Free radicals tyrosyl

The reaction of peroxynitrite with the biologically ubiquitous C02 is of special interest due to the presence of both compounds in living organisms therefore, we may be confident that this process takes place under in vivo conditions. After the discovery of this reaction in 1995 by Lymar [136], the interaction of peroxynitrite with carbon dioxide and the reactions of the formed adduct nitrosoperoxocarboxylate ONOOCOO has been thoroughly studied. In 1996, Lymar et al. [137] have shown that this adduct is more reactive than peroxynitrite in the reaction with tyrosine, forming similar to peroxynitrite dityrosine and 3-nitrotyrosine. Experimental data were in quantitative agreement with free radical-mediated mechanism yielding tyrosyl and nitric dioxide radicals as intermediates and were inconsistent with electrophilic mechanism. The lifetime of ONOOCOO was estimated as <3 ms, and the rate constant of Reaction (42) k42 = 2 x 103 1 mol 1 s 1. 

Other very convincing evidences for free radical-mediated mechanism of decomposition and reactions of peroxynitrite and nitrosoperoxocarboxylate were demonstrated by Lehnig [140] with the use of CIDNP technique. This technique is based on the effects observed exclusively for the products of free radical reactions their NMR spectra exhibit emission characterizing a radical pathway of their formation. Lehnig has found the enhanced emission in the 15N NMR spectra of N03- formed during the decomposition of both peroxynitrite and nitrosoperoxocarboxylate. This fact indicates that N03- was formed from radical pairs [ N02, H0 ] and [ N02, C03 ]. Emission was also observed in the reaction of both nitrogen compounds with tyrosine supposedly due to the formation of radical pair [ N02, tyrosyl ]. 

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

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],... 

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). 

Multireference character, complete active space calculation, 12, 29 Multivariate analysis analytical methods, 624 SIMPLISMA program, 624, 702 Mutations, DNA oxidative damage, 616 Myeloperoxidase, tyrosyl free radicals, 610 Myocardial ischemia, reactive oxygen species in blood, 612... 

Dinuclear iron centres occur in several proteins. They either bind or activate dioxygen or they are hydrolases. Ribonucleotide reductase (RR) of the so-called class I type contains one such centre in the R2 protein in combination with a tyrosyl radical, both being essential for enzymatic activity which takes place in the R1 protein subunit. The diiron centre activates dioxygen to generate the tyrosyl radicals which in turn initiate the catalytic reaction in the R1 subunit. The interplay between the tyrosyl free radical in R2 and the formation of deoxyribonucleotides in R1 which also is proposed to involve a protein backbone radical is a topic of lively interest at present but is outside the scope of this review. Only a few recent references dealing with this aspect are mentioned without any further discussion.158 159 1 1,161... 

Ribonucleotide reductase is notable in that its reaction mechanism provides the best-characterized example of the involvement of free radicals in biochemical transformations, once thought to be rare in biological systems. The enzyme in E. coli and most eukaryotes is a dimer, with subunits designated R1 and R2 (Fig. 22-40). The R1 subunit contains two lands of regulatory sites, as described below. The two active sites of the enzyme are formed at the interface between the R1 and R2 subunits. At each active site, R1 contributes two sulfhydryl groups required for activity and R2 contributes a stable tyrosyl radical. The R2 subunit also has a binuclear iron (Fe3+) cofactor that helps generate and stabilize the tyrosyl radicals (Fig. 22-40). The tyrosyl radical is too far from the active site to interact directly with the site, but it generates another radical at the active site that functions in catalysis. 

Stubbe, J. Riggs-Gelasco, P. (1998) Harnessing free radicals formation and function of the tyrosyl radical in ribonucleotide reductase. Trends Biochem. Sci. 23, 438-443. 

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