REVIEW: ENDOTHIOPEPTIDES ALIAS PEPTIDE THIOAMIDES

Ni-Catalyzed Synthesis of Thiocarboxylic Acid Derivatives

A Ni-catalyzed cross-coupling of readily accessible O-alkyl xanthate esters or thiocarbonyl imidazolides and organozinc reagents for the synthesis of thiocarboxylic acid derivatives has been developed. This method benefits from a fast reaction time, mild reaction conditions, and ease of starting material synthesis. The use of transition-metal catalysis to access a diverse range of thiocarbonyl-containing compounds provides a useful complementary approach when compared with previously established methodologies.

Synthesis of thioester peptides for the incorporation of thioamides into proteins by native chemical ligation

Thioamides can be used as photoswitches, as reporters of local environment, as inhibitors of enzymes, and as fluorescence quenchers. We have recently demonstrated the incorporation of thioamides into polypeptides and proteins using native chemical ligation (NCL). In this protocol, we describe procedures for the synthesis of a thioamide precursor and an NCL-ready thioamide-containing peptide using Dawson's N-acyl-benzimidazolinone (Nbz) process. We include a description of the synthesis by NCL of a thioamide-labeled fragment of the neuronal protein α-synuclein.

Probing substrate backbone function in prolyl oligopeptidase catalysis--large positional effects of peptide bond monothioxylation

Site-specific effects on the catalytic activity of prolyl oligopeptidase from human placenta were studied using oligopeptide substrates in which a peptide bond has been replaced by a thioxo peptide bond. Two series of tetrapeptide-4-nitroanilides, Ala-Gly-Pro-Phe-NH-Np and Ala-Ala-Pro-Phe-NH-Np, along with all possible monothioxylated derivatives, were synthesised and k(cat) and Km values were determined for proteolytic cleavage at the Pro-Phe bond. Regardless of either Gly or Ala in the P2 subsite, tetrapeptides were rendered uncleavable by thioxylation at the Pro-Phe linkage. As a result, Ala-Xaa-Pro-psi[CS-NH]-Phe-NH-Np (Xaa = Gly or Ala) displayed competitive inhibition with Ki-values of 12 microM and 44 microM, respectively. Furthermore, in controlling proteolytic susceptibility of the substrates, cooperation of the P3-P2 thioxylation site and the side chain at the P2 subsite was obtained. Thioxylation at this position enhanced k(cat)/Km fivefold in the Gly series, but led to a 1.7-fold decrease in the Ala series of substrates. With respect to the Xaa-Pro peptide bond, all of the substrates underwent cis/trans isomerisation, thus presenting two stable conformers to the protease. However, the magnitudes of the isomerisation constants suggested that neither isomerisation rates nor cis/trans equilibria can explain the effect of thioxylation on the steady-state constants of proteolysis.