Enzyme affinity of the 5,6-dihydro derivatives of the substrate and product of thymidylate synthetase catalysis.

The 5,6-dihydro derivatives of 2'-deoxyuridine 5'-phosphate (2) and 2'-deoxythymidine 5'-phosphate (3) were synthesized and characterized. The affinities of 2 and 3 were compared to those of the substrate (2'-deoxyuridine 5'-phosphate) and product (2'-deoxythymidine 5'-phosphate) of the reaction catalyzed by thymidylate synthetase. In both cases, the enzyme affinity of the 5,6-dihydro derivatives was 50 times less than that of the substrate or product. The conclusions from this study are that a noncovalent complex of enzyme and a dihydro substrate or dihydro product is improbable in thymidylate synthetase catalysis and the covalent enzyme--substrate complex is more reasonable.