Tryptophan radical reactions

The COs radical anion is a strong one-electron oxidant ( 7-1.7 V vs NHE [15]) that oxidizes appropriate electron donors via electron transfer mechanisms [103]. Detailed pulse radiolysis studies have shown that carbonate radicals can rapidly abstract electrons from aromatic amino acids (tyrosine and tryptophan). However, reactions of CO3 with S-containing methionine and cysteine are less efficient [104-106]. Hydrogen atom abstraction by carbonate radicals is generally very slow [103] and their reactivities with other amino acids are negligible [104-106]. 

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... 

Deeble DJ, Schuchmann MN, Steenken S, von Sonntag C (1990) Direct evidence for the formation of thymine radical cations from the reaction of SO/" with thymine derivatives a pulse radiolysis study with optical and conductance detection. J Phys Chem 94 8186-8192 DeFelippis MR, Murthy CP, Faraggi M, Klapper MH (1989) Pulse radiolytic measurement of redox potentials the tyrosine and tryptophan radicals. Biochemistry 28 4847-4853 Delatour T, Douki T, D Ham C, Cadet J (1998) Photosensitization of thymine nucleobase by benzo-phenone through energy transfer, hydrogen abstraction and one-electron oxidation. J Photo-chem Photobiol 44 191-198... 

Morimoto A, Tanaka M, Takahashi S et al (1998) Detection of a tryptophan radical as an intermediate species in the reaction of horseradish peroxidase mutant Phe221Trp and hydrogen peroxide. J Biol Chem 273 14753-14760... 

Hiner ANP, Martmez JI, Amao MB et al (2001) Detection of a tryptophan radical in the reaction of ascorbate peroxidase with hydrogen peroxide. Eur J Biochem 268 3091-3098... 

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. 

Putsch, S., Lendzian, F., Ingemarson, R., Homberg, A., Thelander, L., Lubitz, W., Lassmann, G., and Gr%oslund, A., 1999, The iron-oxygen reconstitution reaction in protein R2-tyr-177 mutants of mouse ribonucleotide reductaseoEPR and electron nuclear double resonance studies on a new transient tryptophan radical. J. Biol. Chem. 274 17696nl7704. 

Santus R., Patterson L.K., Hug G.L., Bazin M., Maziere J.C., Morliere P., Interactions of superoxide anion with enzyme radicals kinetics of reaction with lysozyme tryptophan radicals and corresponding effects on tyrosine electron transfer. Free Rad. Res., 2000,33,383-391. 

Reactions are fast with aromatic residues (k= 10 mol 1 s f). They begin by OH addition to the ring. With phenylalanine, addition occurs preferentially on the ortho position relative to -CH2CHCONH group (55). For tyrosine, addition takes place mostly at the ortho position relative to the phenol function. However, in both cases additions can also be expected on other positions. In tryptophan, the indole ring is more reactive than the phenyl one. OH-adducts may release OH giving phenoxyl (from tyrosine) or indolyl (from tryptophan) radicals whose absorption spectra are well characterized (table 3). 

A. Morimoto, M. Tanaka, S. Takahashi, K. Ishimori, H. Hori, I. Morishima, Detection of a Tryptophan Radical as an Intermediate Species in the Reaction of Horseradish Peroxidase Mutant (Phe-221 Trp) and Hydrogen Peroxide. J. Biol. Chem., 273 (1998) 14753-14760. 

P. J. O Malley and D. A. Ellson, Chem. Phys. Lett., 260,492 (1996). The Calculation of H, C, N Isotropic and Anisotropic Hyperfine Interactions for the 3-Methyl Indole Cation and Neutral Radicals Using Hybrid Density Functional Methods Models for In Vivo Tryptophan-Based Reactions. 

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