Synthesis of potential substrates and inhibitors for pyridoxal-phosphate dependent enzymes

Abstract

Pyridoxal 5'-phosphate dependent aspartate aminotransferase catalyses the conversion of (2S)-aspartic acid and a-ketoglutaric acid into (2S)-glutamic acid and oxaloacetate. Both the mono and dimethylesters of (2S)-aspartic acid, together with the corresponding amides were synthesised in order to assess the influence of charge neutralisation of the beta-amino pK of aspartate bound to aspartate aminotransferase. A novel method was developed for the synthesis of the (2S)-aspartic acid alpha-methylester (57) starting from the beta-allyl ester of aspartic acid. The required ester (57) was obtained in 25% yield over 4 steps. pKa Studies carried out in collaboration with Profs. Schnackerz and Cook, have revealed that, although both of the carboxylates of aspartate form hydrogen-bonds to Arg-292 and Arg-386, upon binding to the enzyme, charge is not completely neutralised at either of the two. Glutamate 1-semialdehyde aminomutase is an exceptional PLP-dependent enzyme in that, although it is classified as a mutase (transfering amino and oxo functions within the molecule) it functions as an aminotransferase. We have devised a new route to 2,3-diaminopropyl hydrogen sulfate, a substrate/ inhibitor which should shed more light on the mechanism of this enzyme. The key steps in the synthesis were the conversion of the amide (68) into the amine (69) using the Waki modification of the Hoffman reaction and reduction of the alpha-carboxylate into the primary alcohol (53). Sulfation of the diamino alcohol (66) using chlorosulfonic acid in DCM gave the required sulfate (56), contaminated with up to 20% of starting material. The material was purified by ion exchange chromatography and was used in kinetic studies in the laboratory of Prof. R. John. We have also attempted to develop a new synthetic route to 3-[2H]-(2S)-serine starting from (2S)-aspartic acid. This compound should be useful in delineating the mechanism of a number of other PLP-dependent enzymes. Although we have synthesised a number of Baeyer-Villiger precursors we have been unable to find conditions in which the oxidation proceeds in more than 20% yield.