E1 bound ubiquitin Ub:E2s Transfer of ubiquitin from E1 to E2 Activated ubiquitin is transferred from E1 to the active site cystine of ubiquitin conjugating enzymes (E2s) via a trans-esterification reaction. E2s catalyze covalent attachment of ubiquitin to target proteins. They all share an active-site ubiquitin-binding cysteine residue and are distinguished by the presence of a ubiquitin-conjugating catalytic (UBC) fold required for binding of distinct ubiquitin ligases or E3s. Once conjugated to ubiquitin, the E2 molecule binds one of several E3s (Glickman et al. 2002). Authored: Garapati, P V, 2010-10-29 Edited: Garapati, P V, 2010-10-29 Pubmed11917093 Pubmed19452197 Pubmed19851334 Pubmed19940261 Reactome Database ID Release 43983152 Reactome, http://www.reactome.org ReactomeREACT_75860 Reviewed: Elliott, T, 2011-02-10 Activating enzymes E1 E2 Ubiquitin-conjugating enzyme