Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologyeasymcq1 pt

Serine carries a hydroxyl on its side chain, while leucine carries a branched hydrocarbon side chain. In a folded protein dissolved in water, where is leucine most likely to sit, and why?

A.Buried in the interior core, clustered with other nonpolar side chains away from water
B.On the outer surface, where its side chain can hydrogen-bond with the surrounding water
C.Spread evenly through the protein, because side-chain chemistry does not influence folding
D.At the amino and carboxyl termini, where hydrophobic groups are chemically neutralized

Explanation

Core Concept

Leucine's side chain is a nonpolar hydrocarbon that cannot hydrogen-bond with water, so contact with the aqueous surroundings is unfavorable. During folding, the hydrophobic effect drives such nonpolar side chains together in the protein's core, releasing ordered water molecules and stabilizing the folded state. Serine, being polar, commonly occupies solvent-exposed surfaces instead. Side-chain chemistry is a major driver of folding, and position along the chain has nothing to do with this sorting.

Correct Answer

ABuried in the interior core, clustered with other nonpolar side chains away from water

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