A Levels Biology (9700)•9700/12/M/J/19

Explanation
Active Site Formation via Protein Folding Levels
Steps:
- Primary structure is the linear amino acid sequence, which sets the foundation but does not position residues spatially for the active site.
- Secondary structure forms local folds like alpha helices and beta sheets, positioning nearby residues correctly.
- Tertiary structure folds the chain into a 3D shape, bringing distant residues together to create the active site.
- Quaternary structure assembles multiple polypeptide chains, allowing active site formation across subunits in multisubunit enzymes.
Why B is correct:
- Active site requires spatial arrangement of residues via secondary (local motifs), tertiary (overall fold), and quaternary (subunit interactions) structures, per protein folding hierarchy.
Why the others are wrong:
- A includes primary (sequence alone lacks 3D positioning) and excludes quaternary (needed for multisubunit enzymes).
- C excludes tertiary (essential for 3D pocket formation).
- D excludes all higher levels (ignores folding necessary for active site).
Final answer: B
Topic: Proteins
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