[[Ubiquitin conjugating enzyme]]s, also known as E2 enzymes and more rarely as ubiquitin-carrier enzymes, perform the second step in the [[ubiquitination]] reaction that targets a protein for degradation via the [[proteasome]]. The ubiquitination process covalently attaches [[ubiquitin]], a short protein of 76 amino acids, to a [[lysine]] residue on the target protein. Once a protein has been tagged with one ubiquitin molecule, additional rounds of ubiquitination form a polyubiquitin chain that is recognized by the proteasome's 19S regulatory particle, triggering the ATP-dependent unfolding of the target protein that allows passage into the proteasome's 20S core particle, where proteases degrade the target into short peptide fragments for recycling by the cell.

see [[UBE2C]].