Amino acids Tyrosyl radical

The realization of the widespread occurrence of amino acid radicals in enzyme catalysis is recent and has been documented in several reviews (52-61). Among the catalytically essential redox-active amino acids glycyl [e.g., anaerobic class III ribonucleotide reductase (62) and pyruvate formate lyase (63-65)], tryptophanyl [e.g., cytochrome peroxidase (66-68)], cysteinyl [class I and II ribonucleotide reductase (60)], tyrosyl [e.g., class I ribonucleotide reductase (69-71), photosystem II (72, 73), prostaglandin H synthase (74-78)], and modified tyrosyl [e.g., cytochrome c oxidase (79, 80), galactose oxidase (81), glyoxal oxidase (82)] are the most prevalent. The redox potentials of these protein residues are well within the realm of those achievable by biological oxidants. These redox enzymes have emerged as a distinct class of proteins of considerable interest and research activity. 

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). 

ELECTROSTRICTION DIETHYL PYROCARBONATE Diferric-tyrosyl radical cofactor, REDOX-ACTIVE AMINO ACIDS... 

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. 

Peroxidases oxidize tyrosines, both as a free amino acid and as a residue in peptides and proteins. When proteins are treated with HRP in the presence of hydrogen peroxide, protein dimers are obtained through the coupling of tyrosyl radicals. HRP can also be used for cross-linking of proteins with polysaccharides [35]. In this case, coupling occurs between a tyrosyl radical in the protein and a radical species on the saccharide ... 

On the basis of crystallographic structures and amino acid sequence analysis, a possible electron transfer pathway connecting the tyrosyl radical in the R2 subrmit... 

Proton ENDOR is a useful probe of the origin of the EPR signals from organic-based radicals. In a series of cw ENDOR studies including samples with isotopically labeled amino acids, proton ENDOR was used to identify the amino acid radical in cytochrome-c peroxidase (CcP) as a tryptophan species. Similarly, isotopic labeling was used to identify the tyrosyl... 

The active site of this enzyme in its Cu(II) form contains fom amino acids, two tyrosins one of which is a tyrosyl radical, two histidines, and one solvent molecule that together form a five-coordinate metal complex (54,76). During the reduction-oxidation reactions, the coordination sphere changes. In the Cu(I) state, only three amino adds are coordinated to the central cation. The unusual two-electron oxidation reaction involves the reduction of the central Cu(II) cation and the tyrosyl radical. 

Tyrosine is one of the key amino acids to form covalent linkages to DNA bases, upon exposure to ionizing radiation or in the presence of fi ee radicals [40], The mechanisms are believed to start by the formation of a tyrosyl radical, through removal of the phenolic hydrogen, which then reacts with either a non-radical DNA base, or via a radical-radical elimination reaction with a radical center on the DNA base in question [40], Of the bases, thymine displays the hipest reactivity. 

Lippard et al. proposed several possible mechanisms of the alkane hydroxylation [9]. One possible mechanism is shown in Scheme 3, in which an T 2,T 2-peroxo-bridged diiron(III) acts as an active intermediate which directly transfers oxygen to an alkane substrate [9]. This mechanism suggests the participation of a mercapto radical of Cys-151. This amino acid occupies the equivalent region of space to the tyrosyl radical of ribonucleotide reductase, as indicated by sequence homology and the X-ray crystallographic results [53, 55]. 

The photoionization of the amino acid tyrosine in alkaline solution was studied by CW TR EPR. The photoionization of deprotonated tyrosine leads to a spin-polarized emissive/absorptive CIDEP spectrum produced by the radical-pair mechanism, with the tyrosyl radical in emission and the solvated electron in absorption, which implies a triplet precursor. The exchange interaction J is found to be negative for this radical pair. The triplet photoionization channel is determined to be monophotonic. The singlet channel of the photoionization of deprotonated tyrosine is only seen upon the addition of the electron acceptor 2-bromo-2-methylpropionic acid (BMPA) to the sample. The singlet channel is isolated by performing TREPR on a sample containing tyrosine, BMPA and a triplet quencher (2,4-hexadienoic add). This channel is also found to be monophotonic. 

---