Proteins · Paper 2 — Free Biology SL Practice Questions

A point mutation in the $\beta$-globin gene changes the sixth codon from GAG to GTG, substituting glutamic acid (hydrophilic, negatively charged) with valine (hydrophobic, non-polar) in the $\beta$-globin polypeptide. Individuals homozygous for this allele develop sickle cell anaemia; heterozygous individuals carry one normal and one mutant allele.

The enzyme lysozyme is a globular protein that hydrolyses bacterial cell walls. Its structure includes regions of $\alpha$-helix and $\beta$-pleated sheet, stabilised by hydrogen bonds, disulfide bridges, and hydrophobic interactions. A researcher treats lysozyme with a reducing agent that breaks all disulfide bridges (—S—S— bonds) but leaves peptide bonds intact.

Bovine pancreatic trypsin inhibitor (BPTI) is a protein of 58 amino acids. In its native state it has a well-defined tertiary structure stabilised by three disulfide bridges ($-\text{S}-$) between cysteine residues at positions 5–55, 14–38, and 30–51, and it is a potent inhibitor of the enzyme trypsin. A research team treats native BPTI with Compound X, which specifically reduces disulfide bridges to free thiol groups ($-\text{SH}$). After treatment, trypsin-inhibitory activity falls to $5\%$ of the original value. The treated BPTI is then allowed to refold in the presence of oxygen (which can re-oxidise thiol groups to disulfide bridges); after 24 hours, activity recovers to only $12\%$ of the original value. In a separate control experiment, native BPTI is denatured with $8\,\text{M}$ urea — which disrupts hydrogen bonds and hydrophobic interactions but leaves disulfide bridges intact — and then allowed to refold by dialysis to remove the urea. Activity recovers to $98\%$ of the original value.