| Summary: | Dihydrofolate reductase (DHFR) is a key enzyme in cellular anabolism. It catalyses the reduction of 7,8-dihydrofolate (H2F) to 5,6,7,8-tetrahydrofolate (H4F) via hydride transfer from the C4 position of NADPH to the C6 position of H2F accompanied with protonation at the N5 position of H2F. Due to the importance of DHFR as an anticancer and antimicrobial target, the catalytic mechanism of DHFR has long been the focus of intense research. Kinetic isotope effect (KIE) measurements can provide insight into the mechanism of DHFR catalysis and guide the rational design of novel anti-DHFR drugs. However,because of a lack of a practical strategy to introduce heavy atoms (15N, 13C) into H2F, current research is mostly restrained to the study of hydrogen isotope effects. In this thesis, a fourteen step, one-pot chemoenzymatic synthesis of labelled H2F is reported. The flexibility of this synthetic approach enables the production of various isotopically enriched H2Fs from simple starting materials such as D-glucose. The labelled substrates were used to measure, for the first time, heavy atom KIEs and to derive information about the transition state of the chemical step during DHFR catalysis. This methodology is widely applicable to other biochemically important substrates and cofactors and it can be used for a wide variety of in vitro and in vivo investigations.
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