Structural basis for recognition of polyglutamyl folates by thymidylate synthase.

Thymidylate synthase (TS) catalyzes the final step in the de novo synthesis of thymidine. In vivo TS binds a polyglutamyl cofactor, polyglutamyl methylenetetrahydrofolate (CH2-H4folate), which serves as a carbon donor. Glutamate residues on the cofactor contribute as much as 3.7 kcal to the interaction between the cofactor, substrate, and enzyme. Because many ligand/receptor interactions appear to be driven largely by hydrophobic forces, it is surprising that the addition of hydrophilic, soluble groups such as glutamates increases the affinity of the cofactor for TS. The structure of a polyglutamyl cofactor analog bound in ternary complex with deoxyuridine monophosphate (dUMP) and Escherichia coli TS reveals how the polyglutamyl moiety is positioned in TS and accounts in a qualitative way for the binding contributions of the different individual glutamate residues. The polyglutamyl moiety is not rigidly fixed by its interaction with the protein except for the first glutamate residue nearest the p-aminobenzoic acid ring of folate. Each additional glutamate is progressively more disordered than the previous one in the chain. The position of the second and third glutamate residues on the protein surface suggests that the polyglutamyl binding site could be utilized by a new family of inhibitors that might fill the binding area more effectively than polyglutamate.