Tyrosine-phenoxyl radical

Phenoxyl radicals are oxidizing radicals (for a compilation of redox potentials see Wardman 1989). Thus, in their reactions with 02 (E7 = -0.3 V) there is ample driving force for a reduction by ET [cf. reaction (16)], and this has been thought for a long time to be the only (Hunter et al. 1989) or at least a major process, depending on the reduction potential of the (substituted) phenoxyl radical (Jonsson et al. 1993). Yet in the tyrosine system, despite of the high reduction potential of tyrosine phenoxyl radical (E7 = 0.64 V), the by far dominating process is addition, and the intermediate adduct is locked in by a Mannich reaction [reactions (14) and (15) Jin et al. 1993],... 

Hug GL, Bonifacic M, Asmus K-D, Armstrong DA (2000a) Fast decarboxylation of aliphatic amino adds induced by 4-carboxybenzophenone triplets in aqueous solutions. A nanosecond laser flash photolysis study. J Phys Chem B 104 6674-6682 Hug GL, Carmichael I, Fessenden RW (2000b) Direct EPR observation of the aminomethyl radical during the radiolysis of glycine. J Chem Soc Perkin Trans 2 907-908 Hunter EPL, DesrosiersMF, Simic MG (1989) The effect of oxygen, antioxidants and superoxide radical on tyrosine phenoxyl radical dimerization. Free Rad Biol Med 6 581-585 Ito O (1992) Flash photolysis study for reversible addition reactions of thiyl radicals with olefins and acetylenes. Trends Phys Chem 3 245-266... 

Hunter EPL, Desrosiers MF, Simic MG (1989) The effect of oxygen, antioxidants and superoxide radical on tyrosine phenoxyl radical dimerization. Free Rad Biol Med 6 581-585... 

EPR spectroscopy has demonstrated that the oxidation of the globin occurs ultimately at a tyrosine residue, resulting in the formation of a tyrosine-phenoxyl radical this species is postulated to react subsequently with oxygen to give a tyrosine-peroxyl radical. Studies have shown that both of these species are accessible to components in bulk solution, i.e. they are located on the surface of the protein [47-49]. Ferryl haem protein radical from myoglobin and haemoglobin can react with membranes [33,45,50] and lipoproteins [51-... 

It has been proposed that the second oxidizing equivalent is accepted by tyrosine-103 on the surface of the protein (Ortiz de Montel-lano, 1983) forming a tyrosine phenoxyl radical which rapidly takes up oxygen forming the peroxyl species (Davies, 1990) as depicted in Fig. 4.4. 

It has been shown by pulse radio lysis that in peptides and enzymes containing both tryptophan (Trp) and tyrosine (Tyr) the radical produced by oxidation of tryptophan can efficiently oxidize tyrosine to yield the tyrosine phenoxyl radical TyrO (equation 17) ... 

Fit . 4. EPR spectrum observed immediately after mixing soybean Fe(III) Lb (250 fj.M) with HA (250 p,M) at pH 7.4. Reaction studied using a two-way stopped-flow mixing system inserted into the cavity of the EPR spectrometer. The signal is assigned to a sterically constrained tyrosine phenoxyl radical formed at position 133 (reproduced with permission from Davies, M. J. Puppo, A. Biochem. J. 1992, 281, 197—201). 

The main source of thiyl radicals in cells is expected to be reaction (1), because of the relative abundance of C-H bonds. Some biological radicals are nitrogen-or oxygen-centered, such as the tryptophan (indolyl) radical in DNA photolyase [45], and indolyl and tyrosine (phenoxyl) radicals in ribonucleotide reductase [46-49]. Whether oxygen-centred radicals such as phenoxyl radicals (PhO, e.g. from tyrosine) oxidize GSH by hydrogen transfer ... 

The poly (amino acid)s with aromatic side chains behave somewhat differently. In poly(phenylalanine) the a-carbon radical is the major radical species observed, but radicals formed by hydrogen atom addition to the ring are also found. Benzyl radicals formed by side-chain cleavage are present, but only in very low yield. In poly (tyrosine) the only radical species observed is the tyrosyl phenoxyl radical formed by loss of the hydroxyl hydrogen. There is no evidence for formation of significant concentrations of a-carbon radicals. Thus, the nature of the substituents can strongly influence the radiation sensitivity of the backbone chain. 

Jin F, Leitich J, von Sonntag C (1993) The superoxide radical reacts with tyrosine-derived phenoxyl radicals by addition rather than by electron transfer. J Chem Soc Perkin Trans 2 1583-1588 Jonsson M, Lind J, Reitberger T, Eriksen TE, Merenyi G (1993) Free radical combination reactions involving phenoxyl radicals. J Phys Chem 97 8229-8233 Jovanovic SV, Simic MG (1985) Repair of tryptophan radicals by antioxidants. J Free Rad Biol Med 1 125-129... 

A case in point is the combination of a Thy -OH-adduct with a tyrosine-derived phenoxyl radical [reaction (186) Simic and Dizdaroglu 1985],... 

IRadical cofactors in biological systems have become a subject of increasing interest in recent years (1-3). Tyrosine-based radicals, in particular, have now been identified in several enzymes (4). The tyrosine residue functions as a redox-active cofactor by interconverting between the oxidized phenoxyl radical and the normal phenol or phenolate states. More commonly known redox-active cofactors include transition metal ions, and a few enzymes use both tyrosine residues and metals as partners in effecting redox chemistry. 

More recently, MPO-mediated oxidation of tyrosine to dityrosine (o o -dityrosine, or 3,3 -diiyrosine) focused attention as a marker reaction of neutrophile-dependent oxidative damage of proteins and peptides (G11, H14, S3). The reaction occurs both with free tyrosine as well as with tyrosyl residues incorporated into polypeptide structures. The mechanism of dityrosine formation utilizes a relatively long-lived phenoxyl radical that cross-links to dimeric and polymeric structures by formation of carbon-carbon bonds between the aromatic moieties of phenolic tyrosine residues (H14) (Fig. 9). 

Similarly, it is possible that tyrosine radicals are stabilized in part by being in a protective environment. Phenoxyl radicals in general are quite reactive and it is surprising that this particular radical should be readily detected in various biological systems (Ehrenberg and... 

Fig. 4.4. Scheme for postulated formation of the tyrosine phenoxyl and peroxyl radical... 

Fig. 16. Spectroscopic characterization of the oxidized apogalactose oxidase free radical, (a) Optical absorption spectrum for the radical-containing apoprotein, (b) X-band EPR spectrum of the metal-free protein following Ir(IV) oxidation, (c) Expansion of the region near g = 2 comparing experimental data (Exp) with a theoretical simulation (Sim) based on coupling of the unpaired electron spin with one and one Hp proton of a tyrosine phenoxyl. Simulation parameters g = 2.0017, g2 = 2.0073 Ai Ha) = 8.4 G, A2(Hc,) = 8.8 G di(Hp) = 12.7 G, A2(Hp) = 13.8 G.

This is markedly different from the behavior of a simple tyrosine phenoxyl, such as that found in ribonucleotide reductase, whose spectrum exhibits a strong rhombic splitting (Fig. 17, line c) but precisely the same as observed for the 0-methylthiocresyl model radical (Fig. 17, line b). This clearly identihes the Tyr-Cys side chain as the site of the oxidized apoGAOX radical and demonstrates that the electronic structure of the thioether-substituted phenoxyl is distinct from that of a simple phenoxyl radical. 

Jin F, Leitich J, von Sonntag C. (1993) The superoxide radical reacts with tyrosine-derived phenoxyl radicals by addition rather than by electron transfer. / Chem Soc, Perkin Trans 2, 1583-1588. 

There are, however, many pieces of evidence that the cysteine link only causes small perturbations in the electronic structure and energetics of tyrosine. Electrochemical experiments by Whittaker et al [31] showed that the p fiTa of o-methylthiocresol was only 0.7 pH units lower than for cresol (9.5 vs. 10.2). Babcock and co-workers have shown, based on EPR and ENDOR experiments on both apo-enzyme and model alkylthio-substituted phenoxyl radicals, that the sulfur cross-link only induces small perturbation in the spin distribution of the tyrosyl radical [32]. No big shift in the g-tensors between unsubstituted and methylthio-substituted radicals was observed. Since this kind of shift is expected when heavy elements carry some of the spin in organic radicals, the conclusion was that the sulfur center possesses only a small part of the unpaired spin. We have conducted ab initio multiconfigurational linear response g-value calculations of unsubstituted and sulfur-substituted phenoxyl radicals and shown that the shift in g-tensor is as small as 0.0008 in the gxx-component (2.0087 vs. 2.0079 in t). The other components were virtually unchanged, thus confirming the experimental results [33]. 

Phenol serves as a basic unit of larger molecules, e.g. tyrosine residues in proteins. The phenoxyl radical is treated as a model system for the tyrosyl radical whose formation via abstraction of the hydrogen atom from the hydroxyl group of tyrosine is a typical feature of oxidative stress in the physiological pH range -... 

All the phenyl radicals, phenoxyl radicals and hydroxycyclohexadienyl radicals produced from phenols by various reactions react with each other and with other radicals to form, at least in part, new C—C or C—O bonds. As a result of these reactions, irradiation of phenols can lead to dimeric and polymeric products and irradiation of phenols in mixtures with other compounds can lead to crosshnking of the two materials. For example, irradiation of tyrosine or dopa with albumin in aqueous solutions leads to binding of these phenols to the protein . Similarly, irradiation of tyrosine and its peptides - or mixtures of tyrosine and thymine led to various dimerization products. The latter case was studied as a model for radiation-induced crosshnking between proteins and DNA. 

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