Figure 11.

E2 chemical reactions (A) Scheme illustrating how the Ubl moiety of a E2∼Ubl thioester can be transferred to the primary amine group of a lysine residue of a protein substrate (top), to the primary amine group of PE (middle), or to a HECT or RBR E3 for subsequent transfer to the lysine residue of a protein substrate (bottom). Transfer to PE is performed by E2_ATG3. Ubl transfer to E3s of the HECT or RBR families are limited to E2_UBCH8, and this only allows the ISGylation of a limited number of protein substrates. (B) Mechanism for the aminolysis reaction. In this case, the primary amines are presented in their deprotonated states.