Protein disulfide isomerase (PDI) is a protein folding enzyme that catalyzes disulfide–sulfhydryl exchange reactions. Ribonuclease is an enzyme with two β‑sheets, three α‑helices, and four disulfide bridges critical to the stability of its structure. PDI rapidly converts inactive, scrambled ribonuclease to enzymatically active ribonuclease. This process is driven by the decrease in free energy as the scrambled conformations are converted into the stable, active conformation of the enzyme. In contrast, PDI rapidly inactivates insulin, a protein that forms an hexamer of alpha and beta subunits.

What does PDI's effect on insulin imply about the structure of insulin?

Question

Protein disulfide isomerase (PDI) is a protein folding enzyme that catalyzes disulfide–sulfhydryl exchange reactions. Ribonuclease is an enzyme with two β‑sheets, three α‑helices, and four disulfide bridges critical to the stability of its structure. PDI rapidly converts inactive, scrambled ribonuclease to enzymatically active ribonuclease. This process is driven by the decrease in free energy as the scrambled conformations are converted into the stable, active conformation of the enzyme. In contrast, PDI rapidly inactivates insulin, a protein that forms an hexamer of alpha and beta subunits.

What does PDI's effect on insulin imply about the structure of insulin?

BlackTom AI · Accepted Answer

Let’s break down what the question is telling us about PDI’s effects on different proteins to infer insulin’s structural properties.
Option 1: 'The active conformation of insulin has more β‑sheets than α‑helices.' The statement about PDI altering ribonuclease to its active form reveals that disulfide bond rearrangements can stabilize active conformations that may differ in secondary-structure composition from the inactive state. However, this option specifically claims a relative increase in β‑sheets versus α‑helices for insulin, which isn’t directly supported by the described data about disulfide exchange driving activation or by the general principle that PDI reshapes disulfide connectivity. Thus, this conclusion is not a necessary implication of PDI’s effect.
Option 2: 'The three‑dimensional shape of insulin does not contain any disulfide bonds.' This is contradicted by insulin’s known structure, which includes disulfide bonds that are important for its stability and maturation. Since the question hinges on PDI’s ability to refold or remodel via disulfide exchange, denying the presence of disulfide bonds in insulin is factually incorrect and would undermine the premise of PDI acting on disulfide chemistry.
Option 3: 'The amino acid sequence of insulin is similar to the amino acid sequence of ribonuclease.' While both are proteins, sequence similarity is not a direct inference from PDI’s differential effects. PDI acts through disulfide exchange and folding pathways rather than reflecting sequence homology between distinct proteins. Therefore, concluding sequence similarity from this observation would be a misinterpretation of the mechanism described and is not logically supported by the given data.
Option 4: 'The active conformation of insulin is not the most thermodynamically preferred form.' The key idea is that PDI accelerates folding toward an active ribonuclease conformation by forming or rearranging disulfide bonds, moving the protein toward a functional state that may or may not coincide with the global thermodynamic minimum. The fact that PDI rapidly inactivates insulin suggests that, for insulin, the active state accessible via disulfide rearrangements is not the most stable (thermodynamically favored) form. This aligns with the observation that the PDI-driven process can push a protein toward an active conformation that is not the lowest free-energy state for that protein, revealing a discrepancy between functional activity and thermodynamic stability.
So, when considering these options, the only statement that coherently explains PDI’s contrasting effects on ribonuclease versus insulin is that the active insulin conformation is not the thermodynamically preferred form, which accounts for why disulfide‑exchange activity can both generate activity in one protein and inactivate another.

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