Radicals amino acid

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. 

Enzymes containing amino acid radicals are generally associated with transition metal ions—typically of iron, manganese, cobalt, or copper. In some instances, the metal is absent it is apparently replaced by redox-active organic cofactors such as S -adenosylmethionine or flavins. Functionally, their role is analogous to that of the metal ion in metalloproteins. 

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). 

The next five transition metals iron, cobalt, nickel, copper and zinc are of undisputed importance in the living world, as we know it. The multiple roles that iron can play will be presented in more detail later in Chapter 13, but we can already point out that, with very few exceptions, iron is essential for almost all living organisms, most probably because of its role in forming the amino acid radicals required for the conversion of ribonucleotides to deoxyribonucleotides in the Fe-dependent ribonucleotide reductases. In those organisms, such as Lactobacilli6, which do not have access to iron, their ribonucleotide reductases use a cobalt-based cofactor, related to vitamin B12. Cobalt is also used in a number of other enzymes, some of which catalyse complex isomerization reactions. Like cobalt, nickel appears to be much more extensively utilized by anaerobic bacteria, in reactions involving chemicals such as CH4, CO and H2, the metabolism of which was important... 

Once activated, MV-CCP reacts with 1 equiv of H2O2 in a bimolecu-lar reaction, presumably to form compound 0. In YCCP and HRP this species is referred to as compound ES or compound I, respectively, and contains oxyferryl heme and either a porphyrin n -cation radical (HRP) or an amino acid radical (YCCP). However, the presence of an extra reducing equivalent on the second heme in CCP suggests that such an oxidizing radical species close to the active site heme will be very shortlived and readily form compound I (Fig. 10), which is formally Fe(HI) Fe(IV)=0. The bimolecular rate constant for compound I formation is reported to be very close to the diffusion limit (84). 

Barlow, C. K. Moran, D. Radom, L. McFadyen, W. D. O Hair, R. A. J. Metal-Mediated Formation of Gas-Phase Amino Acid Radical Cations. J. Phys. Chem. A 2006, 110, 8304-8315. 

In photosynthesis radical-ions and triplet states of the pigments, radical-pairs and biradicals involving various chlorophylls and quinones, amino acid radicals, hemes in cytochromes, metal clusters of low and higher nuclearity and even coupled metallo-radical species have been observed. Thus the field of photosyn-... 

Scission of alkoxy radicals (12) generated from nitrate esters and BU3S11H furnish a-amino acid radicals (13) (Scheme 4). This new method for forming a-amino acid radicals may be useful for generating site-specific radicals in peptides.25 The reactions of C(2) glyceryl radicals (14) have been observed by EPR. Whereas the phosphate derived radical (14a) gave the reduced product (15a) in 70% yield, the unsubstituted... 

It is necessary that an antioxidant protects cells at all stages of oxidative stress, and therefore an antioxidant should be able to scavenge the secondary radicals produced by the reaction of primary radicals with biomolecules. Radiation chemists designed methods to study reactions of secondary radicals from amino acids of proteins and base and sugar radicals of DNA with antioxidants.The most commonly employed aromatic amino acid radicals generated by radiation chemical experiments are the indolyl radicals of tryptophan (TRP ), the... 

Filipe P, Morliere P, Patterson LK, Hug GL, Maziere J-C, Maziere C, Freitas JP, Fernandes A, Santus R. (2002) Repair of amino acid radicals of apolipopro-tein BlOO of low-density Kpoproteins by flavonoids. A pulse radiolysis study with quercetin and rutin. Biochemistry A 11057-11064. 

Free radical species that are capable of initiating vinyl polymerization reactions have been identified as Cu -co-ordinated amino-acid radicals, and these are produced in the primary photoreactions of the complex. An examination of the quantum yield and photodecomposition stoicheiometry of Cu (H2Aib)3 (H2Ajb= a-aminoisobutyric acid) as a function of irradiation wavelength and medium conditions has shown that 7r-copper -amidyl radicals are the primary photoproducts. The behaviour of other Cu -peptide complexes suggests that these photochemical parameters are dependent on the peptide chain-length and the number of a-carbon methyl substituents. ... 

Electron transfer from the amine to flavin would result in the aminium radical which is expected to rearrange rapidly to radical 61. Inactivation of the enzyme would then occur via coupling of the radical with the flavin radical anion resulting in the formation of 66. Coupling of the aminium radical with an amino acid radical would result in the formation of 65. By use of radioactive labeling techniques Silverman et al. have confirmed the formation of 65 and 66 this confirms the role of electron transfer in the oxidation process. Similar studies have been performed using 1-phenylcyclobutylamine (Scheme 18) [198]. 

Organic free radicals are key intermediates in a number of reactions of biological significance. For istance, there is strong evidence that the biosynthesis of several natural substances and many enzymatic reactions involve amino acid radicals[126-129], and radiation damage to DNA is known to proceed through a number of base-centered radicals[130-132]. Furthermore organic free radicals can be exploited as spin probes in the study of macromolecular systems by means of EPR spectroscopy [133]. 

Chiral auxiliary-mediated diastereoselective allylations of a-bromoglycine derivatives 65 have also been established. 8-Phenylmenthol has been successfully employed as a chiral auxiliary in glycine allylations (Eq. (13.19)) [29]. The captoda-tive radical intermediate generated in this reaction benefits from the observation that a-amino acid radicals prefer an s-cis geometry about the single bond, presum-... 

Fe =0 moiety to Fe" =0 thus, one oxidizing equivalent remained at the heme iron and one was stored on the polypeptide as an amino acid radical in CCP compound I. The formation of Fe =0 has not been validated experimentally for CCP, or any other heme peroxidase, possibly because of its rapid intramolecular reduction to Fe =0. The formation of Fe =0 is not, however, a requirement in the mechanisms of compound I formation discussed later, because the porphyrin or protein (via the porphyrin. Section IV) could directly donate an electron to the peroxide. 

Nature provides striking examples of each of the types of PCET discussed in this chapter. Enzymes often rely on PCET to affect primary metabolic steps involving charge transport and catalysis. Amino acid radical generation and transport is synonymous with PCET [187], as is the activation of substrate bonds at enzyme active sites [29]. PCET is especially prevalent for metallo-cofactors that activate... 

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