Here we would like to introduce Biotin-Ahx-Ub-MTC (UbiQ-378) as a new type of activity-based probe for deubiquitinating enzymes (DUBs). Reported recently by Chanda et al., it contains a C-terminal azaglycine ester (MTC= methyl hydrazinecarboxylate, Figure 1) as electrophile. This has the unique property to react with a DUB active site cysteine in a similar fashion as a ubiquitinated protein substrate, forming a tetrahedral transition state in which the oxygen anion is stabilized by the oxyanion hole. UbiQ-378 contains an N-terminal biotin with a 6-aminohexanoic acid (Ahx) linker to create extra space for biotin binding entities. 

Figure 1. Mode of action of DUB activity-based probes containing a methyl hydrazinecarboxylate electrophile.

Reference: Chanda et al. Ubiquitin Azapeptide Esters as Next-Generation Activity-Based Probes for Cysteine Enzymes in the Ubiquitin Signal Pathway, J Am Chem Soc 2025, 147, 17817.