Molecule shown in the Illustration is a precursor of:
Ans. A. InsulinMolecule shown is pro-insulina. Proinsulin is the prohormone precursor to insulin made in the beta cells of the islets of Langerhans in the pancreas. In humans, proinsulin is encoded by the INS gene. The islets of Langerhans only secrete between 1% and 3% of proinsulin intact. However, because proinsulin has a longer half-life than insulin, it can account for anywhere from 5- 30% of the insulin-like structures circulating in the blood. There are higher concentrations of proinsulin after meals and lower levels when a person is fasting. Additionally, while proinsulin and insulin have structural differences, proinsulin does demonstrate some affinity for the insulin receptor. Due to the relative similarities in structure, proinsulin can produce between 5% and 10% of the metabolic activity similarly induced by insulin.b. Proinsulin is the final single chain protein structure secreted by cells before cleavage into mature insulin. Proinsulin was discovered by Professor Donald F. Steiner of the University of Chicago in 1967.c. Proinsulin is made up of 81 residues and formed by three distinct chains. The A chain, B, chain and the area connecting the two named the C peptide. The correct structure of proinsulin is crucial for the correct folding of mature insulin as the placement of the C peptide sets the molecule up to create correctly positioned disulfide bonds in and between the A and B chains. There are three disulfide bonds that are necessary for mature insulin to be the correct structure. Two of these disulfide bonds are between the A and B chains, and one is an intra-A chain bond. The disulfide bonds occur between the seventh residues of the A and B chain, the 19th residue of the A chain and the 20th residue of the B chain, and the 6th and 11th residues of the A chain.d. The C peptide is between the A and B chains of proinsulin. The connection between the A chain and C peptide is much more stable than the junction between the C peptide and B chain with alpha helical features being exhibited near the C peptide-A chain connection. The C peptide-A chain junction occurs between residues 64 and 65 of proinsulin. These are lysine and arginine molecules, respectively. The C peptide-B chain connection is between two arginine residues at positions 31 and 32 of proinsulin.e. There is conservation of much of the structure of proinsulin among mammalian species, with much of the residue changes seen from one species to another present in the C peptide. The residues of the C peptide that are conserved across species interact with similarly conserved residues on the A and B chains. Thus, it is hypothesized that these conserved residues are important for the functionality of mature insulin.
Insulin
Glucagon
Somatostatin
Lipase
Endocrinology