Catalyzes the covalent attachment of ubiquitin to other proteins (PubMed:1869573) (By similarity). Required for the adaptation to the presence of glucose in the growth medium; mediates the degradation of enzymes involved in gluconeogenesis when cells are shifted to glucose-containing medium (PubMed:9737955, PubMed:10811607, PubMed:12686616). Required for proteasome-dependent catabolite degradation of fructose-1,6-bisphosphatase (FBP1) (PubMed:9737955, PubMed:10811607, PubMed:12686616, PubMed:28126757). S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [E2 ubiquitin-conjugating enzyme]-L-cysteine = [E1 ubiquitin-activating enzyme]-L-cysteine + S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Present with 1590 molecules/cell in log phase SD medium. Belongs to the ubiquitin-conjugating enzyme family.