Final answer:
The form of insulin that is secreted is different from the preproinsulin synthesized on the ribosomes because it undergoes post-translational modifications, including the removal of the 'C' chain by proteolytic enzymes to become active insulin, which is a dimer of A and B chains.
Step-by-step explanation:
The form of insulin that is secreted by the pancreas is different from the preproinsulin that is synthesized on membrane-bound ribosomes. Insulin is first synthesized as preproinsulin, which then converts to proinsulin through the removal of 23 amino acids from the N-terminal end. Proinsulin, which contains an additional 'C' chain composed of 35 amino acids, serves as the storage form. The active form of insulin, however, emerges after the 'C' chain is cleaved off by proteolytic enzymes. This results in the active insulin being a dimer composed of two polypeptide chains, known as the A and B chains. The assembly of these chains is facilitated by disulfide bonds that stabilize the structure and ensure the functional activity of the hormone.