Receptor proteins are chemical transfer substances, chemicals that cause olfactory and taste sensations, and a variety of drugs, which can cause their effects only after specific binding with the corresponding substances on the cell membrane. The substances on this type of cell membrane are mostly proteins, and are called receptor proteins. At present, the receptor protein can be separated and purified, and its characteristics have been studied. For example, the receptor for acetylcholine is obtained by extracting pure protein from an electric fish discharge organ rich in this kind of protein. The characteristics of its molecular weight and molecular structure, as well as its ability to block acetylcholine, are now being studied.
Cell membrane receptors
Cell membrane receptors are one or a class of molecules on the surface of cells. They can recognize and bind specific biologically active substances (called ligands). The complexes they generate can activate and initiate a series of physical and chemical changes, which leads to the ultimate biological effect of the substance. The changes in various factors in the cellular environment are the corresponding changes in the physiological processes that affect the cells through the action of cell membrane receptors.
Figure 1. The seven-transmembrane α-helix structure of a G-protein-coupled receptor.
The factors and pathways of receptor regulation are complex. Under normal physiological conditions, the number of receptors is affected by the microenvironment and rises or falls, which is called ascending or descending regulation. The concentration of the ligand that binds to the receptor has a more important effect on regulating the receptor. For example, when the insulin concentration in the blood of an animal or a human is high, the insulin receptor concentration on the target cell decreases, and if the insulin concentration decreases the number of receptors will rise rapidly. Receptor regulation can also be performed through "negative synergistic effects", that is, receptor-receptor interactions result in changes in receptor affinity. For example, the binding of a receptor to an insulin molecule will promote the interaction between the receptor and other surrounding receptors, leading to a decrease in the affinity of the receptor, thereby dissociating the insulin molecule from the receptor.
There are more than a dozen neurotransmitters, each of which has one or more receptors. As far as acetylcholine is concerned, there are at least three receptors in vertebrates, of which nicotinic cholinergic receptors and muscarinic cholinergic receptors have been studied more. Nicotinic cholinergic receptors are distributed in the cell membranes of autonomic ganglia, central nerves, and electrical organs of electric eels. When the receptor is activated by binding to nicotine, the ion channel opens quickly and the duration of opening is short (ms level). Muscarinic cholinergic receptors exist in tissues such as parasympathetic nerves and smooth muscles. When the receptor is activated by binding to muscarinic venom, the ion channel opens slowly and lasts for a long time (seconds). Each of the above two types of receptors can bind to acetylcholine because they each have a structure matching a different group from acetylcholine.
Cholera toxin receptor
Many toxins have been found to work by binding to receptors on the cell membrane. For example, cholera toxin is an exotoxin produced by Vibrio cholerae, with a molecular weight of 84000 and composed of two subunits, A and B. The A subunit has two peptide chains, A1 and A2, connected by a pair of disulfide bonds. Subunit B binds to a receptor on the cell membrane. Subunit A1 has the function of activating adenylate cyclase on the membrane.
Figure 2. Cholera toxin mechanism.
The receptor for cholera toxin is a ganglioside. After binding toxin, a series of reactions may occur, such as first triggering the conformational change of the receptor, and then the subunit A1 will activate the adenylate cyclase in the process Adenosine diphosphate in nicotinamide adenine dinucleotide (NAD) is transferred to a protein in the cell membrane. Under normal circumstances, guanosine phosphate activates adenylyl cyclase by combining with this protein. When guanosine is hydrolyzed by guanosine, the activation stops. However, if ornithine is bound to adenosine diphosphate-containing protein, it will not be easily hydrolyzed, thus prolonging the action time of adenylate cyclase and the duration of the biological effect will be longer.
Lectins are a special class of proteins isolated from various plant or lower animal tissues, and can specifically bind to receptors on the surface of animal and plant cells to produce a series of physiological effects. They interact with the oligosaccharide structure determinants on the cell surface to cause agglutination of cells, so they are also called lectins. It was first discovered that they can induce agglutination of red blood cells for clinical blood group classification, so they are also known as phytohemagglutinins. As many as 500 lectins have been reported. Receptors that specifically bind to lectins are located on the cell surface and have complex oligosaccharide chain structures. They have special junctions, side chains, and peptide attachments. There are also non-covalent (such as hydrogen) interactions between these oligosaccharide chains. The binding site of lectin and receptor oligosaccharide chain has been considered as the terminal sugar residue in the past, which proves that it can also be combined with the polysaccharide core site. Different lectins may have specific receptors, but may also have common receptors. In general, the specificity of the lectin receptor is worse than that of the antigen-antibody. Lectins cause cell agglutination because a single agglutination molecule can bind to two or more binding points on the cell surface. When lectin binds to its specific receptor, it can trigger a variety of physiological effects of cells, such as cell agglutination, especially high-value agglutination of tumors or transformed cells; induce mitosis of lymphocytes and membrane permeability and fertilization Process, phagocytosis, and cell proliferation can all have some impact.
Figure 3. Lateral hemagglutinine.