Ubiqutin (Ub)- and Ubiqutin-like (Ubl)-dependent metabolism is a multi-enzyme process involving the successive activities of distinct conjugating enzymes. The E1 activating enzymes are central to Ub/UBL conjugation in vivo and in vitro since they initiate the cascade of conjugation by activating their respective modifiers.
These enzymes are part of a larger E1-like enzyme superfamily which includes roles in the biosynthesis of cysteine, thiamine, molybdenum cofactor (MoCo), and several secondary metabolites. The basic catalytic mechanism for these enzymes involved adenylation and sulfotransfer along with specific domains and structures to dictate specificity in substrate binding and other relevant protein-protein interactions. All E1 enzymes have an adenylation domain (two ThiF-homology motifs) that bind to and adenylate the Ub/UBL, a catalytic cysteine domain to which the Ub/UBL transiently attaches, and a C-terminal Ub-fold domain (UFD) which recruits E2 enzymes.