MCQ – 100689

Correct Answer » A Explanation

Detailed Explanation:

Clinical Reasoning for the Correct Answer:
– IGF-1 and IGF-2 are named for their structural similarity to insulin.
– All three peptides share homologous domains and overall tertiary structure (A and B chains with disulfide bonds).

Why Option A is Correct (Insulin):
– Insulin, IGF-1, and IGF-2 are single-chain polypeptides with significant sequence and structural similarity.
– They all utilize a similar alpha-helix and beta-sheet framework connected by disulfide bridges.

Why Option B is Incorrect (Preproinsulin):
– Preproinsulin is a precursor form containing a signal peptide in addition to the actual proinsulin sequence.
– The presence of the signal peptide makes preproinsulin structurally different from IGF-1/IGF-2, which both lack such a segment.
– Preproinsulin undergoes cleavage to form proinsulin, then insulin.

Why Option C is Incorrect (Proinsulin):
– Proinsulin contains the A, B, and C peptide segments in a single chain (precursor to insulin).
– IGFs lack a connecting C-peptide domain; their structure more closely mimics mature insulin.
– The folding and structure of IGFs do not include the full proinsulin chain arrangement.

Why Option D is Incorrect (C-peptide):
– C-peptide is a small connecting peptide, released from proinsulin cleavage, found in blood alongside insulin.
– C-peptide alone does not resemble the full insulin/IGF-1-like structural fold.
– IGF-1 and IGF-2 are functionally and structurally unrelated to C-peptide.

Differential Structural Features
– Insulin: Mature, two-chain polypeptide post-cleavage of C-peptide.
– Proinsulin: Precursor with A, B, and C segments; needs cleavage.
– Preproinsulin: Proinsulin + signal peptide; not present in circulation.
– C-peptide: Cleaved connector; not structurally similar to IGFs or mature insulin.

Key Discriminators
– IGF-1/IGF-2 structure is most homologous to mature insulin, not precursor forms or peptide fragments.