Figure 1.

Schematic representation of tandem ABPP as site specific and activity-based technique to covalently tag active enzymes in situ and in vivo. A small molecule ABP equipped with one partner of a bioorthogonal reaction pair forms a covalent bond with active site residues of a subset of enzymes in a cell. After lysis, tandem labeling, such as click chemistry, introduces a tagging moiety on the enzyme-ABP complex. Depending on the tag, different analysis methods can be used, such as in-gel scanning, or affinity enrichment with mass spectrometry detection.