Structure-Based Affinity Spectra

Despite their simplified structures, most of the tripodal based chelators exhibit iron binding with pronounced affinity, facilitate synthesis and allow fast screening of a variety of related structures with a number of bacterial types in a relatively short time. There is, however, a clear distinction between the flexible and the rigid templates. While the former generally exhibit broad spectrum microbial activity, the latter are generally less active and frequently show no biological activity at all. 

In contrast, 5-benzyl-6-chloro-2-pyrone is not a substrate for chymotrypsin, but inactivates the enzyme with the formation of a shoulder in the absorbance spectrum at 320 nm (Westkaemper and Abeles, 1983). The chromophore, which is similar to that of the pyrone ring, appears with kinetics corresponding to the rate of inactivation and is lost on reactivation of the enzyme. The crystal structure and C NMR studies of the modified enzyme demonstrate that the benzyl group is bound in the specificity pocket of the active site and that serine-195 is covalently attached to C-6 of the intact pyrone (Ringe et ai, 1985) (Fig. 49). Inactivation by the 5-benzyI analog is therefore an example of affinity labeling rather than mechanism-based inactivation and emphasizes the importance of the side chain of protease inactivators in proper orientation of the compound in the active site. 

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