Questions: Protein Tertiary Structure

Question 1

Which force is generally considered the dominant driving force in the folding of a water-soluble protein into its tertiary structure?

• A. Disulfide bond formation between cysteine residues
• B. Hydrogen bonds between backbone amide groups
• C. The hydrophobic effect — burial of nonpolar side chains away from water
• D. Ionic interactions (salt bridges) between charged side chains

Answer: (C)

While all four interactions contribute to tertiary structure, the hydrophobic effect is typically the dominant driving force for water-soluble proteins. Nonpolar side chains are thermodynamically unfavorable when exposed to water (they disrupt hydrogen-bond networks), so folding buries them in the protein core. This is an entropy-driven process at the level of the surrounding water. Disulfide bonds, hydrogen bonds, and salt bridges fine-tune and stabilize the structure once folded.

Question 2

Disulfide bonds between cysteine residues can form anywhere inside a cell, including in the cytoplasm.

• T. True
• F. False

Answer: False

Disulfide bonds require an oxidizing environment to form — two cysteine thiol (-SH) groups must lose electrons to become a covalent -S-S- bond. The cytoplasm of most cells is a reducing environment, which keeps cysteines in their -SH form. Disulfide bonds are therefore found primarily in extracellular proteins and proteins that pass through the endoplasmic reticulum (ER), which provides the oxidizing conditions necessary for their formation.

Question 3

What is the key difference between secondary structure and tertiary structure in proteins, in terms of what determines each level?

Model answer: Secondary structure (alpha-helices and beta-sheets) is determined by hydrogen bonds between atoms of the polypeptide backbone and depends on the local geometry of consecutive residues. Tertiary structure is the overall 3D fold of the entire chain and is determined by interactions between amino acid side chains (R groups) — hydrophobic clustering, disulfide bonds, salt bridges, and hydrogen bonds between non-adjacent residues.

The distinction matters because secondary structure patterns can be predicted from backbone geometry alone, while tertiary structure requires knowing the specific amino acid sequence (which side chains are present and where). This is why tertiary structure is harder to predict computationally and why the protein-folding problem — figuring out the 3D structure from sequence — was considered one of biology's grand challenges until AlphaFold.