Inhibitor studies on para-aminobenzoic acid synthase.
Davies, Glyn Daniel
University of Cambridge
Department of Chemistry
Doctor of Philosophy (PhD)
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Davies, G. D. (2003). Inhibitor studies on para-aminobenzoic acid synthase. (Doctoral thesis). https://doi.org/10.17863/CAM.11639
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Para-aminobenzoic acid (PABA) synthase is a three-subunit enzyme system that catalyses the conversion of chorismic acid top-aminobenzoic acid in plants and microorganisms. P ABA is then incorporated into folic acid an essential nutrient for mammals and utilised in the transfer of one-carbon units. For this reason it is a potential target for herbicidal and antibiotic development. A glutamine amidotransferase and ADC synthase form a heterodimeric complex and carry out the amino substitution reaction to yield 4-amino-4-deoxychorismate (ADC). A separate subunit, ADC lyase, then carries out the elimination of pyruvate to yield P ABA. My research describes a number of advances in the purification and assay system used and the organic synthesis of inhibitors of ADC synthase. The purification and characterisation was carried out of newly cloned 6-His tagged PABA synthase enzymes from E. coli. engineered to overexpress these proteins. The assay system used was refined and substrate kinetic data obtained. Results were compared to 'wild-type' enzyme and data obtained when the assay system has ammonium in place of glutamine and glutamine amidotransferase. The crystal structures of 'wild-type' ADC synthase and ADC Lyase enzymes are only recently available in the literature. Inhibition data for compounds synthesised was obtained initially via a fluorescence screen and then to greater accuracy via a continuous UV assay. My inhibitor studies revealed a close structural analogue of 4-amino-4- deoxychorismate, 4-amino-3-[1-carboxyethoxy] benzoate as the best inhibitor (ea. 20 ?M) of P ABA synthase so far. The synthesis of a number of aromatic compounds to act as m1m1cs of ADC was accomplished. An aromatic nitro group was reduced to give an amine using tin(II) chloride and hydrogenation reactions. Other forms of nitrogen protection using carbamates and bissilyl protection were also utilised. The formation of an enol pyruvyl group was accomplished in an original manner using lithium diisopropylamine to deprotonate a proton alpha to an ester. This was followed by selenation and elimination/oxidation to form the required methylene group as an a,,B-unsaturated ester. Alternative methods, involving a rhodium catalysed hydroxy insertion followed by either a Wadsworth-Horner-Emmons modified Wittig reaction with formaldehyde or alkylation with Eschermosers' Salt, were also carried out. Further synthesis of ADC and chorismate analogues was performed using the aza-Cope and oxy-Cope rearrangements of oxygen-alkylated hydroxybenzoates and N-alkylated aromatic amines. The organic synthesis of an inhibitor containing vinyl fluoride functionality has been the focus of previous studies. The biosynthesis of 6-fluorochorismate led to the discovery that this species was an irreversible inhibitor of the P ABA synthase system. Synthesis of a vinyl fluoride was achieved via a vinyl triflate and vinyl trialkylstannane utilising a lithium cuprate reagent. Fluorination was carried out using XeF2 in the presence of a catalytic amount of Ag(OTf). The mild conditions achieved form a foundation for the synthesis of more complex 6-fluorochorismate analogues. Further synthesis was carried out to try to synthesise a Michael acceptor for an enzyme active-site nucleophile based on a proposed mechanism for the irreversible inhibitor. Dianion alkylation chemistry using �sodium hydride followed by n-BuLi was used to control regioselectivity in the alkylation of a cyclic ,8-ketoester. In a one-pot synthesis, selenyl elimination/oxidation of the alkylated cyclic ,8-ketoester was used to form an extended delocalised system with the potential to act as a Michael acceptor.
This record's DOI: https://doi.org/10.17863/CAM.11639