A polypeptide is composed almost entirely of amino acids containing hydrophobic residues. How would you most likely describe the structure of the resultant protein?

Question

BlackTom AI · Accepted Answer

Hydrophobic amino acids are sequestered away from water by the hydrophobic effect, so a polypeptide rich in hydrophobic residues will fold to minimize those residues’ exposure to the aqueous environment.

1) Relevant knowledge points and concepts
- Hydrophobic effect: nonpolar side chains cluster together to reduce ordered water contact and lower system free energy.
- Protein structural levels: secondary structure (α-helix, β-sheet) is stabilized mainly by backbone hydrogen bonds; tertiary structure is the overall 3D arrangement that places hydrophobic residues inward and polar residues outward in soluble proteins.
- Soluble versus membrane proteins: in water-soluble proteins hydrophobic residues are buried in a core; membrane proteins may display hydrophobic residues exposed to the lipid bilayer.

2) Step-by-step reasoning
- Given an aqueous environment, hydrophobic side chains are energetically unfavorable when exposed to water.
- During folding, the polypeptide will rearrange so hydrophobic residues are packed together, reducing the exposed hydrophobic surface area.
- This packing produces a compact, globular tertiary structure with a hydrophobic core and polar/charged residues on the surface.

3) Calculations / intermediate steps
- No numeric calculation is required; the argument is thermodynamic: burying hydrophobic residues reduces ordered water shells (increases entropy) and lowers Gibbs free energy, so it is favored.

4) Why the correct answer is correct
- Answer b (“A structure largely clustered towards the centre of the molecule”) describes the classical hydrophobic core of soluble proteins. A polypeptide composed almost entirely of hydrophobic residues will fold to place those residues centrally to avoid contact with water.

5) Why the other options are incorrect
- a (many β-sheets): Secondary structure type is determined by backbone geometry and local sequence propensities, not solely by hydrophobicity; hydrophobicity doesn’t guarantee β-sheet content.
- c (open and spread out): An open structure would expose hydrophobic side chains to water, which is thermodynamically unfavorable.
- d (many α-helices): While α-helices can host hydrophobic residues (especially in membranes), hydrophobic composition alone does not necessitate many helices; the defining feature is burial of hydrophobic residues, not a specific secondary motif.

6) Conclusion
- Therefore, the most likely description is that hydrophobic residues cluster towards the center of the molecule, confirming answer b.

A polypeptide is composed almost entirely of amino acids containing hydrophobic residues. How would you most likely describe the structure of the resultant protein?Single choice

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