📝 Protein folding and chaperones (10 MCQs)
📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 10 questions available
What is Protein folding and chaperones?
Definition:
Protein folding is the process by which a polypeptide chain acquires its three-dimensional native conformation, driven by the thermodynamic requirement to minimize free energy, and chaperones are proteins that assist in folding by preventing misfolding and aggregation, ensuring that proteins reach their functional shape; this process is essential because protein function is highly dependent on its precise structure.
Working:
Protein folding works through a series of interactions, including hydrogen bonds, hydrophobic interactions, and disulfide bonds, and the folding pathway is guided by the Anfinsen principle, which states that the native structure is determined by the amino acid sequence, and chaperones like Hsp70 and chaperonins (e.g., GroEL/GroES) provide an isolated environment for folding, preventing aggregation; the free energy of folding is negative (), and misfolded proteins are often targeted for degradation, with chaperones also assisting in refolding under stress.
Example:
A simple example is the folding of the protein lysozyme, which in the presence of chaperones folds into its active conformation, but without chaperones, it may aggregate and become inactive; another example is the chaperonin GroEL, which forms a barrel-like structure that encapsulates unfolded proteins and allows them to fold correctly, illustrating how chaperones assist in proper protein folding.
Reason:
Protein folding and chaperones are essential for understanding protein function, cellular homeostasis, and diseases like Alzheimer's (where protein misfolding and aggregation occur), and they are key targets for therapeutic strategies, making this a critical area in biochemistry and molecular biology.
📝 All Protein folding and chaperones MCQs
Q1. A newly synthesized polypeptide reaches a stable three-dimensional structure in a cell. Which observation best indicates that this structure is its native conformation?
📖 Explanation: The native conformation is the biologically functional three-dimensional state favored under appropriate cellular conditions. It generally corresponds to a thermodynamically favorable arrangement, although cellular factors can influence how efficiently the protein reaches that state.
Q2. Two proteins have identical amino-acid compositions but different sequences. Protein X folds efficiently into one functional structure, whereas protein Y forms several aggregates. What is the best explanation?
📖 Explanation: Amino-acid sequence determines the positions and chemical properties of side chains, influencing interactions such as hydrophobic packing, hydrogen bonding, ionic interactions, and steric effects. Therefore, different sequences can produce different folding pathways and aggregation tendencies.
Q3. A researcher mutates several hydrophobic residues buried inside a protein to charged residues while leaving the catalytic residues unchanged. The enzyme loses activity and becomes unstable. Which reasoning best connects the mutation to the functional change?
📖 Explanation: Hydrophobic residues commonly contribute to the nonpolar interior of folded proteins. Replacing them with charged residues can disrupt packing and favorable interactions, destabilizing the native conformation. Loss of the correct structure can consequently impair catalytic function.
Q4. A cell produces a mutant protein that can eventually become functional but frequently aggregates before reaching its functional state. Increasing the concentration of a molecular chaperone greatly reduces aggregation without changing the protein sequence. What is the most reasonable interpretation?
📖 Explanation: Molecular chaperones can assist proteins during folding by reducing inappropriate intermolecular interactions and aggregation. They generally do not encode the protein's sequence or permanently become part of the final protein structure.
Q5. A student argues: 'If a protein reaches its native conformation spontaneously, molecular chaperones cannot be useful because they would have nothing to do.' Which correction is strongest?
📖 Explanation: A protein may be capable of reaching its native state spontaneously yet still encounter competing interactions or aggregation pathways. Chaperones can increase the probability of productive folding by shielding exposed regions or providing controlled folding environments.
Q6. A folding experiment measures the fraction of protein in the native state as temperature increases. The curve rises from 0.10 at 20°C to 0.85 at 35°C, then falls to 0.20 at 60°C. Which conclusion is most justified?
📖 Explanation: The graph indicates that the native-state population increases initially and reaches a maximum around the middle of the tested range before declining. This pattern is consistent with temperature affecting the balance between folded and non-native conformations.
Q7. A second experiment compares aggregation with and without a molecular chaperone. At every temperature tested, the aggregation curve is lower when the chaperone is present, while the final amount of native protein is similar. What does this pattern most strongly suggest?
📖 Explanation: If native protein levels remain similar but aggregation decreases in the presence of the chaperone, the chaperone is most plausibly reducing an alternative, unproductive pathway. This illustrates how chaperones can improve folding efficiency without specifying primary sequence.
Q8. Protein A and Protein B begin with comparable unfolded concentrations. Protein A rapidly becomes functional, whereas Protein B remains partly unfolded and aggregates. Addition of a chaperone rescues Protein B. Which sequence of reasoning is most appropriate?
📖 Explanation: The comparison suggests that Protein B is capable of forming its functional structure but competes with aggregation during folding. A chaperone can interact with vulnerable folding intermediates, reducing inappropriate contacts and increasing productive folding.
Q9. A scientist observes that a protein remains folded after removal of a chaperone, but the chaperone was essential for preventing aggregation during synthesis. Which conclusion best reconciles these findings?
📖 Explanation: Chaperones often assist the pathway by preventing aggregation or inappropriate interactions rather than serving as permanent structural components. Thus, a protein can require chaperone assistance to fold efficiently while its final native conformation remains stable independently.
Q10. A protein has two possible folding routes. Route 1 produces the native structure through several intermediate states. Route 2 forms a compact aggregate that is difficult to reverse. A mutation increases the probability of Route 2. Which prediction is most defensible?
📖 Explanation: A mutation can alter the relative stability or accessibility of folding intermediates without directly destroying the functional sequence information. If aggregation becomes more favorable, less protein may reach the native conformation, reducing functional protein abundance.