Deficient Process in Recombinant Insulin Missing C-peptide
The early forms of recombinant insulin produced in bacteria that were missing the C-peptide were most likely to be deficient in the stability of the fully folded protein (option D).
Understanding C-peptide's Role in Insulin Structure and Function
The C-peptide (connecting peptide) plays a crucial role in insulin biosynthesis and protein stability:
- In normal insulin production, preproinsulin is synthesized and translocated into the endoplasmic reticulum where the signal peptide is cleaved to form proinsulin 1
- Proinsulin consists of the insulin B-chain, C-peptide, and A-chain in a continuous single polypeptide 2
- The C-peptide facilitates proper folding of proinsulin and formation of the three disulfide bonds critical for insulin's structure 1
Why C-peptide Absence Affects Protein Stability
- The C-peptide serves as a molecular chaperone that ensures proper spatial alignment of the A and B chains during folding 3
- Without the C-peptide, recombinant insulin has reduced stability of the fully folded protein structure 3
- Studies show that constructs with shortened or absent C-peptide linkers tend to be synthesized at lower levels and exhibit improper disulfide bonding 3
Evidence Against Other Options
A. Binding to insulin receptor by fully folded protein: The binding of insulin to its receptor is primarily determined by specific amino acid residues in the A and B chains, not by the C-peptide which is cleaved before secretion 4
B. Rapid formation of the appropriate S-S bonds: While C-peptide does assist in disulfide bond formation, this is a secondary effect of its primary role in ensuring proper spatial alignment for stable folding 3
C. Release from secretory granules: C-peptide is cleaved from proinsulin in secretory granules, but its absence in recombinant insulin wouldn't affect release mechanisms since bacterial expression systems don't use secretory granules 4
E. Translation of the A and B fragments: Translation itself isn't affected by C-peptide absence; rather, it's the post-translational folding and stability that are compromised 2
Clinical Relevance of C-peptide
- C-peptide is secreted equimolarly with insulin into circulation and is used clinically to assess endogenous insulin production 4
- Low or undetectable levels of plasma C-peptide are indicative of little or no insulin secretion, as seen in advanced type 1 diabetes 5
- C-peptide measurements help distinguish between type 1 and type 2 diabetes in ambiguous cases 5
- Beyond its role in insulin biosynthesis, C-peptide has been found to have biological activities including binding to cell membranes and triggering intracellular signaling 6
In conclusion, the absence of C-peptide in early recombinant insulin primarily affected the stability of the fully folded protein, leading to challenges in producing properly structured and functional insulin molecules.