Diiron cofactor

Bar, G., M. Bennati et al. (2001). High-frequency (140-GHz) time domain EPR and ENDOR spectroscopy The tyrosyl radical-diiron cofactor in ribonucleotide reductase from yeast. J. Am. Chem. Soc. 123 3569-3576. 

Under conditions of copper deficiency, some methanotrophs can express a cytosolic, soluble form of MMO (sMMO) (20-23), the properties of which form the focus of the present review. The sMMO system comprises three separate protein components which have all been purified to homogeneity (24,25). The hydroxylase component, a 251 kD protein, contains two copies each of three subunits in an a 82y2 configuration. The a subunit of the hydroxylase houses the dinuclear iron center (26) responsible for dioxygen activation and for substrate hydroxylation (27). The 38.6 kD reductase contains flavin adenine dinucleotide (FAD) and Fe2S2 cofactors (28), which enable it to relay electrons from reduced nicotinamide adenine dinucleotide (NADH) to the diiron center in the... 

The corresponding high-valent intermediate in the assembly of the di-iron(III) center-tyrosyl radical cofactor of RNR R2 has also been identified by Stubbe and coworkers and designated as X [86,89], This intermediate decays to the (p,-oxo)diiron(III) form at a rate commensurate with the appearance of the tyrosyl radical. Intermediate X, formally Fe(III)Fe(IV), exhibits an isotropic S = 1/2 spin EPR signal near g = 2, which is split by the introduction of 57Fe and broadened by 1702 in the assembly reaction. These observations as well as Mossbauer results show that the unpaired spin must be associated with the diiron center [88,89],... 

Bollinger, J. M., Tong, W. H., Ravi, N., Huynh, B. H., Edmondson, D. E., and Stuhhe, J., 1994a, Mechanism of assembly of the tyrosyl radical-diiron(III) cofactor of E. coli rihonucleotide reductase II. Kinetics of the excess Fe reaction by optical, EPR, and M ssbauer spectroscopies. /. Am. Chem. Soc. 116 8015n8023. 

Although the carboxylate-bridged diiron center in the hydroxylase protein of sMMO activates dioxygen for CH4 oxidation, two other protein components are required for catalytic turnover. These include a small coupling protein (MMOB) which, despite containing no cofactors, exhibits profound effects on catalysis. In addition, a reductase (MMOR), which binds FAD and a [2Fe-2S] cluster, is re-... 

Some FeNO complexes have been synthesized to mimic the structure and function of nitric oxide reductase enzymes, which can be separated into two classes. One class utilizes a heme/nonheme active site to reduce two equivalents of NO into N2O and is found in denitrifying bacteria (NorBC) (18,19). Another class is found in pathogenic bacteria such as Helicobacter pylori. Neisseria meningitides, and Salmonella enterica. These microbes have evolved ways to handle attack by NO by converting it to relatively ben p N2O through the expression of nitric oxide reductases utilizing a nonheme diiron protein that has a flavin cofactor within 4 A of the active site metals (FNORs Figure 7) (16,17). A few recent (2011—2014) FeNO complexes have been constructed to model FNORs in order to probe the mechanism of this di-Fe enzyme. These complexes will be discussed below. 

A combination of spectroscopies including Mossbauer and resonance Raman were used to identify the presence of a diiron-oxo cluster with properties similar to those identified in ribonucleotide reductase (RB2) and methane monooxygenase (MMO). These enzymes all share the ability to break unactivated carbon-hydrogen bonds with a nonheme diiron cluster cofactor. Fatty acid desaturation and methane oxidation require a two-electron reduction of the diiron cluster to initiate the oxygen activation reaction. Identification of a diiron cluster in the desaturase allowed us to propose a consensus diiron-oxo binding motif consisting of two repeats of (D/E)EXXH. 

Zhou B, Shao J, Su L, Yuan Y-C, Qi C, Shih J, Xi BX, Chu B, Yen Y. 2005. A dityro-syl-diiron radical cofactor center is essential for human ribonucleotide reductases. Mol Cancer 4 1830-1836. 

Sturgeon BE, Burdi D, Chen SX, Huynh BH, Edmondson DE, Stubbe J, Hoffman BM. 1996. Reconsideration of X, the diiron intermediate formed during cofactor assembly mE. coli ribonucleotide reductase. JAm Chem Soc 118 7551-7557. 

Regeneration of the catalytic cycle occurs following reduction of the oxidized diiron center to the active diferrous utihzing electrons coming through the enzyme flavin cofactor... 

There are three classes of proteins that reversibly bind dioxygen. Hemoglobins occur in a wide variety of organisms, having active sites with a single iron pro-toprophyrin IX cofactor. Hemerythrins are nonheme diiron proteins present in certain species of seaworms. The hemocyanins are dinuclear copper proteins found in the hemolymph of mollusks and arthropods. There have been extensive studies on the physical and structural properties of these proteins, and only a brief mention of their active site structures will be discussed here [9]. 

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