Ubiquitination is a crucial biological process involving the covalent attachment of a small protein called ubiquitin to target proteins. Ubiquitin is highly conserved and found in nearly all eukaryotic organisms, from yeast to humans, consisting of 76 amino acid residues. This process is energy-intensive and ATP-dependent, meaning it requires energy from ATP to facilitate the attachment of ubiquitin to the target protein.
During ubiquitination, ubiquitin peptides are covalently linked to the target protein primarily through the R group of lysine amino acid residues, which are particularly susceptible to this modification. The attachment occurs via an amide linkage, where the carboxylate group of ubiquitin interacts with the amino group of lysine's R group. Although this bond resembles a peptide bond, it is specifically referred to as an isopeptide bond because it involves the side chain of lysine rather than the backbone of the protein.
Moreover, multiple ubiquitin molecules can be linked together, forming a polyubiquitin chain on the target protein. This chain formation is significant as it plays a vital role in regulating protein activity, marking proteins for degradation or altering their function. Understanding ubiquitination is essential for grasping how cellular processes are controlled and how proteins are managed within the cell.