<p> Ubiquitin-activating enzyme (E1 enzyme) activates ubiquitin by first adenylating with ATP its C-terminal glycine residue and thereafter linking this residue to the side chain of a cysteine residue in E1, yielding an ubiquitin-E1 thiolester and free AMP. Later the ubiquitin moiety is transferred to a cysteine residue on one of the many forms of ubiquitin-conjugating enzymes (E2) [<cite idref="PUB00044523"/>]. This domain carries the last of five conserved cysteines that is part of the active site of the enzyme, responsible for ubiquitin thiolester complex formation, the active site being represented by the sequence motif PICTLKNFP [<cite idref="PUB00044524"/>]. Not all proteins in this entry contain a functional active site.</p>