MCAT® ExamChemical and Physical Foundations of Biological SystemsEasy
A biochemist is analyzing the structure of a novel protein. They observe that the protein contains several cysteine residues that form disulfide bonds. Which of the following levels of protein structure is directly stabilized by these disulfide bonds?
- ATertiary structure
- BQuaternary structure
- CSecondary structure
- DPrimary structure
Show answer & explanationAnswer & explanation
Correct answer: A. Tertiary structure
Disulfide bonds are covalent bonds formed between the thiol groups of cysteine residues, and they play a crucial role in stabilizing the three-dimensional folding of a single polypeptide chain, which is characteristic of tertiary structure. While quaternary structure can also involve disulfide bonds between different polypeptide chains, the question refers to 'a novel protein' implying a single protein's folding.
Why the other options are wrong
- B. Quaternary structure involves the arrangement of multiple polypeptide subunits, which can be stabilized by disulfide bonds, but tertiary structure specifically refers to the folding of a single chain.
- C. Secondary structure (alpha-helices and beta-sheets) is stabilized primarily by hydrogen bonds between backbone atoms.
- D. Primary structure is the linear sequence of amino acids, stabilized by peptide bonds, not disulfide bonds.
Protein Tertiary Structure
The overall three-dimensional shape of a single polypeptide chain, resulting from interactions between the side chains of amino acids.
- Determined by interactions between R-groups (side chains).
- Stabilized by hydrophobic interactions, hydrogen bonds, ionic bonds, and disulfide bonds.
- Crucial for protein function as it creates the active site in enzymes.
Memory trick: Primary: Peptide sequence. Secondary: Spirals & Sheets. Tertiary: Three-dimensional folds. Quaternary: Q-tip multiple units.