The attachment of the small protein ubiquitin to substrates regulates a vast array of processes in eukaryotes. Substrates can be modified with a single ubiquitin (Ub) or with a polyubiquitin chain in which one ubiquitin is conjugated to the next Ubiquitin is covalently attached to substrate lysines in a three-enzyme cascade catalyzed by E1, E2 and E3 enzymes4, thus resulting in an isopeptide linkage between the ubiquitin C terminus and the ε-amino group of lysine. The E3 ligase binds to both the E2~Ub thioester and the substrate, catalyzing transfer of the ubiquitin from the active site cysteine of the E2 to the substrate lysine or N terminus. E3 ligases thus have dual roles as both molecular matchmaker and catalyst, bringing together the right E2 with the right substrate and greatly increasing the rate of ubiquitin transfer. 
All E3 ligases bind an E2~Ub thioester and either catalyze transfer of ubiquitin from the E2 to a substrate lysine via an aminolysis reaction, as is the case for RING E3s, or to another cysteine in the E3 via a transthioesterification reaction, as is the case for HECT and RBR E3s. RING E3 ligases contain a RING (or RING-like) domain that is responsible for both binding to the E2 and stimulating ubiquitin transfer. In contrast to RING E3 ligases, which catalyze direct attack of the substrate lysine on the E2~Ub thioester, HECT-domain E3 ligases catalyze two distinct reactions. The RBR family has only recently burst upon the scene as a mechanistically distinct class of E3s that share features of both RING and HECT E3s yet catalyze ubiquitination and autoregulate their activity in a distinct manner.

References

1.Berndsen CE, et al. Nat Struct Mol Biol. 2014;21(4):301–307.