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Which of the following statements is not correct?
Proinsulin has an extra C-peptide which is removed during maturation. The functional mature A (21 aa) and B (30 aa) polypeptide chains are held together by covalent DISULPHIDE bonds (-S-S- bridges), not weak hydrogen bonds. Statement (3) is false!
In humans and mammals, insulin is synthesized as a pro-hormone (proinsulin) that requires proteolytic processing before it becomes fully active and functional.
Proinsulin contains an extra polypeptide segment called the C-peptide (connecting peptide). During maturation into functional insulin, this C-peptide is cleaved and removed; it is completely absent in mature active insulin.
Mature functional insulin consists of two polypeptide chains: Chain A (21 amino acids) and Chain B (30 amino acids). These two chains are covalently linked together by two interchain disulphide bonds (and one intrachain bond in Chain A), NOT by weak hydrogen bonds.
In 1983, American company Eli Lilly prepared two DNA sequences corresponding to chains A and B of human insulin, introduced them into plasmids of E. coli to produce chains A and B separately, extracted them, and combined them by creating disulphide bonds in vitro.
Diagnose every choice: Know exactly why the right option works and why distractors fail.
This statement is factually true, hence not the required incorrect answer.
Factually true; proinsulin possesses the connecting C-peptide which is excised during maturation.
This statement is FALSE and therefore the correct answer. Chains A and B are linked by covalent disulphide (-S-S-) bonds, not hydrogen bonds.
Factually true; Eli Lilly successfully used recombinant Escherichia coli to produce chains A and B in 1983.
Proteins have secondary structures stabilized by hydrogen bonds, but the covalent linkage holding the separate A and B polypeptide chains of insulin together is strictly DISULPHIDE (-S-S-) bridges.
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