First discovered by Gideon Goldstein in 1975,ubiquitin is a small regulatory protein that exists in almost all eukaryoticcells. It can be found in the nucleus, cytoplasm, and cell-surface membranes ofthese eukaryotes. Ubiquitin has an amino acid sequence of 76 amino acids thattogether aid in DNA repair, signal transduction and degradation of proteins.Ubiquitin acts as a tag in the protein transport system to which proteins arebrought to the proteasome for digestion into short polypeptides or amino acids.
This process is known as ubiquitination, which is the principal mechanism forprotein catabolism in mammalian cells throughout the animal kingdom. Ubiquitin is a small moleculethat can attach itself to substrates within the human body to perform variousbiological processes. Ubiquitin contains seven lysine residues, lysine-6,lysine-11, lysine-27, lysine 29, lysine-33, lysine-48, and lysine-63.
Dependingon the number of ubiquitin containing molecules bonded to which lysine residues,monoubiquitin or polyubiquitin can be formed. Monoubiquitin, illustrated inFigure 1, is a single ubiquitin molecule that is attached to one lysine residuewithin the substrate. Figure 1.
3D Structure of Mono-Ubiquitin.(PDB ID IUBQ) Ubiquitin structure shown in yellow cartoonformat. All lysine residues labeled and shown in stick format with nitrogengroups highlighted in blue. Lysine-48 is highlighted in red. C-terminus andN-terminus are labeled.Polyquitination, on the other hand, is a chain of ubiquitin-ubiquitin linkedmolecules attached to a specific lysine residue of the same substrate.
Monoubiquitinationof a protein usually functions in the regulation of protein activation, signaltransduction, and DNA repair, whereas polyubiquitination primarily deals withthe degradation of proteins. Studies have shown that tagging of Lys-48 inpolyubiquitination mostly is utilized to target proteins for degradation by theproteasome. Ubiquitination is anATP-dependent process orchestrated by three classes of enzymes known as E1, E2,and E3. These enzymes function in the attachment of ubiquitin to proteinsdestined for degradation. First, ubiquitin is activated by conjugation of E1 atits C-terminus.
Ubiquitin activating enzyme (E1), the initial enzyme in the ubiquitinationpathway, then allows for binding of ubiquitin to a ubiquitin conjugating enzyme(E2) through the formation of a thiolester linkage. E2 then forms an isopeptidebond with the C-terminus of ubiquitin and a lysine residue of the substrateprotein. Finally, through high specificity of the ubiquitin-protein ligase(E3), the enzyme is able to recognize specific protein substrates and catalyzethe transfer of activated ubiquitin them. Ubiquitin primarilyfunctions to target proteins for degradation, which plays a key role in the regulationof the immune system. Therefore, deficiencies in ubiquitin can lead to certain diseasesor conditions within an individual.