The UBA2:SAE1 complex catalyzes the formation of a thioester bond between SUMO1 and cysteine-173 of UBA2 (Desterro et al. 1999, Okuma et al. 1999, Werner et al. 2009, Olsen et al. 2010, Wang and Chen 2010). ATP reacts with the C-terminal glycine residue of SUMO1 to yield pyrophosphate and a transient intermediate, SUMO1 adenylate, which then reacts with the thiol group of the cysteine residue on UBA2.