Hormones can be divided into two groups based on their binding with receptor. Some hormones are water soluble like protein or peptide hormone (example is insulin) and catecholamines. These water soluble hormones cannot cross the plasma membrane of the target cell and thus bind to the membrane receptor. Binding to the membrane receptor leads to activation of intracellular enzymes which bring the physiological change. Steroid hormones in the other case can cross the plasma membrane and then bind to the intracellular receptors. The hormone receptor complex then either binds or activates specific portion of DNA to bring the physiological change.

Insulin is made by beta cells islets of Langerhans in the pancreas. It controls the levels of glucose. It is a small protein made of two polypeptides, one of 21 amino acids and the other of 30 amino acids. These two are called A chain and B chain and are linked together by disulphide bonds. The pro-hormone contains an extra stretch called C-peptide. During the maturation process the proinsulin looses the c-peptide to become mature insulin protein hormone. An american company called Eli Lilly, for the first time made recombinant human insulin.

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