Activation of ubiquitin by UBA1 proceeds through 3 steps: adenylation of ubiquitin, conjugation of ubiquitin from adenyl-ubiquitin to an internal cysteine residue of UBA1, and adenylation of a second molecule of ubiquitin. In the second step, the thiol group of an internal cysteine residue of UBA1 attacks the acyl-adenyl bond of the C-terminal glycine of adenylated ubiquitin, resulting in a thioester bond between the cysteine residue and the C-terminal glycine of ubiquitin (Jin et al. 2007, inferred from the rabbit homologue in Haas et al. 1982, Hershko et al. 1983, reviewed in Groettrup et al. 2008).
ubiquitin activating enzyme activity of UBA1:ubiquitin adenylate [cytosol]