Precision Bioconjugation: Chemical Strategies for Site-Selective Cysteine Conjugation Advance Peptide and Protein Engineering

Complex peptides and proteins require precise engineering to maintain native function and activity. Traditional chemoselective bioconjugation methods modify specific amino acid side chains, but often lack ultimate control over the exact conjugation position. Achieving site-selective modification is crucial for constructing well-defined architectures with preserved biological function. Cysteine residues, with their highly nucleophilic thiols and typically low natural abundance in their reduced form, are ideal targets for such precise chemical transformations, offering excellent chemoselectivity for introducing novel functionalities.

This review systematically examines advanced chemical strategies specifically targeting cysteine residues for site-selective bioconjugation. It focuses on transformations refined to occur exclusively at predefined positions within peptides or proteins, achieving high site-selectivity. The authors discuss approaches leveraging neighboring functional groups, such as the α-amine or carboxylate, to enable selective N- or C-terminal modifications. The review aims to guide future synthetic developments in precision chemistry by exploiting the local environment of targeted cysteines.

The review highlights that achieving high site-selectivity in cysteine bioconjugation requires advanced chemical strategies that exploit the unique local environment of the targeted cysteine. One key approach involves leveraging neighboring functional groups, for example, engaging the thiol together with the α-amine or carboxylate to enable selective N- or C-terminal modification, respectively. In such designs, the cysteine side chain may contribute through transient interactions, direct incorporation into the covalent linkage, or the stabilization of the desired product. > This level of precision allows for the construction of atomically tailored complex peptides and proteins, preserving native folding and activity crucial for chemical biology, therapeutics, and biomaterials science. The authors emphasize that ultimate control is achieved by precisely defining both the nature of the linkage and the exact position of conjugation on elongated peptide sequences or fully assembled proteins, moving beyond general chemoselectivity. A promising strategy attracting increasing attention involves exploiting specific local environments to achieve unprecedented precision.