Radicals tyrosyl

Ribonucleotide reductase is responsible for the conversion of the four biological ribonucleotides (RNA) into their corresponding deoxy forms (DNA). Although RNR is not an oxygenase during its primary catalyzed reaction (the conversion of ribonucleotides), it activates oxygen to generate a stable tyrosyl radical that is essential to the overall mechanism [46 49]. The common link between the chemistry of MMO and RNR is the activation of O2 by the di-iron active site. 

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. 

Un, S., M. Atta et al. (1995). g-values as a probe of the local protein environment High-field EPR of tyrosyl radicals in ribonucleotide reductase and photosystem II. J. Am. Chem. Soc. 117 10713-10719. 

M. Bennati, A. Weber, J. Antonie, D.L. Perlstein, J. Robblee and J. Stubbe, Pulsed ELDOR spectroscopy measures the distance between the two tyrosyl radicals in the R2 subunit of the E. coli ribonucleotide reductase, J. Am. Chem. Soc., 2003, 125, 14988. 

Heinecke et al. [191] studied the oxidation of L-tyrosine by the H202-MP0 system and showed that the main product of this reaction is dityrosine. They have also found that tyrosine successfully competed with chloride as a substrate for MPO that points out at the possibility of in vivo oxidation of tyrosine by MPO even in the presence of big physiological concentration (0.10-0.1 mol l-1) of chloride in human blood. It was also suggested that the tyrosyl radical formed at the catalytic oxidation of tyrosine by peroxidases may interact with... 

HRP-catalyzed oxidation of tyrosine. ESR spectra of the spin-adducts of tyrosyl radical with 2-methyl-2-nitrosopropane were also obtained in the reactions catalyzed by MPO and LPO. 

Then, tyrosyl radical reacts with bound AA to form a bound A A free radical ... 

In the last step of cyclooxygenase cycle tyrosyl radical is regenerated and PGG2 is released ... 

It has been suggested that only one tyrosine residue, tyrosine 385, is oxidized into tyrosyl free radical in cyclooxygenase cycle. This suggestion was confirmed by NO trapping of this tyrosyl radical generated by prostaglandin H synthase [81]. It was found that the stoichiometry of AA oxidation by prostaglandin H synthase AA/02 is equal to ca. 2 [82,83],... 

Recently, Gunther et al. [41] proposed that nitric oxide may directly react with enzymes without intermediate formation of peroxynitrite. It is known that the oxidation of arachido-nic acid by prostaglandin H oxidase is mediated by the formation of enzyme tyrosyl radical (see Chapter 26). Correspondingly, it has been suggested that NO is able to react with this radical to form the tyrosine iminoxyl radical and then nitrotyrosine. Therefore, the NO-dependent nitration of protein tyrosine residue may occur without the formation of peroxynitrite or other nitrogen oxides. 

Zuckerman SH, Bryan N (1996) Inhibition of LDL oxidation and myeloperoxidase dependent tyrosyl radical formation by the selective estrogen receptor modulator raloxifene (LY139481 HCL). Atherosclerosis 126 65-75... 

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. 

Here, we will first review the physical organic chemistry and spectroscopic features of the ligand phenoxyl and introduce briefly some well-characterized metalloproteins known to contain tyrosyl radicals and then systematically describe the coordination chemistry of uncoordinated and coordinated phenoxyls. Finally, we will describe the reactivity of coordinated phenoxyls toward some organic substrates. 

Since the phenoxyls possess an S = ground state, they have been carefully studied by electron paramagnetic spectroscopy (EPR) and related techniques such as electron nuclear double resonance (ENDOR), and electron spin-echo envelope modulation (ESEEM). These powerful and very sensitive techniques are ideally suited to study the occurrence of tyrosyl radicals in a protein matrix (1, 27-30). Careful analysis of the experimental data (hyperfine coupling constants) provides experimental spin densities at a high level of precision and, in addition, the positions of these tyrosyls relative to other neighboring groups in the protein matrix. 

One of the most powerful spectroscopic techniques for the detection and characterization of persistent and transient phenoxyls is time-resolved resonance Raman (RR) spectroscopy. Vibrational frequencies and the relative intensities of the resonance-enhanced bands have proven to be sensitive markers for tyrosyl radicals in proteins. For example, Sanders-Loehr and co-workers (31) detected the tyrosyl radical in native ribonucleotide reductase from Escherichia coli by a resonance-enhanced Raman mode at 1498 cm 1 that they assigned to the Ula Wilson mode of the tyrosyl, which is predominantly the u(C=0) stretching mode. 

As already mentioned, RNR is the metalloenzyme in which the first definitively characterized stable amino acid radical (1), later identified as a tyrosyl radical, was found in 1972. The RNR enzymes catalyse the reduction of ribonucleotides to their corresponding deoxyribonucleotides utilized in DNA biosynthesis. There are three unique classes of this enzyme, differing in composition and cofactor requirements all of them, however, make use of metal ions and free radical chemistry. Excellent reviews on RNRs are available (60, 61, 70, 89-97). 

The active site of GO contains a covalently modified tyrosyl radical coordinated to a Cu(II) ion constituting thereby a novel mononuclear active site in which both the metal ion and a ligand are redox active. Recently, evidence was provided for the same type of active site in glyoxal oxidase (82), which catalyzes the oxidation of aldehydes to carboxylic acids, Eq. (3). 

In the resonance Raman spectra of GO0X (125), vibrational modes have been assigned to both the tyrosinate ligand (Tyr 495) as well as the tyrosyl radical (Tyr 272). The spectrum does not provide evidence for the speculation that the tyrosyl radical is delocalized onto the jr-stacked tryptophan residue (Trp 290) (126, 127). Recent results of high-frequency EPR measurement (30) on the apogalactose oxidase radical are also consistent with the radical spin density being localized on the modified Tyr 272 moiety only. 

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