What are hormone receptors ?
Hormone receptors are proteins present mostly on the cell membrane or surface of cells on the body, where the hormones bind and carry out an action.
Hormones act specifically and with high affinity with certain allosteric proteins on the cell surface (hormone receptor proteins). This binding with hormone receptors then triggers the hormone response.
The structure of the binding site on the receptor is highly specific for a hormone e.g. Glucagon receptors have a high affinity for glucagon but not insulin.
However, some hormones, similar in structure may bind to each others receptors e.g. aldosterone weakly binds to glucocorticoid receptors. The hormone receptor interaction leads to a cascade of events in the cell which results in the hormonal response.
The insulin receptor contains 4 polypeptide chains linked by disulphide bonds. The receptor is glycosylated and the sugar moiety is critical for the receptors function.
Occupancy of only a small proportion of receptor’s results in a full hormonal response. In these cases “spare receptors” are said to be present and the limiting factor in determining the magnitude of the hormonal response is distal to the initial hormone-receptor interaction.
At specialized regions of the cell-membrane a protein clathryn accumulates on the inside portion of the membrane. Vesicles from these specialized regions are called endosomes where the hormone is separated from the receptor.
The endosome can be returned to the cell membrane or can differentiate into other cellular compartments where the hormone and receptor can be separated. Sometimes, the hormone-receptor complex comes to the cell surface whereas at other times, it can be fused with the lysosomes when the hormone or receptor or both can be degraded
E.g. Most of the internalized insulin is degraded whereas most of the Epidermal growth factor (EGF) and its receptors are degraded. This provides one of the mechanisms whereby hormones can down regulate the levels of their receptors.