Tyrosyl radical generation

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],... 

H15. Heinecke, J. W., Li, W., Francis, G. A., and Goldstein, J. A., Tyrosyl radical generated by myeloperoxidase catalyzes the oxidative cross-linking of proteins. J. Clin. Invest. 91, 2866-2872 (1993). 

Figure 10 The proposed reaction pathways for the tyrosyl radical generated upon phenolic hydrogen atom abstraction by the 5 -deoxyadenosyl radical in ThiH. Heterolytic and homolytic Ca-C/3 bond cleavage events are shown. Both pathways converge in the ultimate formation of glyoxylate and cresol.

By oxidation with copper or hypochlorous acid, HDL loses its ability to remove cholesterol from cultured cells. This effect would be expected to be detrimental and promote cardiovascular disease. In contrast, however, HDL becomes more effective in protecting mice against atherosclerosis by exposing it to tyrosyl radicals generated by myeloperoxidase, a heme protein seereted by macrophages in human atherosclerotic lesions. The resulting tyrosylated HDL... 

Savenkova, M. L, Mueller, D. M., and Heinecke, J. W., 1994, Tyrosyl radical generated by myeloperoxidase is a physiological catalyst for the initiation of lipid peroxidation in low density lipoprotein, J. Biol. Chem. 269 20394-20400. 

With regard to the diiron site, its most interesting features involve the mechanism of tyrosyl radical generation. In its fully oxidized form the site is diferric and EPR mute. The reaction starts with the diferrous site which is proposed to be carboxylate bridged. In this form oxygen is activated and forms, via a possible peroxo intermediate, after electron transfer a species denoted cluster X . In this form the diiron site is formally Fe(IV)Fe(III) and EPR active. From cluster X the tyrosyl radical at a close-by residue is formed. These mechanisms, which had been well studied in the past for the E. coli RNR, have recently been characterized also for RNR from mouse by stopped-flow absorption and rapid-freeze EPR spectroscopy. A combined EPR and crystallographic study in an E. coli mutant... 

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. 

Figure 13.3 The three classes of RNRs utilize their metallo-cofactors to generate an active site thiyl radical (S). The diferric-tyrosyl radical in Class I enzymes is on the R2 subunit, at best 35 A from the thiyl radical (on subunit Rl). (FromStubbeetal., 2001. Copyright 2001, with permission from Elsevier.)...

RNR catalyzes the conversion of ribonucleotides to deoxyribonucleo-tides. Because of this central position in generating precursors for DNA synthesis, the enzyme has been the target for pharmacological modification. The reaction has been proposed to proceed through a radical mechanism involving the cyclic transfer of a preexisting radical in the enzyme to the substrate and back (Stubbe, 1989). Several classes of RNR have been identified on the basis of the type of radical and associated metallocenter. For the purposes of this chapter, only the E. coli enzyme, which has a tyrosyl radical and a binuclear iron cluster, is considered. 

Conversion of ribonucleotides to deoxyribonucleotides is an important process that occurs by several pathways, and in one of these, tyrosyl radicals 71 are formed, which serve to generate thiyl radicals from cysteine residues (equation A mechanism for this process has been proposed by... 

For several years it has been known that a tyrosyl radical is generated on cyt c... 

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... 

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