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Tyrosine radicals

In order for the cyclooxygenase to function, a source of hydroperoxide (R—O—O—H) appears to be required. The hydroperoxide oxidizes a heme prosthetic group at the peroxidase active site of PGH synthase. This in turn leads to the oxidation of a tyrosine residue producing a tyrosine radical which is apparendy involved in the abstraction of the 13-pro-(5)-hydrogen of AA (25). The cyclooxygenase is inactivated during catalysis by the nonproductive breakdown of an active enzyme intermediate. This suicide inactivation occurs, on average, every 1400 catalytic turnovers. [Pg.152]

H. Mino and T. Ono, Applications of pulsed ELDOR-detected NMR measurements to studies of photosystem II Magnetic characterization of Yd tyrosine radical and Mn2+ bound to the high-affinity site, Appl. Magn. Reson., 2003, 23, 571. [Pg.167]

Langen, P., Quenching of tyrosine radicals of M2 subunit from ribonucleotide reductase in tumor cells by different antitumor agents an EPR study, Free Radical Biol. Med. 9 (1990), p. 1-4... [Pg.280]

In the fully reduced model, four electrons are transferred to dioxygen through sequential one-electron oxidations of heme as s iron ion, the Cub ion, the heme a iron ion, and one of the bimetallic center s Cua ions. The sequence of electron transferal differs in the mixed valence model, and a tyrosine radical (tyr) is generated. The proposed formation of a tyrosine radical during catalytic turnover arises from the known post-translational modification in most CcO s in which a covalent bond is formed between the his240 ligand of Cub... [Pg.434]

The question of the molecular basis for the S states has existed since the original proposal by Kok and coworkers. As first formulated, the S state designation referred to the oxidation state of the O2-evolving center which could, in principle, include all of photosystem II and its associated components. Indeed, there are a number of redox-active components on the electron-donor side of photosystem II in addition to the Mn complex, such as the tyrosine radical that gives rise to EPR signal, and cytochrome b jg. [Pg.222]

In eukaryotes, ribonucleotide reductase is a tetramer consisting of two R1 and two R2 subunits. In addition to the disulfide bond mentioned, a tyrosine radical in the enzyme also participates in the reaction (2). It initially produces a substrate radical (3). This cleaves a water molecule and thereby becomes radical cation. Finally, the deoxyribose residue is produced by reduction, and the tyrosine radical is regenerated. [Pg.190]

Hydroxyurea selectively inhibits ribonucleotide reductase (see p. 190). As a radical scavenger, it removes the tyrosine radicals that are indispensable for the functioning of the reductase. [Pg.402]

Figure 6 High Field EPR spectra of radicals occurring in PS II ( Yz obtained at 245 GHz, all other spectra at 285 GHz). For comparison, the spectra of the tyrosine radical in ribonucleotide reductase (RNR) and of irradiated tyrosine hydrochloride crystals n are also shown (for details see reference 30). Note the striking differences in g values for the tyrosyl radicals. Figure reproduced from reference 30 with permission. Figure 6 High Field EPR spectra of radicals occurring in PS II ( Yz obtained at 245 GHz, all other spectra at 285 GHz). For comparison, the spectra of the tyrosine radical in ribonucleotide reductase (RNR) and of irradiated tyrosine hydrochloride crystals n are also shown (for details see reference 30). Note the striking differences in g values for the tyrosyl radicals. Figure reproduced from reference 30 with permission.
The Tyrosine Radicals. - In PS II two tyrosine radicals Y,D and Y z are known. The position of YD and Yz within PS II has been determined by X-ray crystallography 7,19,341 see Fig. 1. Yz is located in between P680 and the tetranuclear Mn-cluster (Mm) in the D1 protein. It is involved in ET between these 2 components and is only observed as a transient radical or in the form of a split signal (see below). YD is found in a symmetry equivalent position to Yz in a hydrophobic pocket of the D2 protein. The Y D is stable in the dark for hours. Y D undergoes slow redox reactions with the lower S states of the OEC (see 4.7) in the dark but is not involved in the main ET pathway that leads to water oxidation. Its detailed function is not fully understood. [Pg.214]

EPR and other spectroscopic work on the tyrosine radicals and their function in PS II has recently been reviewed 345-385-387 in a special issue of Biochim. Biophys. Acta. The authors give their specific views on the research efforts which led to our present knowledge of these highly interesting radicals that are also found in many other radical enzymes .385,388 389 In the following some of the important results described in recent EPR papers are summarized. For further details and references to the earlier literature the reader is referred to the cited review articles. [Pg.214]

The g-tensor and the hfcs of tyrosine radicals have also been calculated by DFT methods.393,406 412 The agreement between the calculated and experimental data is very satisfying in general, which shows the great improvement of modern quantum chemical approaches. [Pg.215]

Miscellaneous Applications of EPR to PS II. - A number of authors used EPR to detect changes of the S-state cycle and the tyrosine radicals and correlated this with other functional aspects of the OEC and PS II. In particular, the prominent S2 state MLS and the tyrosine signals have been used for these investigations. A representative fraction of such work and some other EPR applications from the last years are discussed below. [Pg.222]

Structure originally proposed for form P. Movement of one electron to form tyrosinate radical is indicated by single-headed arrows (see p. 1030)... [Pg.1032]

The emphasis on the study of hemoproteins and the iron-sulfur proteins often distracts attention from other iron proteins where the iron is bound directly by the protein. A number of these proteins involve dimeric iron centres in which there is a bridging oxo group. These are found in hemerythrin (Section 62.1.12.3.7), the ribonucleotide reductases, uteroferrin and purple acid phosphatase. Another feature is the existence of a number of proteins in which the iron is bound by tyrosine ligands, such as the catechol dioxygenases (Section 62.1.12.10.1), uteroferrin and purple acid phosphatase, while a tyrosine radical is involved in ribonucleotide reductase. The catecholate siderophores also involve phenolic ligands (Section 62.1.11). Other relevant examples are transferrin and ferritin (Section 62.1.11). These iron proteins also often involve carboxylate and phosphate ligands. These proteins will be discussed in this section except for those relevant to other sections, as noted above. [Pg.634]

The iron protein has been found in animals, viruses and some prokaryotic organisms. The best studied example is from Escherichia coli.S19 The enzyme consists of two subunits, Bi and B2. Subunit B, has a molecular weight of 160 000 and is made up of two polypeptide chains. It contains redox-active SH groups, which play a role in the reaction, and has two binding sites for substrates and sites for effector molecules. It can bind any of the four ribonucleotide substrates. Subunit B2 has a molecular weight of 78 000 and also has two polypeptide chains. It contains a dimeric oxo-bridged iron site associated with a remarkably stable tyrosine radical. The formation of the active enzyme from B, and B2 requires the presence of magnesium ions. [Pg.635]

The radical is destroyed by hydroxyurea and hydroxylamine, which accounts for the ability of the former compound to inhibit the synthesis of the DNA. Subtraction of the UV-vis spectrum of the hydroxyurea-treated protein B2 from that of the native protein thus gives the spectrum of the tyrosine radical, which shows considerable similarities with the known spectrum of the 2,4,6-tri(2-methylpropane)phenoxy radical. [Pg.635]

The reduction of the 2 -hydroxyl group in the ribose ring is coupled to the cysteine-to-cystine oxidation occurring in the Bt subunit. This process cannot occur directly and other intermediates are necessary, with NADPH as the final reductant. Reduced thioredoxin (or glutaredoxin) transfers electrons to an enzyme disulfide bridge, which in turn reduces the tyrosine radical which then interacts with the 2 -OH group. Little is known about the way in which the 2 -OH group is activated... [Pg.635]


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Radical peroxyl tyrosine phenoxyl

Reduction of Tryptophan radicals by Tyrosine in proteins

Ribonucleotide reductase tyrosine radical

The Tyrosine Radicals

Tyrosine radical reactions

Tyrosine radical, in ribonucleotide

Tyrosine, proton transfer to histidine radicals, in photosystem

Tyrosine-peroxyl radical

Tyrosine-phenoxyl radical

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