Molecular model of ubiquitin-conjugating enzyme complexed with ubiquitin. Ubiquitin is found in all eukaryotic cells. When a protein is damaged or old it will be tagged by several ubiquitin molecules. The protein is then moved to a proteosome, a hollow barrel shaped protein that degrades other proteins into amino acids and small polypeptides. Ubiquitin-conjugating enzymes play a role in the tagging of proteins for degradation.