EC 6.2.1.55     
Accepted name: E1 SAMP-activating enzyme
Reaction: ATP + [SAMP]-Gly-Gly + [E1 SAMP-activating enzyme]-L-cysteine = S-[[SAMP]-Gly-Gly]-[[E1 SAMP-activating enzyme]-L-cysteine] + AMP + diphosphate (overall reaction)
(1a) ATP + [SAMP]-Gly-Gly = diphosphate + [SAMP]-Gly-Gly-AMP
(1b) [SAMP]-Gly-Gly-AMP + [E1 SAMP-activating enzyme]-L-cysteine = S-[[SAMP]-Gly-Gly]-[[E1 SAMP-activating enzyme]-L-cysteine] + AMP
Glossary: SAMP = small archaeal modifier protein = ubiquitin-like small archaeal modifier protein
Other name(s): UbaA; SAMP-activating enzyme E1
Systematic name: [SAMP]:[E1 SAMP-activating enzyme] ligase (AMP-forming)
Comments: Contains Zn2+. The enzyme catalyses the activation of SAMPs (Small Archaeal Modifier Proteins), which are ubiquitin-like proteins found only in the Archaea. SAMPs are involved in protein degradation, and also act as sulfur carriers involved in thiolation of tRNA and other metabolites such as molybdopterin. The enzyme catalyses the ATP-dependent formation of a SAMP adenylate intermediate in which the C-terminal glycine of SAMP is bound to AMP via an acyl-phosphate linkage (reaction 1). This intermediate can accept a sulfur atom to form a thiocarboxylate moiety in a mechanism that is not yet understood. Alternatively, the E1 enzyme can transfer SAMP from its activated form to an internal cysteine residue, releasing AMP (reaction 2). In this case SAMP is subsequently transferred to a lysine residue in a target protein in a process termed SAMPylation. Auto-SAMPylation (attachment of SAMP to lysine residues within the E1 enzyme) has been observed. cf. EC 2.7.7.100, SAMP-activating enzyme.
References:
1.  Miranda, H.V., Nembhard, N., Su, D., Hepowit, N., Krause, D.J., Pritz, J.R., Phillips, C., Soll, D. and Maupin-Furlow, J.A. E1- and ubiquitin-like proteins provide a direct link between protein conjugation and sulfur transfer in archaea. Proc. Natl. Acad. Sci. USA 108 (2011) 4417–4422. [PMID: 21368171]
2.  Maupin-Furlow, J.A. Ubiquitin-like proteins and their roles in archaea. Trends Microbiol. 21 (2013) 31–38. [PMID: 23140889]
3.  Miranda, H.V., Antelmann, H., Hepowit, N., Chavarria, N.E., Krause, D.J., Pritz, J.R., Basell, K., Becher, D., Humbard, M.A., Brocchieri, L. and Maupin-Furlow, J.A. Archaeal ubiquitin-like SAMP3 is isopeptide-linked to proteins via a UbaA-dependent mechanism. Mol. Cell. Proteomics 13 (2014) 220–239. [PMID: 24097257]
4.  Hepowit, N.L., de Vera, I.M., Cao, S., Fu, X., Wu, Y., Uthandi, S., Chavarria, N.E., Englert, M., Su, D., Söll, D., Kojetin, D.J. and Maupin-Furlow, J.A. Mechanistic insight into protein modification and sulfur mobilization activities of noncanonical E1 and associated ubiquitin-like proteins of Archaea. FEBS J. 283 (2016) 3567–3586. [PMID: 27459543]
[EC 6.2.1.55 created 2018]