Synthesis of alpha-methyl selenocysteine and its utilization as a glutathione peroxidase mimic.

Abstract

Selenocysteine (Sec) is the 21st amino acid in the genetic code where this amino acid is primarily involved in redox reactions in enzymes because of its high reactivity toward oxygen and related reactive oxygen species. Sec has found wide utility in synthetic peptides, especially as a replacement for cysteine. One limitation of using Sec in synthetic peptides is that it can undergo β-syn elimination reactions after oxidation, rendering the peptide inactive due to loss of selenium. This limitation can be overcome by substituting Cα-H with a methyl group. The resulting Sec derivative is α-methylselenocysteine ((αMe)Sec). Here, we present a new strategy for the synthesis of (αMe)Sec by alkylation of an achiral methyl malonate through the use of a selenium-containing alkylating agent synthesized in the presence of dichloromethane. The seleno-malonate was then subjected to an enzymatic hydrolysis utilizing pig liver esterase followed by a Curtius rearrangement producing a protected derivative of (αMe)Sec that could be used in solid-phase peptide synthesis. We then synthesized two peptides: one containing Sec and the other containing (αMe)Sec, based on the sequence of glutathione peroxidase. This is the first reported incorporation of (αMe)Sec into a peptide as well as the first reported biochemical application of this unique amino acid. The (αMe)Sec-containing peptide had superior stability as it could not undergo β-syn elimination and it also avoided cleavage of the peptide backbone, which we surprisingly found to be the case for the Sec-containing peptide when it was incubated for 96hours in oxygenated buffer at pH8.0.