Chemoreceptor membranes

Membrane-associated receptors are linked to transducing proteins (like G-proteins) in the inner portion of the membrane. G-protein coupled receptor (GPCR) families comprise a major class of the receptors that are pharmacologically relevant, such as muscarinic acetyl choline receptors, adrenoceptors, dopamine receptors, serotonine, opiate, peptide hormone, purinerg receptors, and also sensory chemoreceptors. A large variety of subtypes are described in the pharmacological literature. 

Figure 10.1. The membrane topology proposed for D. vulgaris Hildenborough DcrH based on hydrophathy analysis and sequence homologies to other bacterial chemoreceptors (Deckers and Voordouw 1996). Shaded boxes, putative membrane-spanning (residues 9-30 and 422 31), excitation (residues 653-692), and methylation (residues 757-764) regions. The C-terminal box (residues 824-959) indicates the Hr-like region. Reprinted with permission from Xiong et al. (2000), copyright 2000 American Chemical Society.

This volume contains the papers presented in a symposium on carbohydrate-protein interaction. The symposium was devoted to an exploration of protein-glycoconjugate interaction in a wide range of biological phenomena the interaction of enzymes, antibodies, and lectins with complementary carbohydrate molecules the recognition of carbohydrate-containing structures by chemoreceptors such as taste and other plasma membrane proteins and the role of carbohydrates in the organization of connective tissue. 

A membrane-puncturing theory has been proposed by Davis (Dravnieks 1967). According to this theory, the odorous substance molecules are adsorbed across the interface of the thin lipid membrane, which forms part of the cylindrical wall of the neuron in the chemoreceptor and the aqueous phase that surrounds the neuron. Adsorbed molecules orient themselves with the hydrophilic end toward the aqueous phase. When the adsorbed molecules are desorbed, they move into the aqueous phase, leaving a defect. Ions... 

Fig. 1. Schematic of a natural chemoreceptor. The natural chemoreceptor possesses a specific site for binding of a substance, e.g. acetylcholine, and an ion channel traversing the membrane. The protein structure is changed as a result of substrate binding so as to allow for temporary opening of the channel and passage of sodium ions. The influx of sodium ions strongly changes the membrane potential. As little as a single binding event leads to a detectable signal.

A similar sequence of events occurs in biological chemoreceptors under the action of very complex biomolecules and membranes. 

Methylation. A group of enzymes, referred to as the protein methyltrans-ferases, use S-adenosylmethionine to methylate certain proteins. For example, one type of methyltransferase found in E. coli and related bacteria methylates glutamate residues in membrane-bound chemoreceptors. The methyltransferase and a methylesterase are components in a methylation/demethylation process, which plays a role in a signal transduction mechanism involved in chemotaxis. (Recall that the capacity of a living cell to respond to certain environmental cues by moving toward or away from specific molecules is referred to as chemotaxis.)... 

Some of the ACh molecules successfully diffuse across the cleft and bind to a special area on the surface membrane of the dendrite of the adjacent cells that can be referred to as a chemoreceptor.4 The binding of the ACh with the receptor changes the membrane s permeability, which causes depolarization and finally a new action potential. As this newly generated impulse (which is actually a continuation of the preganglionic impulse) passes down the axon and reaches the terminal, it stimulates the release of another chemical mediator (or the same). It in turn may combine with receptors in the next cell—either another nerve cell or one in the organ or tissue innervated by the fiber. Such chemical transmission at the synapse is believed to be a general phenomenon in mammals. An alternate mechanism of total electrical impulse transmission where pre- and postsynaptic membranes touch is, of course, conceivable and has been found to exist in lower life forms, such as the crayfish. 

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