Methyltetrazine as a small live-cell compatible bioorthogonal handle for imaging enzyme activities .

Bioorthogonal chemistry combines well with activity-based protein profiling, as it allows for the introduction of detection tags without significantly influencing the physiochemical and biological functions of the probe. In this work, we introduced methyltetrazinylalanine (MeTz-Ala), a close mimic of phenylalanine, into a dipeptide fluoromethylketone cysteine protease inhibitor. Following covalent and irreversible inhibition, the tetrazine allows vizualisation of the captured cathepsin activity by means of inverse electron demand Diels Alder ligation in cell lysates and live cells, demonstrating that tetrazines can be used as live cell compatible, minimal bioorthogonal tags in activity-based protein profiling.

Bioorthogonal protein labelling enables the study of antigen processing of citrullinated and carbamylated auto-antigens.

Proteolysis is fundamental to many biological processes. In the immune system, it underpins the activation of the adaptive immune response: degradation of antigenic material into short peptides and presentation thereof on major histocompatibility complexes, leads to activation of T-cells. This initiates the adaptive immune response against many pathogens.