlap (Fig. 4). The histidine ligands of the nonheme Fe are located toward the cytoplasmic ends of helices D and E in each subunit the glutamyl ligand in the M subunit is in the connecting region between D and E.

Reaction centers isolated from the Rhodospirillaceae contain four molecules of BChl, two molecules of BPh, one or two quinones (depending on the isolation procedure), and one atom of nonheme Fe [21, 116]. As mentioned above, the quinones can be either ubiquinone or menaquinone, depending on the species. The Fe can be replaced by Mn, Zn or other metals with only minor effects on photochemical activity [42,117,118]. In reaction centers from Rp. viridis the BChl and... 

The nonheme Fe atom appears to have five ligands two histidine residues of the L subunit, two histidiftes of the M subunit, and a glutamyl residue of M (Fig. 2). The coordination to four histidine nitrogens and the finding that the Fe is not attached directly to either of the quinones are in accord with measurements of the EXAFS spectrum of the Fe [121,122]. 

Extracting the nonheme Fe from the reaction center slows electron transfer from Qa to Qb by about a factor of 2 [168], a remarkably modest effect in view of the Fe s location between the two quinones (Fig. 4). Electron transfer from EPFl to... 

A second missing link is that the critical driver responsible for the dramatically increased lipid oxidation rate is the Fe" itself, not radicals from the decomposition of contaminating hydroperoxides. Ferryl iron is a strong oxidant, kinetically equivalent to HO in reactivity (154) but more selective due to its lower redox potential (168). Ferryl iron rapidly abstracts H from the doubly allylic C-11 of linoleate (now conveniently oriented toward the heme iron core) (144) and it abstracts hydrogens from hydroperoxides even more rapidly (154), in contrast to the very slow oxidation with nonheme Fe ... 

In the book, the section on homogeneous catalysis covers soft Pt(II) Lewis acid catalysts, methyltrioxorhenium, polyoxometallates, oxaziridinium salts, and N-hydroxyphthalimide. The section on heterogeneous catalysis describes supported silver and gold catalysts, as well as heterogenized Ti catalysts, and polymer-supported metal complexes. The section on phase-transfer catalysis describes several new approaches to the utilization of polyoxometallates. The section on biomimetic catalysis covers nonheme Fe catalysts and a theoretical description of the mechanism on porphyrins. 

The reaction of nonheme Fe complexes that are analogues of Rieske dioxygenase enzymes have been studied by the group of Que [234,235] (see Chapter 18). A possible mechanism has been suggested [241] (Fig. 1.31) for a bispidme ligand. An [Fe oOH] intermediate is formed initially, which undergoes 0-0 bond homolysis to form a ferryl [Fe =O] oxidant and HO. The oxidant was formed independently by reaction of the complex with lodosylbenzene as indicated by the appearance of an expected electronic transition in the near infrared of e = 400 M cm . This was able to epoxidize czs-cyclooctene in an Ar atmosphere. 

The challenge of pinning down the exact nature of the reaction mechanism, whether it is a synchronous PCET or not, from BDEs is highlighted by the oxidation of hydrocarbons by the nonheme Fe =0 complex [61-64] shown in Table 17.1(c). Figure 17.7(a) plots the measured second order rate constants for the oxidation of hydrocarbons by [(N4Py)Fe =0]2+ against the BDEs of the hydrocarbons... 

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