Binding sites, enantiomer selectivity

The binding sites of most enzymes and receptors are highly stereoselective in recognition and reaction with optical isomers (J, 2 ), which applies to natural substrates and synthetic drugs as well. The principle of enantiomer selectivity of enzymes and binding sites in general exists by virtue of the difference of free enthalpy in the interaction of two optical antipodes with the active site of an enzyme. As a consequence the active site by itself must be chiral because only formation of a diasteromeric association complex between substrate and active site can result in such an enthalpy difference. The building blocks of enzymes and receptors, the L-amino acid residues, therefore ultimately represent the basis of nature s enantiomer selectivity. 

The space arrangement of the important feature is also to be considered. Indeed, many SAR studies include length modifications in scaffold side-chains to estimate the influence of flexibility and steric tolerance of the binding site (see, for example, [29, 30]). Chirality of the model is another spatial criterion that is necessary if two enantiomers of different activity level are to be discriminated. At least four points are necessary, but not sufficient to assure enantio-selectivity. The use of directional features (H-bond acceptor/donor) or shape criteria can help in obtaining chiral models. 

The template la can be split off by water or methanol to an extent of up to 95% (see Scheme 4.II). The accuracy of the steric arrangement of the binding sites in the resulting imprinted cavity can be tested by the ability of the polymer to resolve the racemate of the template, namely phenyl-a-D,L-mannopyranoside. The polymer was equilibrated in a batch procedure with a solution of the racemate under conditions that allowed a thermodynamically controlled partition of the enantiomers between polymer and solution. The enrichment of the antipodes in the polymer and in solution was determined and the separation factor a, i.e. the ratio of the distribution coefficients of the d- and L-enantiomer between polymer and solution, was calculated. After extensive optimisation of the procedure, a values between 3.5 and 6.0 were obtained [4]. This is an extremely high selectivity for racemic resolution that cannot be reached by most other methods. 

Finally, researchers have also noted that the selection of non-aqueous solvents can affect enzyme activity in more subtle ways. Enzymes that bind with a dual pocket binding site (secondary alcohol dehydrogenases) possess enantiomeric conformations capable of binding (Cowan, 1997). It was observed that increasing the dielectric constant increased protein flexibility, which increased binding and catalysis. These conditions may also allow increased binding of the usually less favored enantiomer. 

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