Receptor site coordinates

A persistent idea is that there is a very small number of flavor quaUties or characteristics, called primaries, each detected by a different kind of receptor site in the sensory organ. It is thought that each of these primary sites can be excited independently but that some chemicals can react with more than one site producing the perception of several flavor quaUties simultaneously (12). Sweet, sour, salty, bitter, and umami quaUties are generally accepted as five of the primaries for taste sucrose, hydrochloric acid, sodium chloride, quinine, and glutamate, respectively, are compounds that have these primary tastes. Sucrose is only sweet, quinine is only bitter, etc saccharin, however, is slightly bitter as well as sweet and its Stevens law exponent is 0.8, between that for purely sweet (1.5) and purely bitter (0.6) compounds (34). There is evidence that all compounds with the same primary taste characteristic have the same psychophysical exponent even though they may have different threshold values (24). The flavor of a complex food can be described as a combination of a smaller number of flavor primaries, each with an associated intensity. A flavor may be described as a vector in which the primaries make up the coordinates of the flavor space. 

A coordinate system to identify stack and receptor site positions. 

Fig. 8 Typical excimer probes utilizing two chelating sites and two fluorophores (20), a flexible central composite receptor site and three fluorophores (21) and a single receptor site with two pendant arms and two fluorophores (22). (a) Na+-induced excimer alignment in 20 and (b) respective spectroscopic response (c) selective probes for Fe3+ (21) and Cu2+ and Ni2+ (22) that show quenching of monomer and excimer emission upon binding. Color code fluorophores in red and atoms responsible for coordination in blue. (Reprinted in part with permission from [83]. Copyright 1995 American Chemical Society)...

Another topographical model, advanced for the renal vascular DA receptor by Erhardt (92) is described in greater detail in another section of this monograph (93). This model locates important receptor sites on Cartesian coordinates. It extends the McDermed model by suggesting a second site of steric hindrance about 2.0 A above the plane of the ethylamine chain and an auxiliary binding site, alluded to previously, opposite the principal site of bulk intolerance. As this model, which is consistent with the structures of most DA receptor agonists, specifically locates the amine and "meta"-0H it can be utilized to rationalize the enantioselectivity of known chiral DA receptor agonists (94). 

Recently, organometalhc mthenium chemistry has found its way into biological apphcations. Treating [Cp Ru(MeCN)3][CF3S03] with amino acids in THF yields -coordinated amino acid complexes of the type [Cp Ru(jj -Haa0H)][Cp3S03], where HaaOH = L-HPheOH (phenylalanine) or L-HTrpOH (tryptophan), as shown in equation (24). Similarly, dipeptide and cyclopeptide complexes can be sjm-thesized under parallel conditions. These types of complexes are essentially Cp Ru labeled peptides, which could have a notable impact on the analysis of protein receptor sites. ... 

Figure 1. Stereoview of the active site of ADH looking through the protein and out through the entrance cavity of the receptor site. The zinc atom ( ) is tetra-hedrally coordinated to a his and two cys residues. The forth position is occupied by the O atom of the substrate, here, cyclohexanol. The cofactor, NAD-, is shown from front left to center, where the nicotinamide moiety is placed in bonding proximity with the substrate. 

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