Explanation:
During the processing of proinsulin into the mature insulin, the C-peptide is removed from proinsulin.

Proinsulin is a single chain polypeptide consisting of three regions: the N-terminal B-chain, the C-terminal A-chain, and a connecting peptide (C-peptide) in the middle. The connecting peptide or C-peptide is important because it connects the B-chain and A-chain of proinsulin.

During the processing of proinsulin, the C-peptide is cleaved enzymatically from the precursor molecule. This cleavage reaction is catalyzed by proteolytic enzymes called prohormone convertases (PCs) in the secretory granules of pancreatic beta-cells.

Once the C-peptide is removed, the remaining A and B chains of proinsulin are held together by disulfide bonds to form mature insulin. Mature insulin is then secreted into the bloodstream in response to increased glucose levels.

The removal of C-peptide is important for two reasons:

1. Regulation of insulin secretion: C-peptide is co-secreted with insulin in equimolar amounts, and its removal allows for more precise control of insulin secretion.

2. Diagnostic value: C-peptide levels can be measured in blood to assess insulin secretion and to differentiate between type 1 and type 2 diabetes. Since C-peptide has a longer half-life than insulin, measuring C-peptide levels can provide a more accurate assessment of insulin secretion over time.

In conclusion, during the processing of proinsulin into mature insulin, the C-peptide is enzymatically cleaved from the precursor molecule, leaving the A and B chains of insulin held together by disulfide bonds.