Interaction mechanisms receptor structure

Steroid receptors control gene expression in target cells. An understanding of the molecular interactions between receptors and specific nuclear components is crucial for elucidating the mechanism of hormone action. Receptor binding to at least three structural elements of nuclei has been described. These include the nuclear matrix, nucleoacidic chromatin proteins (acceptor proteins), and specific DNA sequences in 5 upstream elements of hormone responsive genes. 

The previously mentioned quantities are completely general, and their importance holds for any kind of sensor. For chemical sensors an additional parameter of great importance is the selectivity. The selectivity defines the capability of a sensor to be sensitive only to one quantity rejecting all the others. In case of physical sensors, the number of quantities is limited to a dozen and the selectivity can be achieved in many practical applications. For chemical sensors, it is important to consider that the number of chemical compounds is of millions and that the structural differences among them may be extremely subtle. With these conditions the selectivity of chemical sensor can be obtained only in very limited conditions. Lack of selectivity means that the sensor responds with comparable intensity to different species and with such a sensor it is not possible to deduce any reliable information about the chemical composition of the measured sample. Selectivity is a straightforward requisite for analytical systems where sensors and its related measurement technique are addressed to the detection of individual compounds. As mentioned in the previous section, selectivity is not found in olfactory receptors. As a consequence, artificial olfaction systems are not based on individual selective sensors, but on sensors whose selectivity can be oriented towards molecular families, or better, towards interaction mechanisms. Figure 22.5 shows a typical selectivity map related to an array of quartz microbalances (see next section) coated with different metalloporphyrins based on the same macrocycle (tetraphenyl-porphyrin) but with different metal atoms. Figure 22.5 depicts well the concept of combinatorial selectivity, namely each compounds is identified by a unique sensitivity pattern that makes possible the identification. 

A condition in which a receptor is unresponsive despite the presence of agonist also referred to as a refractory state . Typically this state is the consequence of prolonged exposure to agonist, and occurs after receptor activation it is a built in mechanism to limit a receptor s effects. Mechanistically the desensitised state differs from the resting, closed state of a receptor because in the latter state, a receptor can respond to agonist. This difference predicts that these states are structurally distinct. The desensitised state may also be stabilised by very low concentrations of agonist, such that no measurable activation of the receptor precedes it. Desensitisation is an intrinsic property of many receptors but can also be influenced by other interactions or modifications, such as phosphorylation. 

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