Explain how covalent modification of an enzyme can regulate its activity, using phosphorylation as an example.

IB DP Biology Higher Level (2023 syllabus) — C1.1 Enzymes and metabolism · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Covalent modification changes an enzyme’s activity by adding or removing a chemical group. Phosphorylation is a common example: a protein kinase transfers a phosphate from ATP to a specific residue (serine, threonine or tyrosine) on the enzyme. The added phosphate alters the enzyme’s tertiary structure, especially the active‑site conformation, which can either activate or inhibit the enzyme. The change is reversible because a phosphatase can remove the phosphate, restoring the original shape and allowing rapid control of metabolic pathways.

Examiner tips

Use the word ‘covalent modification’ first, then give the specific example of phosphorylation.
Mention the kinase, ATP donor and target residue.
Explain the structural change and its effect on activity.
State that phosphatases reverse the modification for rapid regulation.

Common mistakes

Confusing phosphorylation with other modifications (e.g. acetylation).
Forgetting to mention the role of ATP or the specific amino acid residues.
Failing to note that the change is reversible via phosphatases.

Mark scheme (4 marks)

Covalent modification involves the addition or removal of a chemical group to/from an enzyme, changing its activity
Phosphorylation involves the transfer of a phosphate group from ATP to a specific amino acid residue (e.g. serine, threonine, or tyrosine) on the enzyme, catalysed by a protein kinase
Addition of the phosphate group changes the shape of the enzyme's active site / tertiary structure, either activating or inhibiting the enzyme
The modification is reversible because a phosphatase enzyme can remove the phosphate group, restoring the original enzyme conformation and allowing rapid regulation of metabolic pathways

Key terms in this question

covalent modification · phosphorylation