Biochemistry and Molecular Biology - Belyasova N.A. 2002

Structure and Functions of Cellular Components
Biomembranes
Receptor Functions of Membranes

All Cells must possess systems that allow them to detect environmental conditions and changes in order to adapt to them. These systems consist of various receptor molecules located in surface structures—most commonly in Plasma Membranes, less frequently in Cell walls, and in the outer membrane in the case of Gram-negative Bacteria. The function of receptor molecules and their assemblies is to interact with extracellular components and initiate a specific cellular response.

In most cases, receptor molecules are Proteins, but other molecules can also perform this function, such as Glycolipids, Glycoproteins, or Sphingolipids. For example, gangliosides have been shown to serve as binding sites for cholera and tetanus toxins, as well as to participate in the Regulation of Cell growth and differentiation processes.

Among the vast diversity of cellular receptors, several main types can be distinguished. The Surface structures of bacterial, Yeast, and animal cells contain receptors that determine the ability of cells to recognize one another, interact to form clusters, and bind to insoluble Components of the Extracellular matrix. An example of this type of receptor is found in the protein pili (fimbriae) discovered in pathogenic strains of E. coli that cause human Urinary Tract infections. These pili are anchored in the outer membrane and carry a receptor protein at their tip—an adhesin—capable of specifically binding to digalactoside-containing glycolipids. These Lipids are present On the surface of epithelial cells lining the urinary tract, where the bacteria multiply.

Another Class of receptors is represented by molecules located in the plasma membranes of organisms that bind nutrients and metabolites. These receptors are involved in Endocytosis and Exocytosis, determining the Specificity of these transport modes.

More complex receptor responses involve the binding of a receptor to a metabolite, hormone, or neurotransmitter, the transmission of the signal into The Cell, and the subsequent cellular response. This class of receptors includes, for example, bacterial proteins responsible for chemotaxis. The Plasma Membrane of E. coli contains a receptor for aspartate, which is a transmembrane protein. This protein binds aspartate, triggering a conformational change in the cytoplasmic domain of the molecule. This change acts as a signal that indirectly (via the phosphorylation of another protein component of the system) causes the flagella to rotate. As a result, the cell moves along an aspartate concentration gradient, enabling it to utilize aspartate as a nutrient substrate. The cellular response to a signal elicited by the reception of a specific substance can also be expressed as the activation of METABOLISM/31.html">Transcription of individual genes. Such a receptor system includes a regulatory protein that is presumably present in the soluble cytoplasmic form. It is believed that receptors somehow modify regulatory proteins, which subsequently activate transcription.

Signal Transduction occurs in a similar manner when a Ligand (a neurotransmitter or hormone) binds to a specific receptor on the outer surface of an animal cell membrane. This event initiates a conformational transition in the receptor molecule, followed by a cascade of intracellular events that may include channel opening (such as the nicotinic Acetylcholine Receptor), the phosphorylation of cellular proteins accompanied by changes in their activity, and complex formation with G proteins. In the latter case, G proteins are activated, released from the complex, and diffuse to cellular targets, eliciting a specific response. One of the most common targets of G proteins is adenylate cyclase (which catalyzes The formation of cAMP). A conformational change in this enzyme leads to an alteration in the intracellular concentration of cAMP, which, as is well known (Chapter 3), serves as a second messenger, influencing a multitude of intracellular processes.

Finally, many cells feature membrane-bound receptors capable of generating a Nerve Impulse in response to a stimulus (external signal). A nerve impulse generated in the membrane of a specialized receptor cell is transmitted via synapses along the processes of afferent (sensory) Nerve Cells to the Central Nervous System, and then via the processes of efferent (motor) nerve cells to a Muscle or gland. In Skeletal Muscle cells, this excites the acetylcholine receptor and triggers an Action Potential, followed shortly thereafter (in about 35 ms) by contraction driven by the sliding of Actin and Myosin within intracellular myofibrils.

Specialized receptor cells in higher animals and humans can form Sensory Organs. The functioning of these organs is based on Changes in the electrical properties of receptor cells in response to a specific stimulus—that is, the ability of cells to generate a nerve impulse. These processes are examined in greater detail in Chapter 13 using the visual system as an example.