Thiyl radical reaction with amino acid side chains: rate constants for hydrogen transfer and relevance for posttranslational protein modification.

Thiyl radicals are prominent intermediates during biological conditions of oxidative stress and have been suggested to be involved in the mutagenic effects of thiols. While several enzymatic processes rely on the formation and selective reactions of protein thiyl radicals with substrates, such reactions may represent a source for biological damage when occurring uncontrolled during oxidative stress. For example, intramolecular hydrogen transfer reactions to protein cysteine thiyl radicals may lead to secondary amino acid oxidation products, which may represent starting points for protein aggregation and/or fragmentation. Here, we have used a kinetic NMR method to determine rate constants, k(sc), for hydrogen transfer reactions between thiyl radicals and amino acid side chain C-H bonds at 37 degrees C. Rate constants cover a range between k(sc) <or= 1 x 10(3) M(-1) s(-1) (Val) and k(sc) = 1.6 x 10(5) M(-1) s(-1) (Ser). On the basis of these values and earlier data, model calculations are performed, which will demonstrate that protein thiyl radicals may attack protein C-H bonds via intramolecular hydrogen transfer at physiological conditions, potentially resulting in irreversible protein damage.