Biomolecules and enzymes: hydrophobic effect in folding: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Biomolecules and enzymes: hydrophobic effect in folding

Sample item

In a water-soluble globular protein, nonpolar side chains such as leucine, isoleucine and phenylalanine are mostly buried in the interior. Which force chiefly drives this arrangement?

AIonic bonds between buried acidic and basic residuesBThe hydrophobic effect, increasing entropy of surrounding waterCCovalent cross-linking of the buried side chainsDHydrogen bonding of every buried residue to water

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. The hydrophobic effect, increasing entropy of surrounding water

Burying nonpolar side chains releases ordered water molecules that surrounded them, raising the entropy of the solvent. This hydrophobic effect is the main driving force of folding, while hydrogen bonds, ionic bonds and disulfides refine and stabilise the structure.

Option by option

A. Ionic bonds between buried acidic and basic residues
Ionic interactions occur between charged residues, mostly at the surface, and do not explain burial of nonpolar chains.
B. The hydrophobic effect, increasing entropy of surrounding water (correct)
Correct. Burying nonpolar groups frees ordered water, and the resulting entropy gain drives folding.
C. Covalent cross-linking of the buried side chains
Covalent cross-links such as disulfides involve cysteine and do not hold leucine or isoleucine in the core.
D. Hydrogen bonding of every buried residue to water
The buried residues are nonpolar and cannot hydrogen bond with water, which is exactly why they leave the aqueous phase.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, The hydrophobic effect, increasing entropy of surrounding water. Burying nonpolar side chains releases ordered water molecules that surrounded them, raising the entropy of the solvent.

Why is "Ionic bonds between buried acidic and basic residues" not the answer?

Ionic interactions occur between charged residues, mostly at the surface, and do not explain burial of nonpolar chains.

Why is "Covalent cross-linking of the buried side chains" not the answer?

Covalent cross-links such as disulfides involve cysteine and do not hold leucine or isoleucine in the core.

Why is "Hydrogen bonding of every buried residue to water" not the answer?

The buried residues are nonpolar and cannot hydrogen bond with water, which is exactly why they leave the aqueous phase.

What concept does this question test?

It tests Biomolecules and enzymes: hydrophobic effect in folding in Biochemistry. This hydrophobic effect is the main driving force of folding, while hydrogen bonds, ionic bonds and disulfides refine and stabilise the structure.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Biomolecules and enzymes: hydrophobic effect in folding?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.