3.4. Receptors

Receptors at Membrane. Biological receptors are protein molecules with a specific affinity for hormones, antibodies, enzymes, and other biologically active com-pounds; most of them are bound to the cell membrane. A receptor-ligand interaction is transmitted to other molecules inside the cell, where consecutive reactions are triggered.

Hormone Receptors. The currently best-known receptors are those for hormones. Many hormones released into the blood, e.g. insulin, glucagon, or adrenaline, do not penetrate the cell membrane but react with specific receptors at the cell surface. These are present in enormously high amounts, e.g., a single fat cell of about 50 µm diameter carries 160, 000 insulin receptors, which corresponds to about 20 receptors per µm2.

How Receptor Works. The receptor molecules mostly penetrate the cell membrane (Fig. 3.19) and many of them are coupled inside to an enzyme system. A conformational change of the receptor molecule by hormone binding may be directly mediated to the enzyme and result in its activation. Thus, for example the adrenaline receptor at the surface of the liver cell reacts with adrenaline formed in the adrenocortex and released into the blood under stress. The resulting conformational change of the receptor molecule activates associated adenylate cyclase reaching into the internal space of the cell and converting ATP to cyclic adenosine monophosphate (cAMP). cAMP initiates the phosphate transfer from ATP to other enzymes by protein kinase. In this way a number of other enzyme reactions are started, leading to a cascade of activated enzymes. Finally, a single adrenaline molecule will have stimulated several thousands of enzyme molecules, which will themselves liberate about three million glucose molecules from glycogen within a few seconds. An extremely weak chemical signal is thus immediately enzymatically amplified a millionfold and the sugar reserve of the body is mobilized.

Smell Receptor. In addition to hormone receptors, taste and olfactory receptors are typical examples of this biospecific recognition process. Presumably there are about 20 to 30 primary smells. After being bound to the appropriate receptor, their molecules cause conformational changes in the receptor molecule leading to a depolarization of a part of the nerve cell membranes and initiating an action potential.

Light Receptor. Another receptor type is that of light receptors. The retina of the human eye contains about 108 tightly packed receptor cells. Here, biochemical reactions, namely of the rhodopsin molecule, are not in-itiated by the binding of chemical substances but by light quanta. Thereactions are enzymatically amplified and transformed into electrical impulses via membrane potential changes. Because of its light-absorbing chemical group the protein bacteriorhodopsin from salt-tolerant halobacteria has been studied in detail as a photoreceptor model. A single photon is sufficient to give rise to a conformational change of the protein and to transport two protons outside the cell. This 'proton pump' forms a proton and voltage gradient across the cell membrane driving the production of energy-rich ATP.

Current Status. Compared to the investigation of enzymes, that of the structure and function of membrane-bound receptors and their biotechnological application is only at the very beginning. An analogous classification, e.g., according to recognition mechanism or specificity has not yet been attempted. However, any progress in this field will provide impetus to the development of new biosensors based on receptors.

Fig. 3.19. Location of receptor.