Lysozyme contains several intra‑chain disulfide bonds that stabilize its three‑dimensional structure. Explain how reduction of these disulfide bonds by a reducing agent such as dithiothreitol (DTT) affects lysozyme’s structure and its ability to hydrolyse bacterial cell‑wall peptidoglycan.
Answer
Reduction breaks the disulfide bridges, causing loss of the rigid tertiary structure; the active‑site geometry is distorted, leading to a drastic decrease or complete loss of catalytic activity.
Step-by-step solution
- State that disulfide bonds are covalent links between cysteine residues that lock portions of the polypeptide chain into a fixed orientation.
- Explain that a reducing agent donates electrons to break the S–S bond, converting each cystine into two free thiol (–SH) groups.
- Describe that the removal of these covalent restraints allows the protein chains to unfold or adopt many conformations, thus disrupting the native tertiary structure.
- Point out that the active site of lysozyme is formed by residues positioned precisely by the disulfide‑stabilised fold; loss of this fold changes the distances and orientations of catalytic residues.
- Conclude that because the geometry of the active site is essential for binding the peptidoglycan substrate, its distortion greatly reduces the rate of hydrolysis, often rendering the enzyme inactive.
Common mistake: Assuming reduction only changes primary structure, not tertiary conformation
Concept tested: Disulfide bond reduction and protein activity
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