Amino acid free radicals

Fig. 3. Nature of free radical associated with compound I in peroxidases/catalases. Structure of first intermediate, following peroxide addition to ferric peroxidases and catalases. Boxes denote porphyrin ring. The amino-acid free radicals are depicted as protonated (tryptophan) and deprotonated (tyrosine), although this is yet to be conclusively determined.

One might hope to see evidence in the 3D structure for both the ferric-ferryl conversion and the amino-acid free radical. However, X-ray crystallography of proteins cannot resolve individual hydrogen atoms. Therefore it is important to remember [104] that as the oxidation of cytochrome c peroxidase to compound I involves no change in the number of non-hydrogen atoms, it is possible that... 

With ferryl myoglobin, in contrast to peroxidases, the reactions of the protein free radicals and that of the ferryl haem can be considered as uncoupled from each other. The protein has not been designed to form a cation radical for a specific reaction therefore not only is more than one cation free radical generated, but there is no control over their subsequent reactions. A similar situation can be observed in cytochrome c peroxidase mutants that have lost tryptophan-191. A different amino-acid free radical is still formed that is less stable. Indeed, even in the presence of tryptophan-191, small amounts of other free radicals are formed [237] this is further evidence that even in enzymes it is difficult to exclusively control free radical reactions. 

Whenever Compound I is present, the enzyme undergoes slow inactivation by a gradual one-electron reduction of Compound I to Compound II (formal oxidation state of Fe is -i-4) by external electron donors or, in their absence, even by elements of the enzyme molecule itself in this case an amino acid free radical is formed on the tyrosine residue distant from the active site (Tyr 370 in the human enzyme). Compound II is an inactive form of CAT and cannot be recycled by hydrogen peroxide. NADPH prevents formation of Compound II by serving as an electron donor to reduce Compound I. When NADPH is unavailable, the one-electron reduction by an external donor or of the porphyrin TT-cation by an electron from Tyr 370 and formation of Compound II appears to be the next alternative [ 199]. 

The brown pigments which were formed through the oxidative pathway are almost twice those formed through the anaerobic pathway. This effect could be derived from the involvement of H2O2 and oxy--radicals in the aerobic oxidation pathway of ascorbic acid (20,21). The oxy-radicals may generate carbonyls and amino acid free radicals which, by polymerization, could accelerate the formation of more brown pigments. 

Spectral properties of some important amino-acid free radicals in peptides and... 

N.K. King, F.D. Looney, and M.E. Winfield, Amino acid free radicals in Oxidised metmyoglobin, Biochim, Biophys. Acta 133 65 (1967). 

Fig. 1. a-Oxidation of amino acids. Hydroxyl radical (or other reactive radical) abstracts hydrogen atom from the a-carbon. The C-centered free radical formed may react with other amino acid residues or dimerize in the absence of oxygen, which leads to protein aggregation. In die presence of oxygen the carbon-centered radical forms peroxyl radical. Reduction of peroxyl radical leads to protein hydroperoxide. Decomposition of hydroperoxide leads to formation of carbonyl compounds via either oxidative deamination or oxidative decarboxylation. Oxidation of the new carbonyl group forms a carboxyl group. 

Polyethylene (Section 6 21) A polymer of ethylene Polymer (Section 6 21) Large molecule formed by the repeti tive combination of many smaller molecules (monomers) Polymerase chain reaction (Section 28 16) A laboratory method for making multiple copies of DNA Polymerization (Section 6 21) Process by which a polymer is prepared The principal processes include free radical cationic coordination and condensation polymerization Polypeptide (Section 27 1) A polymer made up of many (more than eight to ten) amino acid residues Polypropylene (Section 6 21) A polymer of propene Polysaccharide (Sections 25 1 and 25 15) A carbohydrate that yields many monosacchande units on hydrolysis Potential energy (Section 2 18) The energy a system has ex elusive of Its kinetic energy... 

A waterborne system for container coatings was developed based on a graft copolymerization of an advanced epoxy resin and an acryHc (52). The acryhc-vinyl monomers are grafted onto preformed epoxy resins in the presence of a free-radical initiator grafting occurs mainly at the methylene group of the aHphatic backbone on the epoxy resin. The polymeric product is a mixture of methacrylic acid—styrene copolymer, soHd epoxy resin, and graft copolymer of the unsaturated monomers onto the epoxy resin backbone. It is dispersible in water upon neutralization with an amine before cure with an amino—formaldehyde resin. 

The use of free-radical reactions for this mode of ring formation has received rather more attention. The preparation of benzo[Z)]thiophenes by pyrolysis of styryl sulfoxides or styryl sulfides undoubtedly proceeds via formation of styrylthiyl radicals and their subsequent intramolecular substitution (Scheme 18a) (75CC704). An analogous example involving an amino radical is provided by the conversion of iV-chloro-iV-methylphenylethylamine to iV-methylindoline on treatment with iron(II) sulfate in concentrated sulfuric acid (Scheme 18b)(66TL2531). 

Coating materials may be based on short or medium-oil alkyds (e.g. primers for door and window frames) nitrocellulose or thermoplastic acrylics (e.g. lacquers for paper or furniture finishes) amino resin-alkyd coatings, with or without nitrocellulose inclusions, but with a strong acid catalyst to promote low temperature cure (furniture finishes) two-pack polyurethanes (furniture, flat boards) unsaturated polyester resins in styrene with free-radical cure initiated by peroxides (furniture) or unsaturated acrylic oligomers and monomers cured by u.v. radiation or electron beams (coatings for record sleeves paperback covers, knock-down furniture or flush interior doors). 

One type of fatty liver that has been smdied extensively in rats is due to a deficiency of choline, which has therefore been called a lipotropic factor. The antibiotic puromycin, ethionine (a-amino-y-mercaptobu-tyric acid), carbon tetrachloride, chloroform, phosphorus, lead, and arsenic all cause fatty liver and a marked reduction in concentration of VLDL in rats. Choline will not protect the organism against these agents but appears to aid in recovery. The action of carbon tetrachloride probably involves formation of free radicals... 

Excellent biological arguments exist for a direct impact of fever specifically on neurological outcome. On a local level, fever produces increased levels of excitatory amino acids (e.g., glutamate and dopamine), free radicals, lactic acid, and pyr-uvate. There is an increase in cell depolarizations and BBB breakdown. Enzymatic function is impaired and cytoskeletal stability reduced. These events lead to increased cerebral edema, with a possible reduction in CPP as well as larger volumes of ischemic injury. " ... 

Once the initial event occurs, secondary events occur at the cellular level that contribute to cell death. Regardless of the specific initiating event, the cellular processes that follow may be similar. Excitatory amino acids such as glutamate accumulate within the cells, causing intracellular calcium accumulation. Inflammation occurs and oxygen free radicals are formed ending in the common pathway of cell death. 

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