Question

2. Consider the process of native chemical ligation. Explain why only N-terminal cysteines and not internal or C-terminal cysteines will form native peptide bonds.

Answer 1

Native chemical reaction is a method of constructing large polypeptides by assembling of two or more unprotected peptides segments.

In native chemical ligation, the thiol group of an N-terminal cysteine residue of an unprotected peptide attacks the C-terminal thioester of a second unprotected peptide in an aqueous medium. This reversible transthioesterification step leads to form a thioester intermediate. This intermediate rearranges by an intramolecular S,N-acyl shift that results in the formation of a native amide bond at the ligation site.

For this S,N-acyl shift the N-terminal NH₂ group is required which can be only available at N-terminal cystein. The free amino group will not be available at internal of C-terminal cysteinand hence the reaction will not be possible.