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Family of activating enzymes (E1) of ubiquitin-like proteins related to the E.coli hypothetical protein ygdL. The common reaction mechanism catalyzed by E1-like enzymes begins with a nucleophilic attack of the C-terminal carboxylate of the ubiquitin-like substrate, on the alpha-phosphate of an ATP molecule bound at the active site of the activating enzymes, leading to the formation of a high-energy acyladenylate intermediate and subsequently to the formation of a thiocarboxylate at the C termini of the substrate. The exact function of this family is unknown.