What are post-translational modifications and why are they essential for protein function?

• A. Phosphorylation, glycosylation, etc. modify activity, localization, and stability of proteins.
• B. They occur during transcription.
• C. They are encoded directly in DNA as amino acids.
• D. They occur only in bacteria.

Answer: (A)

Post-translational modifications are chemical changes added to a protein after it has been made. They include phosphorylation, glycosylation, ubiquitination, acetylation, methylation, lipidation, and more. These modifications can dramatically alter a protein’s function by changing its activity, where it goes in the cell, or how stable it is. For example, phosphorylation can switch an enzyme on or off or create a docking site for other proteins; glycosylation often helps with proper folding, stability, and targeting to membranes or secretory pathways; ubiquitination can mark a protein for degradation or modify its interactions. Because these changes happen after translation and are tightly regulated, they greatly expand the functional diversity of the proteome beyond what is encoded in the DNA sequence. They are not added during transcription, they are not encoded directly in DNA as amino acids, and they occur across many organisms, not just bacteria.