Radical cofactors

Interest in this class of coordination compounds was sparked and fueled by the discovery that radical cofactors such as tyrosyl radicals play an important role in a rapidly growing number of metalloproteins. Thus, in 1972 Ehrenberg and Reichard (1) discovered that the R2 subunit of ribonucleotide reductase, a non-heme metal-loprotein, contains an uncoordinated, very stable tyrosyl radical in its active site. In contrast, Whittaker and Whittaker (2) showed that the active site of the copper containing enzyme galactose oxidase (GO) contains a radical cofactor where a Cu(II) ion is coordinated to a tyrosyl radical. 

Recently, a somewhat different synthetic approach has been reported. Halcrow et al. (215) synthesized a series of five-coordinate copper(II) complexes comprising a tridentate tris(pyrazolyl)borate ligand and a bidentate phenol derivative. Neutral complexes [Cun(TpPh)(bidentate phenolate)] were synthesized and structurally characterized [Tpph] = hydrido-tris(3-phenylpyrazol-l-yl)borate. The species [Cun(TpPh)(2-hydroxy-5-methyl-3-methylsulfanylbenzaldehydato)] can electro-chemically be converted to the (phenoxyl)copper(II) monocation, which has been characterized in solution by UV-vis spectroscopy. It displays two intense absorption maxima at 907 nm (e = 1.2 x 103 M 1 cm-1), and 1037 (1.1 x 103 M l cm-1), resembling in this respect the radical cofactor in GO (Fig. 7). 

ELECTROSTRICTION DIETHYL PYROCARBONATE Diferric-tyrosyl radical cofactor, REDOX-ACTIVE AMINO ACIDS... 

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],... 

Riggs-Gelasco, P. J., Shu, L. J., Chen, S. X., Burdi, D., Huynh, B. H., Que, L., and Stubbe, J., 1998, Exafs Characterization of the intermediate X generated during the assembly of the Escherichia coli ribonucleotide reductase R2 diferric tyrosyl radical cofactor, J. Am. Chem. Soc. 120 8499860. 

Type of class Polypeptide composition Gene nomenclature Stable radical/ cofactor Metal site/ cofactor Substrate Reductant (s )"... 

Hogbom M, Galander M, Andersson M, Kolberg M, Hofbauer W, Lassmann G, Nordlund P, Lendzian F. Displacement of the tyrosyl radical cofactor in rihonucleotide reductase obtained by... 

The protein is a single polypeptide with molecular mass of ca 68 kDa. To perform the two-electron chemistry, the enzyme utilizes, in addition to the copper center, a protein radical cofactor, which has been assigned to the Tyr272 residue. GO can exist in three distinct oxidation states the highest state with Cu(II) and tyrosyl radical, the intermediate state with Cu(II) and tyrosine, and the lowest state with Cu(I) and tyrosine. The highest oxidation state is the catalytically active one. The protein radical couples antiferromagnetically with the copper ion, resulting in an EPR silent species. 

Assembly of the (/i-oxo)di-iron(III) and tyrosyl radical cofactor in RNR-R2 has been extensively studied. Reaction of the apoenzyme and iron(II) with dioxygen results in a putative peroxo species that rapidly collapses to intermediate X with injection of an external electron. 

Galactose oxidase (GO) catalyses the two-electron oxidation of primary alcohols to aldehydes. It contains a single type II copper centre. The enzyme employs the metal and a protein radical cofactor to effect the chemistry. The crystal structure shows a square pyramidal five-coordinate copper site with the metal coordinated by two histidines, two tyrosines and a water or acetate ligand. The equatorial tyrosine, Tyr272, has an interesting crosslink to a cysteine group ortho to the tyrosine oxygen. 

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. 

Saleh L, Krebs C, Ley BA, Naik S, Huynh BH, Bollinger Jr JM. 2004. Use of a chemical trigger for electron transfer to characterize a precursor to cluster X in assembly of the iron-radical cofactor of Escherichia coli rihonucleotide reductase. Biochemistry 43 5953-5964. 

Vitamin E (tocopherol) is the most important antioxidant in the body, acting in the lipid phase of membranes and protecting against the effects of free radicals. Vitamin K functions as cofactor to a carboxylase that acts on glutamate residues of clotting factor precursor proteins to enable them to chelate calcium. 

Copper is an essential trace element. It is required in the diet because it is the metal cofactor for a variety of enzymes (see Table 50—5). Copper accepts and donates electrons and is involved in reactions involving dismu-tation, hydroxylation, and oxygenation. However, excess copper can cause problems because it can oxidize proteins and hpids, bind to nucleic acids, and enhance the production of free radicals. It is thus important to have mechanisms that will maintain the amount of copper in the body within normal hmits. The body of the normal adult contains about 100 mg of copper, located mostly in bone, liver, kidney, and muscle. The daily intake of copper is about 2—A mg, with about 50% being absorbed in the stomach and upper small intestine and the remainder excreted in the feces. Copper is carried to the liver bound to albumin, taken up by liver cells, and part of it is excreted in the bile. Copper also leaves the liver attached to ceruloplasmin, which is synthesized in that organ. 

Methylmalonyl-CoA mutase (MCM) catalyzes a radical-based transformation of methylmalonyl-CoA (MCA) to succinyl-CoA. The cofactor adenosylcobalamin (AdoCbl) serves as a radical reservoir that generates the S -deoxyadenosine radical (dAdo ) via homolysis of the Co—C5 bond [67], The mechanisms by which the enzyme stabilizes the homolysis products and achieve an observed 1012-fold rate acceleration are yet not fully understood. Co—C bond homolysis is directly kineti-cally coupled to the proceeding hydrogen atom transfer step and the products of the bond homolysis step have therefore not been experimentally characterized. 

Co within all compounds of the so-called cobalamin (or B12) family. The biological functions of cobalamin cofactors are defined by their axial substituents either a methyl or an adenosyl group. Both cofactors participate in biosynthesis the former in methyl transfer reactions while the latter is a free radical initiator, abstracting H atoms from substrates. Decades after their initial characterization, the fascination with the biological chemistry of cobalamins remains.1109... 

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. 

As presented in Table II, no quinones are obtained with NADPH for dibenz[a,h]anthracene and benz[a]anthracene, whereas with cumene hydroperoxide a trace amount of benz[a]anthracene quinone is observed. For the PAH with low IP, quinones are formed in the presence of both cofactors. The relationship between IP and formation of quinones constitutes further evidence that these metabolites are obtained by an initial one-electron oxidation of the PAH with formation of its radical cation. 

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