Insulin is synthesized in theb-cells of the pancreas as a proinsulin molecule. In order for this proinsulin to be biologically active, which of the following posttranslational modifications of the hormone is required?
A) acetylation of the α-amino group
B) binding of zinc ions
C) cleavage of peptide bonds
D) oxidation of sulfhydryl groups
E) phosphorylation of tyrosine residues

Question

Quizplus · Accepted Answer

Cleavage of peptide bonds is required to convert proinsulin into active insulin by removing the C-peptide, leaving the A and B chains connected by disulfide bonds.  Insulin is synthesized as a preprohormone in the b-cells of the islets of Langerhans. Its signal peptide is removed in the cisternae of the endoplasmic reticulum and it is packaged into secretory vesicles in the Golgi, folded to its native structure, and locked in this conformation by the formation of 2 disulfide bonds. Specific protease activity cleaves the center third of the molecule, which dissociates as C peptide, leaving the amino terminal B peptide disulfide bonded to the carboxy terminal A peptide.

Insulin Is Synthesized in Theb-Cells of the Pancreas as a Proinsulin