SUMO activating enzymes are 'canonical' E1 enzymes that act as heterodimers to activate the ubiquitin-like protein SUMO for subsequent steps along the SUMO conjugation cascade. They perform three distinct chemical reactions: first, they bind ATP, magnesium and SUMO to catalyze formation of a high-energy acyl adenylate intermediate with subsequent release of pyrophosphate; second, an E1 catalytic cysteine attacks the SUMO-adenylate to catalyze formation of a high-energy thioester bond between the E1 and SUMO and release of the AMP product; finally, the E1-SUMO catalyzes transthioesterification to an E2 to form a high energy thioester-linked E2-SUMO product. (Adapted from PMID:28234446.)