Hormone Receptor properties
All receptors are proteins and have these properties-
- Each cell may have receptors for more than one hormone and therefore is a target for several hormones
- Specificity spill over – hormones with similar structure may bind to one another’s receptors usually with lower affinity and ability to activate the receptors.
- Activation of receptor by a ligand is an all or none phenomena
- Message carried by the ligand is in its three dimensional structure which is important to bind to the receptors
- Once a hormone binds to its receptor the message is delivered and the hormone plays no further role
Hormone receptor combination initiates a cascade of reactions
Specificity of Receptors
The binding site of the receptor is highly specific for each signaling molecule(hormone, neurotransmitter); molecules that bind specifically to receptor are called ligands.
However, some molecules having structural similarity to ligands can also bind with receptors and produce a physiological response and such molecules are called as agonists. Some molecules having structural similarity with hormone may bind with receptors without producing a response and inhibit the hormone effect are called as antagonists.
High affinity: Usually the concentration of the hormone in the blood is very low(in pmol/l to μmol/l). Because the receptors have high affinity to hormones, binding occurs rapidly
Binding capacity: Number of receptors available on the target cells at any given time is the limiting factor that determines number of hormone molecules bound to target organs; i.e. the binding capacity of target cells is decided by the receptor numbers.
The concentration of receptors is quite variable. When a target cell is exposed to high level of hormone for a period of time, the number of active receptors will be reduced (by receptor degradation) known as down regulation.
During deficiency of a hormone, there is an increase in the number of active receptors known as up regulation.
When a receptor becomes bound to a hormone, the receptor undergoes a conformational change (known as receptor activation), which allows it to interact productively with other components of the cells, leading ultimately to an alteration in the physiologic state of the cell.