FIGURE 1.

A schematic figure of the ubiquitin proteasome system. Initially, a single free ubiquitin was activated by E1 in an ATP-dependent manner and then the activated monoubiquitin molecule is transferred to a cysteine residue of the E2 enzyme, which is subsequently recruited into the E3 ligases. Working with a specific E3, E2 transferred one or more ubiquitin moieties sequentially to form ubiquitin chains to the substrate protein. Finally, the proteasome system can target the ubiquitinated substrate protein for degradation.