UbiQ-103 is a new and first of its kind activity based probe for Ub E1, E2 and (HECT/RBR) E3 ligases [Mulder et al. 2016]. It is based on the Ub sequence in which the C-terminal Gly76 has been replaced by a dehydroalanine (Dha) residue. The N-terminus is labeled with an His6 tag; two aminohexanoic acid (Ahx) linkers are used to create extra space between the His6 and Ub protein for efficient access of His6 binding entities. It has been prepared by total chemical synthesis and is therefore well-defined in terms of His6 site.
UbiQ-103 is processed in a native manner by Ub E1, E2 and (HECT/RBR) E3 ligases and during this process it forms an electrophilic intermediate that can react with the active site Cys residue of the E1, E2 and (HECT/RBR) E3 enzyme, thereby creating a covalent bond (see figure).