The following points highlight the top five properties of hormone receptor interactions. The properties are: 1. Hormone Receptor Interaction is Rapid and Reversible 2. Receptor Specificity 3. Receptor Affinity 4. Saturation 5. Agonist and Antagonist.
Property # 1. Hormone Receptor Interaction is Rapid and Reversible:
Most hormones bind to their receptors rapidly and reversibly. The interaction also terminates very rapidly. The number of receptors for a given hormone on any cell type varies from zero to more than 1 million. Moreover, non-target cells may also possess receptors. For example, TSH receptors are not only found on thyroid cells but also on adipocytes.
Property # 2. Receptor Specificity:
The most characteristic and essential feature of hormone receptor interaction is the specificity of binding. At the first level, specificity means that each hormone binds to its specific receptor. For example, glucagon binds only to glucagon receptors, insulin to insulin receptors. At a more subtle level, specificity implies that a hormone and its derivatives bind to their receptors with an affinity that is directly related to its bioactivity.
Human insulin is about 50 times as potent in stimulating the metabolism of isolated fat cells as human pro-insulin. It binds to insulin receptor with 50 times the affinity of pro-insulin. Insulin-like growth factor-I has an even lower affinity for the insulin receptors and even lower insulin-like activity.
Property # 3. Receptor Affinity:
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Hormones bind to their receptors with high affinity and specificity, because most peptide hormones are present in the circulation in pico-molar to Nano molar concentrations. Receptors must have approximately high affinities to achieve significant binding at physiological levels. Receptor affinity may be calculated from the kinetics of association and dissociation.
The affinity of a hormone for its receptor results from non-covalent binding, primarily in the form of hydrophobic interactions that provide the driving force for the binding reaction, and from electrostatic interactions.
Property # 4. Saturation:
The binding of hormone to receptor is a saturable process; there are a finite number of receptors on a target cell. In addition, the binding of hormone to receptor must either precede or accompany the biological response, and the magnitude of the biological response must be associated, in some manner, with receptor occupancy.
Property # 5. Agonist and Antagonist:
Hormones or analogues that bind to receptors and elicit the same biological response as the naturally occurring hormone are termed as agonists. Molecules that bind to receptors but fail to elicit the normal biological response are termed competitive antagonists, as they occupy the receptors and prevent the binding of the biologically active molecules.
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Molecules that bind to receptors but are less biologically active than the native hormones are termed partial agonists.