CHORISMATE-UTILIZING ENZYMES ISOCHORISMATE SYNTHASE, ANTHRANILATE SYNTHASE, AND P-AMINOBENZOATE SYNTHASE - MECHANISTIC INSIGHT THROUGH INHIBITOR DESIGN

Three enzymes of the shikimic acid pathway, isochorismate synthase (IS), anthranilate synthase (AS), and p-aminobenzoate synthase (PABS), exhibit significant sequence homology and may be related mechanistically. Compounds 1, 2, and 3 were designed to mimic, in their all-axial conformations, the putative transition state for these enzymes. The inhibitors were prepared in racemic form starting from Diels-Alder addition of a propiolate ester to a protected 1-oxy- or 1-amino-1,3-butadiene in 14%, 4%, and 9%, overall yields, respectively. All three compounds are competitive inhibitors of the three enzymes, binding IS and AS strongly and PABS weakly. For both IS and AS, the affinity of the 6-amino-4-hydroxy isomer 2 is ca. 10-fold that of the 4-amino-6-hydroxy isomer 3, a difference that is largely due to their conformational equilibria; 2 is 25 +/- 2% axial and 3 is 6 +/- 3% axial, as determined by the temperature dependence of their NI?IR spectra. The similarity between IS and AS was extended by the finding that IS, like AS, catalyzes formation of 2-amino-2-deoxyisochorismate (ADIC) in the presence of ammonia. These observations are consistent with direct 1,5-substitution mechanisms for both IS and AS; the weak inhibition of PABS by these inhibitors suggests that it operates by a significantly different mechanism.