On the mechanism of Dehydroquinate synthase : a thesis presented to Massey University in partial fulfilment of the requirements for the degree of Master of Science in Chemistry

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The aim of this thesis is to investigate the influence of fluorine substitution on the second reaction of the shikimate pathway catalysed by the enzyme 3-dehydroquinate synthase. The shikimate pathway is an essential pathway that is required for the synthesis of aromatic compounds in bacteria, microbial eukaryotes and plants. The enzyme, 3-dehydroquinate synthase, catalyses the second step of the shikimate pathway, the conversion of 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) into 3-dehydroquinate (DHQ). Item_03771-1.jpg It has been reported that when (3S)-3-fluoro DAHP (where fluorine occupies the C3 axial position) is treated with the enzyme 3-dehydroquinate synthase, two products, the expected (6S)-6-fluorodehydroquinate (5) and its C1 epimer, (6S)-6-fluoro-1-epideydroquinate (6) are formed in a ratio of 2 : 1. Item_03771-2.jpg The C1 epimer of 3-dehydroquinate was reported to be formed from the natural substrate DAHP in a solution reaction, but not in the enzyme catalysed reaction. Therefore, it has been suggested that fluorine substitution at the axial position on C3 stabilises the fluoroenolpyranose intermediate allowing the intermediate to dissociate from the enzyme and cyclise to complete the formation of (6S)-6-fluoro-1-epideydroquinate free in solution. The results reported in this thesis are from an investigation carried out to understand further the influence of fluorine orientation on the stereochemical outcome of the products in the dehydroquinate synthase reaction. (3S)-3-Fluoro DAHP was synthesised in large amounts using both chemical and enzymatic synthesis. This was achieved by treating the isomers of 3-fluoro phosphoenolpyruvate and D-erythrose 4-phosphate with DAHP synthase, the first enzyme of the shikimate pathway. The erythrose 4-phosphate was prepared by lead tetraacetate oxidation of D-glucose 6-phosphate. The isomers of 3-fluoro phosphoenolpyruvate were prepared from 3-bromo, 3-fluoropyruvic acid by the Perkow reaction. Then (3S)-3-3-fluoro DAHP was purified by anion exchange chromatography. The chemical synthesis of erythrose 4-phosphate and the isomers of 3-fluoro phosphoenolpyruvate and the enzymatic synthesis of (3S)-3- fluoro DAHP and its purification are discussed in Chapter Two. A recombinant Escherichia coli strain (pJB 14) was used to over-express the enzyme dehydroquinate synthase, and partial purification of the enzyme was achieved by anion exchange chromatography. Chapter Three describes the production and purification of the enzyme 3-dehydroquinate synthase. Purified (3S)-3-fluoru DAHP was treated with the E. coli enzyme 3-dehydroquinate synthase. Formation of both (6S)-6-fluorodehydroquinate and its C1 epimer was observed. The reaction was followed at different pH and temperature values. The ratio of products produced in the enzyme-catalysed reaction was monitored by 19 F NMR spectroscopy. No significant change in the ratios was observed with the different conditions employed. The results from these experiments are discussed in Chapter Four. Our results are consistent with the hypothesis that the fluoroenolpyranose intermediate is released to the solution, where it cyclises without the constraint of an enzymatic template. To test this hypothesis unequivocally, further investigations are required and these are discussed in Future Directions. [NB: Mathematical/chemical formulae or equations have been omitted from the abstract due to website limitations. Please read the full text PDF file for a complete abstract.]
Dehydroquinate synthase, Fluorine, Enzymes, Metabolism, Physiological effect