Quenching of thymidylate synthetase fluorescence by substrate analogs.

Quenching of fluorescence occurs when $\text{Lactobacillus casei}$ thymidylate synthetase is titrated with fluorodeoxyuridylate in the presence of 1-L-methylenetetrahydrofolate to form a ternary complex. Neither fluorodeoxyuridylate nor 1-L-methylenetetrahydrofolate added singly has any effect on enzyme fluorescence but d-L-methylenetetrahydrofolate alone causes quenching. Thus ternary complex formation and interaction with d-L-methylenetetrahydrofolate alter the environment of tryptophan residues in thymidylate synthetase in a similar manner.