Radical cysteinyl

Griffin and co-workers (30) measured the high-field EPR spectrum of o-(methylthio)-p-cresyl as a model for the tyrosyl Y272, which is covalently cross-linked to a cysteinyl residue C228 in GO. The radical spin density is localized on... 

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. 

In the vascular system, a particularly important fate of peroxynitrite may be its rapid reaction with Hb-Fe(II)-02. Studies by Romero et al. have led to the proposal of a mechanism for the formation of Hb-Fe(III), involving the displacement of 02 - from Hb-Fe(TI)-02 by ONOO, with the formation of Hb-Fe(III)-ONOO. This transient decays mainly to NO and Hb-Fe(III), but ca. 10 % gives Hb-Fe(IV) = 0 plus N02. Tyrosyl and cysteinyl radicals, trapped using MNP and DMPO, respectively, were proposed to arise via electron transfer from the protein moiety to the ferryl haem.97 In an earlier study, a mechanism involving N02 generation had been suggested for the formation of Hb-Fe(IV) = 0 from Hb-Fe(II)-02 by ONOOH.98... 

Peroxides generated on peptides using l02 can also inactivate enzymes by a non-radical mechanism, possibly involving the oxidation of cysteinyl thiol resi-... 

This reaction is accomplished by several types of iron ribonucleoside reductase (RNR) [1,5,7,13,118], One of the best-characterized RNRs (from E. coli) contains two homodimeric protein components, R1 and R2. The R2 protein comprises an oxygen bridged dinuclear Fe(III) in its oxidized form [119], All RNRs promote the formation of a stable organic radical, which, eventually, leads to the abstraction of a hydrogen atom from the ribose. In the case of E. coli RNR, the latter is accomplished by the R1 protein, specifically by a cysteinyl residue a redox active cystine, also part of Rl, provides the required reducing equivalents (Figure 25). 

The transient cysteinyl radical on the Rl protein is produced by a stable but remote tyrosyl radical on the R2 protein. The role of the dinuclear metal... 

Free radicals through lipid peroxidation can cause membrane damage, induce electrolyte imbalance and edema. Indeed, children with kwashiorkor display low levels of polyunsaturated fatty acids (e.g., linoleic acid) in the erythrocyte membrane compared to the marasmic children, presumably due to increased lipid peroxidation (Leichsenring et al., 1995). Interestingly, cysteinyl leukotrienes, which can cause edema by altering capillary permeability, are also enhanced in those with kwashiorkor but not marasmic children (Mayatepek et al., 1993). 

Recently it was found that the reaction employs intermediary nitrogen-centered free radical formation via thermal homolysis of the N—Cl bond (H11). The stable chloramine T was effectively used for relatively specific oxidation of cysteinyl residues exposed to the surface of the protein molecule (S25). 

The other amino acid residue present in proteins that is susceptible to oxidation is the indole moiety of tryptophan (Fig. 11). The reducing potential of tryptophan is considerably less than that of cysteine and methionine, so oxidation of tryptophanyl residues usually does not occur until all exposed thiol residues are oxidized. Also, the spontaneous oxidation of tryptophanyl residues in proteins is much less probable than that of cysteinyl and methionyl residues. Tryptophan residues are the only chromophoric moieties in proteins which can be photooxi-dized to tryptophanyl radicals by solar UV radiation, even by wavelengths as long as 305 nm (B12). Tryptophanyl residues readily react with all reactive oxygen species, hypochlorite, peroxynitrite, and chloramines. Oxidative modifications of other amino acid residues require use of strong oxidants, which eventually are produced in the cells. Detailed mechanisms of action of these oxidants is described in subsequent sections of this chapter. 

Persson, A. L., Sahlin, M., and Sj berg, B.-M., 1998, Cysteinyl and substiate radical formation in active site mutant E441Q of Escherichia coli class I ribonucleotide reductase. J. Biol. Chem. 273 31016fi31020. 

X. Shi et al., Generation of free radicals from model lipid hydroperoxides and H2O2 by Co(ll) in the presence of cysteinyl and histidyl chelators. Chem. Res. Toxicol., 6 (1993) 277-83. 

In all three classes of ribonucleotide reductases, a cysteinyl radical (in the E. coli RNRl sequence at position Cys ) abstracts a hydrogen atom from the C3 position of the carbohydrate moiety of the ribonucleotide substrate [3]. Biomimetic model studies of this enzymatic process were designed, achieving intramolecular hydrogen transfer within a tetrahydrofurane-appended thiyl radical (Scheme 3.4 Reactions (3.27) and (3.28) [75, 76]. 

In kinetic NMR experiments, rate constants for the intermolecular hydrogen transfer from several carbohydrates to cysteinyl radicals were found to be of the order of k29 = (1-3) x lO s at 37 °G [77]. These values agree with previous, pulse radiolytically determined rate constants for thiyl radical-mediated hydrogen abstraction from various model alcohols and ethers [74, 78, 79]. In contrast, the reverse reaction, hydrogen transfer from thiols to carbohydrate radicals, proceeds with k 29 > 10 s [80, 81], indicating that equilibrium (3.29) is normally lo-... 

---