Regioselective Chemical Modification of Cysteine Residues on Protein Surfaces Focusing on Local Environment around the Conjugation Site

For chemical modification of cysteines in a protein, the regioselectivity among cysteine residues on the protein surface is an issue to be considered. To elucidate the determinants of cysteine reactivities on protein surfaces, we have investigated the chemical modification of the adenylate kinase A55C/C77S/V169C mutant as an experimental model. Although Cys55 and Cys169 are commonly located on the protein surface, Cys55 showed the ca. 3-6-fold higher reactivity compared to Cys169 in a reaction with a pyrene derivative. By a further conjugation of a phenanthroline derivative into the vacant Cys thiol, fluorescence quenching was attained by a pyrene-phenanthroline interaction that occurred by the conformational change of the protein. The K50A mutation further enhanced the regioselectivity of pyrene conjugation in Cys55, which is attributed to the effects of structural flexibility in the vicinity of Cys55 on its reactivity. To regioselectively conjugate different types of synthetic molecules onto the surface of a protein, perturbation in the local structural flexibility around the conjugation sites will be a useful strategy.