Activates ubiquitin by first adenylating its C-terminal glycine residue with ATP, and thereafter linking this residue to the side chain of a cysteine residue in E1, yielding a ubiquitin-E1 thioester and free AMP. Specific for ubiquitin, does not activate ubiquitin-like peptides. Differs from UBE1 in its specificity for substrate E2 charging. Does not charge cell cycle E2s, such as CDC34 (By similarity). Essential for embryonic development. Required for UBD/FAT10 conjugation. ATP + ubiquitin + [E1 ubiquitin-activating enzyme]-L-cysteine = AMP + diphosphate + S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Forms a thioester with UBD in cells stimulated with tumor necrosis factor-alpha (TNFa) and interferon-gamma (IFNg) (PubMed:17889673). Embryonic lethality at early stage. Belongs to the ubiquitin-activating E1 family.