Receptors are chemical signal-receiving protein molecules found inside or on the surface of the target cell. Signaling cells send out chemical signals in the form of ligands, which are small molecules that are usually volatile or soluble. A ligand is a molecule that binds to another molecule and, in some cases, sends a signal as a result of the binding. As a result, ligands can be considered signaling molecules. Ligands and receptors come in a variety of shapes and sizes; however, a specific ligand will typically have a specific receptor that binds only that ligand.
Receptors are classified into two types:
- Cell Surface Receptors
- Intracellular Receptors
Cell Surface Receptors:
- Any cell of a multicellular organism is exposed to a variety of signals.
- At any time, hundreds of different chemical signals in the environment surround the cell.
- The receptor proteins are located on or within the cell having a three-dimensional shape that fits into a specific signal molecule.
- When a signal molecule binds with a receptor protein of the right shape, the activation of the receptor occurs.
- This binding produces a change in the receptor protein’s shape, producing a response in the cell.
- Most of the signal molecules are water-soluble such as peptide hormones, neurotransmitters, and proteins that act as growth factors during development.
- Water-soluble signal molecules cannot diffuse through the cell membranes.
- Therefore, for the generation of responses, they must bind to receptor proteins on the surface of the cell.
- These cell surface receptors convert the extracellular signal to an intracellular signal, by binding with the signal molecule by producing a change within the cytoplasm of the cell.
- These are of three types;
- Chemically gated ion channels
- Enzymatic receptors
- G-protein linked receptors
1. Chemically Gated Ion Channels:
- These are the protein receptors through which ions passes.
- In the plasma membrane, many protein molecules are embedded.
- At the centre of the protein, the pore is present that connects the extracellular fluid with the cytoplasm.
- The size of the pore is big so that ions can easily pass through it, so the protein functions as an ion channel.
- The channel is known as chemically gated because it opens when a chemical (neurotransmitter) binds with it.
- When a chemically gated ion channel gets opens, different varieties of ions such as sodium, potassium, calcium, chloride flows across the membrane depending on the structure of the channel.
2. Enzymatic Receptors:
- Many cell-surface receptors either act as enzymes or are directly linked to enzymes.
- When a signal molecule binds to the enzymatic receptor, it activates the enzyme.
- The enzymes that get activated are called protein kinases, which add phosphate groups to the proteins.
- The enzymatic receptor consists of signal trans-membrane protein; the portion that binds to signal molecule lies outside the cell, and the portion that carries out the enzyme activity is exposed to the cytoplasm.
3. G-protein Linked Receptors:
- G-protein linked receptors acts indirectly on enzymes or ion channels in the plasma membrane with the help of protein, called a guanosine triphosphate (GTP) binding protein, or G-protein.
- In this type, G-proteins are used to transmit signals from the membrane surface into the inside of the cell.
- G-proteins are the mediators that initiate a diffusible signal in the cytoplasm.
- They form a link between the cell surface receptor and signal pathway within the cytoplasm.
- As the signal arrives, it finds the G-protein present in the G-protein-linked receptor on the cytoplasmic side of the plasma membrane.
- Once the signal molecule binds to the receptor, the shape of the G-protein-linked receptor gets changes.
- The G-protein can now diffuse away from the receptor.
- The “activated” complex of a G-protein with attached GTP is then free to initiate a number of events.
- Many cell signals are lipid-soluble or very small molecules that can readily pass across the plasma membrane of the target cell and into the cell, where they interact with a receptor.
- Some cell signals bind to protein receptors present in the cytoplasm whereas; others pass across the nuclear membrane as well and bind to receptors within the nucleus.
- These intracellular receptors may trigger a variety of responses in the cell, depending on the receptor.
Receptors that Act as Gene Regulators:
- Some of the intracellular receptors act as regulators of gene transcription. For example, receptors
for steroid hormones, such as cortisol, estrogen, and progesterone.
- All of these receptors have a DNA binding site.
- In an inactive state, the receptor cannot bind with DNA because an inhibitor protein occupies the binding site.
- As the signal molecule binds to another site on the receptor, the inhibitor is released and the DNA binding site is exposed.
- The receptor then binds to a specific nucleotide sequence on the DNA, which activates a particular gene, usually located adjacent to the regulatory site.
Receptors that Act as Enzymes
- The other intracellular receptors act as enzymes. For example, Nitric oxide (NO).
- A small gas molecule NO diffuses out of the cells where it is produced and passes directly into neighboring cells, where it binds with the guanylyl cyclase enzyme.
- The binding of NO to the receptor activates the enzyme, enabling it to catalyze the synthesis of guanosine monophosphate GMP, an intracellular messenger molecule that produces cell-specific responses such as the relaxation of smooth muscle cells.