Prooxidative chain transfer activity by thiol groups in biological systems

Cysteine is arguably the best-studied biological amino acid, whose thiol group frequently participates in catalysis or ligand binding by proteins. Still, cysteine's unusual biological distribution has remained mysterious, being strikingly underrepresented in transmembrane domains and on accessible protein surfaces, particularly in aerobic life forms (“cysteine anomaly”). Noting that lipophilic thiols have been used for decades as radical chain transfer agents in polymer chemistry, we speculated that the rapid formation of thiyl radicals in hydrophobic phases might provide a rationale for the cysteine anomaly. Hence, we have investigated the effects of dodecylthiol and related compounds in isolated biomembranes, cultivated human cells and whole animals (C. elegans). We have found that lipophilic thiols at micromolar concentrations were efficient accelerators, but not inducers of lipid peroxidation, catalyzed fatty acid isomerization to trans-fatty acids, and evoked a massive cellular stress response related to protein and DNA damage. These effects were specific for lipophilic thiols and were absent with thioethers, alcohols or hydrophilic compounds. Catalytic chain transfer activity by thiyl radicals appears to have deeply influenced the structural biology of life as reflected in the cysteine anomaly. Chain transfer agents represent a novel class of biological cytotoxins that selectively accelerate oxidative damage in vivo.

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Intramembrane thiol groups are prooxidants that promote lipid peroxidation in vivo.

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Intramembrane thiols also cause protein oxidation, DNA damage and cytotoxicity.

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Thiol groups in hydrophobic biological environments act as chain transfer catalysts.

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Thiol groups do not induce radical formation, but accelerate radical propagation.

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Thiyl radicals are the carriers of biological chain transfer catalysis.

Thiyl radicals and induction of protein degradation

Thiyl radicals are important intermediates in the redox biology and chemistry of thiols. These radicals can react via hydrogen transfer with various C-H bonds in peptides and proteins, leading to the generation of carbon-centered radicals, and, potentially, to irreversible protein damage. This review summarizes quantitative information on reaction kinetics and product formation, and discusses the significance of these reactions for protein degradation induced by thiyl radical formation.

Protein thiyl radical reactions and product formation: a kinetic simulation

Protein thiyl radicals are important intermediates generated in redox processes of thiols and disulfides. Thiyl radicals efficiently react with glutathione and ascorbate, and the common notion is that these reactions serve to eliminate thiyl radicals before they can enter potentially hazardous processes. However, over the past years increasing evidence has been provided for rather efficient intramolecular hydrogen transfer processes of thiyl radicals in proteins and peptides. Based on rate constants published for these processes, we have performed kinetic simulations of protein thiyl radical reactivity. Our simulations suggest that protein thiyl radicals enter intramolecular hydrogen transfer reactions to a significant extent even under physiologic conditions, i.e., in the presence of 30 µM oxygen, 1 mM ascorbate, and 10 mM glutathione. At lower concentrations of ascorbate and glutathione, frequently observed when tissue is exposed to oxidative stress, the extent of irreversible protein thiyl radical-dependent protein modification increases.