part of ubiquitin activating enzyme activity https://github.com/geneontology/go-ontology/issues/30193 GO:0004839 Catalysis of the reaction: E1 + ubiquitin + ATP--> E1-ubiquitin + AMP + PPi, where the E1-ubiquitin linkage is a thioester bond between the C-terminal glycine of Ub and a sulfhydryl side group of an E1 cysteine residue. This is the first step in a cascade of reactions in which ubiquitin is ultimately added to a protein substrate. molecular_function Reactome:R-HSA-8852133 Reactome:R-HSA-8852132 Reactome:R-HSA-8852134 Reactome:R-HSA-8865090 Reactome:R-HSA-8865050 Reactome:R-HSA-8865098 EC:6.2.1.45 The ubiquitin activating enzyme catalyzes a ligation reaction. E1 ubiquitin-activating enzyme ubiquitin-like modifier activating enzyme activity protein ubiquitination