Reversible Hydrogen Transfer Reactions of Cysteine Thiyl Radicals in Peptides: the Conversion of Cysteine into Dehydroalanine and Alanine, and of Alanine into Dehydroalanine

Abstract

The photodissociation of disulfide bonds in model peptides containing Ala and Ala-d3 generates a series of photoproducts following the generation of a CysS• thiyl radical pair. These photoproducts include transformations of Cys to dehydroalanine (Dha) and Ala, as well as Ala to Dha. Intramolecular Michael addition of an intact Cys with a photolytically generated Dha results in the formation of cyclic thioethers. The conversion of Cys into Dha likely involves a 1,3-H-shift from the Cys αC-H bond to the thiyl radical, followed by elimination of HS•. The conversion of Dha into Ala most likely involves hydrated electrons, which are generated through the photolysis of Cys, the photoproduct of disulfide photolysis. Prior to stable product formation, CysS• radicals engage in reversible hydrogen transfer reactions with αC-H and βC-H bonds of the surrounding amino acids. Especially for the βC-H bonds of Ala such hydrogen transfer reactions are unexpected based on thermodynamic grounds; however, the replacement of deuterons in Ala-d3 by hydrogens in H2O provides strong experimental evidence for such reactions.

Sulfheme formation during homocysteine S-oxygenation by catalase in cancers and neurodegenerative diseases

“…The disproportionation of two thiyl radicals RS· results in the formation of a thiol RSH and a thione derivative R=S whose hydrolysis can be linked to H 2 S production along with a ketone R=O (ref. 38). Furthermore, H 2 S may be present as a contaminant in the solution of HCys we used during our experiments.…”

Protein thiyl radical reactions and product formation: a kinetic simulation

“…In vitro photochemical studies with several model peptides[31–35, 46, 47] and proteins[48–50], including IgG1 and IgG2, have demonstrated that thiyl radicals can generate intermediary carbon-centered radicals on proteins in a sequence-specific manner. While thiyl radicals were not generated through physiological processes in these studies, they are relevant to physiology as they demonstrate what can happen when thiyl radicals are generated on proteins.…”

Special issue on “Free Radical and Redox Biochemistry of Thiols”

“…Those include thiyl radical repair by ascorbate and glutathione as well as reaction with oxygen, together with hydrogen abstraction from C-H bonds in peptides and proteins to form carbon-centered radicals that could result in protein damage [6]. Mechanisms leading to such protein damage include generation of peroxyl radicals at the a C position followed by additional hydrogen and electron transfer processes and fragmentation reactions as well as thiyl radical elimination promoting the electrophilic dehydroalanine formation, which can further react with nucleophilic protein residues [7,8]. Sulfenic acids are also usually unstable species, particularly when formed in LMM compounds.…”