Structure and Action of Insulin

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The primary role of insulin is to control the absorption of glucose from the bloodstream into cells where glucose is utilized as an energy source or converted into glycogen for storage. Insulin functions to regulate the level of glucose in blood. Carbohydrates, such as starch, taken in the diet are digested into glucose, which is transferred to the blood stream. The high level of blood glucose stimulates the pancreatic p cells to release insulin directly into blood stream. Insulin binds to insulin receptors on the surface of a cell, generating signals for movements of glucose transporters to the cell membrane. The glucose transporters aggregate into helical structures creating channels for entrance of glucose molecules into the cells.

Insulin is produced in pancreatic cells as prepro-insulin which contains 4 segments: (1) an N-terminal signal sequence of 16 amino acids, (2) a B chain of 30 amino acids, (3) a C peptide of 33 amino acids, and (4) an A chain of 21 amino acids. In a later stage of the process, the N-terminal and the C peptides are cleaved. Active mature insulin consists of A- and B-chains held together by disulfide bonds (Fig. 16.1).

Human Insulin s-s

A Chain giveqcctsicslyqlenycn s s

B Chain fvnqhlcgshlvealylvcgergffytpkt

O Porcine f~l Bovine

O Porcine f~l Bovine

Signai sequence

Preproinsulin

Proinsulin

Fig. 16.1. Structure of human insulin and its posttranslational modification.

Signai sequence

Insulin

Preproinsulin

Proinsulin

Fig. 16.1. Structure of human insulin and its posttranslational modification.

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