Which Amino Acids Can Be Ubiquitinated?
Ubiquitination is a post-translational modification in which ubiquitin, a small regulatory protein, is covalently attached to substrate proteins. This modification predominantly occurs on specific amino acid residues:
1. Lysine (K)
The ε-amino group of lysine is the primary site for ubiquitination. Ubiquitin’s C-terminal glycine forms a covalent isopeptide bond with the lysine residue of the substrate protein.
2. Cysteine (C)
Although less common, cysteine residues can also undergo ubiquitination. In such cases, ubiquitin is conjugated via a thioether bond rather than the typical isopeptide linkage.
3. Serine (S) and Threonine (T)
Ubiquitination at these residues occurs through ester bond formation; however, it is significantly less frequent than lysine ubiquitination.
4. Arginine (R)
Although rare, ubiquitination of arginine residues has been documented in specific contexts.
These modifications play critical roles in regulating protein stability, subcellular localization, enzymatic activity, and protein-protein interactions.
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