 Ubiquitin-conjugating enzyme

A ubiquitin-conjugating enzyme, or E2, accepts activated ubiquitin from a ubiquitin-activating enzyme onto a catalytic cysteine. It then works with a ubiquitin ligase to transfer ubiquitin onward. Pairing between E2 and E3 helps determine which modification is assembled. This division of labour separates initial energy-dependent activation from later substrate selection and transfer. ^[ref]