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 (PubMed:35970836, PubMed:35986001). Specific for ubiquitin, does not activate ubiquitin-like peptides. Also activates UBD/FAT10 conjugation via adenylation of its C-terminal glycine (PubMed:17889673, PubMed:35970836, PubMed:35986001). Differs from UBE1 in its specificity for substrate E2 charging. Does not charge cell cycle E2s, such as CDC34. Essential for embryonic development. Isoform 2 may play a key role in ubiquitin system and may influence spermatogenesis and male fertility. {ECO:0000269|PubMed:15202508, ECO:0000269|PubMed:17597759, ECO:0000269|PubMed:17889673, ECO:0000269|PubMed:35970836, ECO:0000269|PubMed:35986001}. This is the function of ENSG00000033178 (UBA6, ubiquitin like modifier activating enzyme 6).