Updated project metadata. Carbon dioxide is an abundant biological gas that drives adaptive physiological responses within organisms from all domains. The basic biochemical mechanisms by which proteins serve as sensors of CO2 are, therefore, of great interest. CO2 is a potent electrophilic, and thus one way it can modulate protein biochemistry is by carboxylation of the primary amines group of lysines. However, the resulting cabamates spontaneously decompose, giving off CO2, which makes studying this modification notoriously challenging. Here we describe a chemoproteomic method to stably mimic CO2-carboxylated lysine residues in proteins by reacting them with OCNH. We leverage this method to develop a competition based strategy to quantitatively identify CO2-carboxylatedlysines of proteins and use it to explore the lysine ‘carboxylome’.