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) [1]. 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 [2,3,4]. The catalytic cysteine domain contains the E1 active site cysteine, and is divided in two half-domains, FCCH and SCCH, for 'first' and 'second' catalytic cysteine half-domain, respectively. This is the SCCH domain in which resides the catalytic cysteine [5].

1: Molecular cloning, sequence, and tissue distribution of the human ubiquitin-activating enzyme E1. Handley PM, Mueckler M, Siegel NR, Ciechanover A, Schwartz AL; Proc Natl Acad Sci U S A. 1991;88:258-262. PMID:1986373

2: Molecular cloning of cDNA encoding a ubiquitin-activating enzyme (E1) from goldfish (Carassius auratus) and expression analysis of the cloned gene. Tokumoto M, Nagahama Y, Tokumoto T; Biochim Biophys Acta. 2000;1492:259-263. PMID:11004499

3: Structural insights into E1-catalyzed ubiquitin activation and transfer to conjugating enzymes. Lee I, Schindelin H; Cell. 2008;134:268-278. PMID:18662542

4: Structures of the SUMO E1 provide mechanistic insights into SUMO activation and E2 recruitment to E1. Lois LM, Lima CD; EMBO J. 2005;24:439-451. PMID:15660128

5: Crystal structure of a human ubiquitin E1-ubiquitin complex reveals conserved functional elements essential for activity. Lv Z, Williams KM, Yuan L, Atkison JH, Olsen SK; J Biol Chem. 2018;293:18337-18352. PMID:30279270