CHEMICAL LIGATION OF CYSTEINE-CONTAINING PEPTIDES - SYNTHESIS OF A 22-KDA TETHERED DIMER OF HIV-1 PROTEASE

Thioester-forming chemoselective reaction of unprotected peptide fragm ents containing cysteine residues has been investigated. This work sho ws that free sulfhydryl groups are compatible with the reactive compon ents of thioester-forming ligation chemistry. This allows conjugation by chemical ligation of cysteine or other thiol-containing peptides, f ollowed by postligation disulfide bond formation to form folded protei n domains, or large multisubunit synthetic proteins. Under acidic cond itions, peptides bearing bromoacetyl or a-thiocarboxylate groups did n ot undergo intermolecular reaction with the sulfhydryl group of cystei ne. Intramolecular reaction also did not occur, provided a sufficient number of intervening residues separated the functionalities. The resu lts of these studies have been used in the design and synthesis of a 2 2 kDa tethered dimer HIV-1 protease analogue, prepared by the converge nt chemical ligation of four unprotected peptide segments. Two pairs o f similar to 50 residue peptides were ligated via formation of thioest er bonds to form the individual monomer polypeptide chains. The ligate d monomers each possessed a two residue extension, either at the N-ter minal or at the C-terminal, containing an unprotected sulfhydryl group . These were subsequently linked via directed formation of a disulfide bond. The resulting enzyme analogue retained full catalytic activity, showing that the placement of the backbone thioesters and the disulfi de bond were in functionally unimportant parts of the molecule.