The hormone insulin provide a good example to explain the mode of human action through the extracellular receptors. The membrane bound receptor of insulin is a protein consisting of four subunits : the two a-subunits and two p-subunits. The following are the steps in this mechanism :
(a) Binding to the receptor : Insulin binds with the outer a-subunits of the receptor. This causes a structural change in the p-subunits, to become an activated kinase, which promotes phosphorylation.
(b) Second Messengers : The hormonal control of cell function is done through adenylate cyclase. The hormone receptor complex does not directly stimulates adenylate cyclase. It is done through a transducer G protein. The hormone receptor
complex induces the release of GDP from the G protein. It makes phosphatidylinositol which activates other processes.
(c) Amplification of signal : Mediates amplifier signals. The P-subunit activates DG and IP3 with second messenger. Some hormones use secondary messenger cAMP. Adenyl cyclase is amplifier. It changes Adenyl cyclase into ATP and then into cAMP. Adrenaline binds to receptors in liver or muscles cells which changes shape and binds to G protein. Then it binds to GTP and activates adenyl cyclase. This adrenaline releases 100 million moles of glucose (C5Hi2Ofi) in one or two minutes.