Context-Dependence of the Reactivity of Cysteine and Lysine Residues

The S-alkylation of Cys residues with a maleimide and the N-epsilon-acylation of Lys residues with an N-hydroxysuccinimide (NHS) ester are common methods for bioconjugation. Using Cys and Lys derivatives as proxies, we assessed differences in reactivity depending on the position of Cys or Lys in a protein sequence. We find that Cys position is exploitable to improve site-selectivity in maleimide-based modifications. Reactivity decreases substantially in the order N-terminal>in-chain>C-terminal Cys due to modulation of sulfhydryl pK(a) by the alpha-ammonium and carboxylate groups at the termini. A lower pK(a) value yields a larger fraction thiolate, which promotes selectivity while somewhat decreasing thiolate nucleophilicity in accord with beta nuc =0.41. Lowering pH and salt concentration enhances selectivity still further. In contrast, differences in the reactivity of Lys towards an NHS ester were modest due to an appreciable decrease in amino group nucleophilicity with a lower pK(a) of its conjugate acid. Hence, site-selective Lys modification protocols will require electrophiles other than NHS esters.

Automated summary

The position of Cys in a protein sequence can be exploited to improve site-selectivity in maleimide-based modifications, while differences in the reactivity of Lys towards an NHS ester are modest.