In the ubiquitination cascade, E3 ubiquitin ligases play a pivotal part in selecting substrates and, together with E2 enzyme-ubiquitin, they coordinate the conjugation of specific ubiquitin linkages onto substrates. As E3 ligases mediate substrate specificity, they are the most heterogeneous class of enzymes in the ubiquitination pathway. More than 600 E3 ligases are found in the human body. Currently, E3 ligases can be classified in three main types depending on the presence of characteristic binding domains and on the mechanism of ubiquitin transfer to the substrate protein: HECT E3s, RING E3s, and RBR E3s.
RING E3s are the most abundant type of ubiquitin ligases. RING E3s can function as monomers, homodimers, or heterodimers. RING E3s catalyze the direct transfer of ubiquitin from E2 enzyme-ubiquitin to the substrate. By contrast, HECT and RBR E3s contain a catalytic cysteine that first receives ubiquitin from E2 enzyme-ubiquitin to form an E3 ligase-ubiquitin thioester intermediate, followed by the transfer of this ubiquitin to the substrate.