Explain how post-translational modifications can alter the function of a protein after it has been synthesised on a ribosome.

Post‑translational modifications are chemical changes that occur to a polypeptide after translation has finished. For example, phosphorylation of serine residues adds a phosphate group. This addition can change the protein’s three‑dimensional conformation, for instance by creating a new charge that alters folding or by blocking a binding site. The altered shape can activate or inhibit the protein’s function – phosphorylation of a kinase, for example, activates its catalytic activity, whereas phosphorylation of a transcription factor can target it for degradation. Thus, post‑translational modifications modify the protein’s structure and thereby its biological activity.