Protein disulfide isomerase (PDI) is a protein folding enzyme that catalyzes disulfide–sulfhydryl exchange reactions. Ribonuclea
se 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. What does PDI's effect on insulin imply about the structure of insulin? The three‑dimensional shape of insulin does not contain any disulfide bonds. The active conformation of insulin has more β-sheets than α-helices. The amino acid sequence of insulin is similar to the amino acid sequence of ribonuclease. The active conformation of insulin is not the most thermodynamically preferred form.
The effect of protein disulfide isomerase on insulin signifies that the active conformation of insulin is not the most thermodynamically favored form. The main reason behind this is that the protein disulfide isomerase seems to decline the free energy, that is, it makes them more steady form predominant.
In the case of insulin, the prevalence of the stable form results in its inactivation. Thus, it signifies that the active form is not thermodynamically stable.
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