Complexes of enzymes that catalyze the covalent attachment of UBIQUITIN to other proteins by forming a peptide bond between the C-terminal glycine of UBIQUITIN and the alpha-amino groups of lysine residues in the protein. The complexes play an important role in mediating the selective-degradation of short-lived and abnormal proteins. The complex of enzymes can be broken down into three components that involve activation of ubiquitin (ubiquitin-activating enzymes), conjugation of ubiquitin to the ligase complex (ubiquitin-conjugating enzymes), and ligation of ubiquitin to the substrate protein (ubiquitin-protein ligases).