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

Strategies for the Synthesis of Selenocysteine Derivatives

β-Seleno-α-amino acids, known as selenocysteine (Sec) derivatives, have emerged as important targets because of their role in chemical biology, not only as part of selenoproteins with important redox properties, but also because of their activity as antivirals or metabolites effective in inhibiting carcinogenesis. In addition, there is demand for this type of compounds due to their use in native chemical ligation to construct large peptides. Therefore, this review summarizes the various synthetic methods that have been published to construct Sec derivatives. Most of them involve the generation of the C–Se bond by nucleophilic substitution reactions, but other reactions such as radical or multicomponent strategies are also reported. Of particular importance is the Se-Michael addition of Se-nucleophiles to chiral bicyclic dehydroalanines, in which the stereogenic center is generated under complete stereocontrol.

1 Introduction

2 Previously Reviewed Synthesis of Sec

3 Retrosynthesis of Sec Derivatives

4 Sec Derivatives by Nucleophilic Substitutions

5 Sec Derivatives by Radical Processes

6 Sec Derivatives by 1,4-Conjugate Additions

7 Conclusion

Can Selenoenzymes Resist Electrophilic Modification? Evidence from Thioredoxin Reductase and a Mutant Containing α-Methylselenocysteine

Selenocysteine (Sec) is the 21st proteogenic amino acid in the genetic code. Incorporation of Sec into proteins is a complex and bioenergetically costly process that evokes the following question: "Why did nature choose selenium?" An answer that has emerged over the past decade is that Sec confers resistance to irreversible oxidative inactivation by reactive oxygen species. Here, we explore the question of whether this concept can be broadened to include resistance to reactive electrophilic species (RES) because oxygen and related compounds are merely a subset of RES. To test this hypothesis, we inactivated mammalian thioredoxin reductase (Sec-TrxR), a mutant containing α-methylselenocysteine [(αMe)Sec-TrxR], and a cysteine ortholog TrxR (Cys-TrxR) with various electrophiles, including acrolein, 4-hydroxynonenal, and curcumin. Our results show that the acrolein-inactivated Sec-TrxR and the (αMe)Sec-TrxR mutant could regain 25% and 30% activity, respectively, when incubated with 2 mM H₂O₂ and 5 mM imidazole. In contrast, Cys-TrxR did not regain activity under the same conditions. We posit that Sec enzymes can undergo a repair process via β-syn selenoxide elimination that ejects the electrophile, leaving the enzyme in the oxidized selenosulfide state. (αMe)Sec-TrxR was created by incorporating the non-natural amino acid (αMe)Sec into TrxR by semisynthesis and allowed for rigorous testing of our hypothesis. This Sec derivative enables higher resistance to both oxidative and electrophilic inactivation because it lacks a backbone C_α-H, which prevents loss of selenium through the formation of dehydroalanine. This is the first time this unique amino acid has been incorporated into an enzyme and is an example of state-of-the-art protein engineering.