Spectroscopic studies of ternary complexes of thymidylate synthetase, deoxyribonucleotides, and folate analogs

Conformational changes accompanying the formation of binary and tightly bound ternary complexes of thymidylate synthetase and all possible combinations of three folate analogs (N-10-ethyl-quinazoline, folic acid triglutamate, and folic acid) and three deoxyribonucleotides (5-fluoro-2'-deoxyuridylic acid (FdUMP), 2'-deoxyuridylic acid (dUMP), and thymidylic acid (dTMP] were studied by means of ultraviolet difference spectroscopy. The amplitudes of the spectral changes upon ternary complex formation were 2-3-fold greater than those generated by formation of binary enzyme-nucleotide and enzyme-folate analog complexes. Difference spectra of the ternary complexes all showed a major increase in absorbance in the region of 320-340 nm, presumably due to perturbations of the folate analog chromophores, whereas decreases in absorbance occurred over a range of 260-310 nm. N-10-ethyl-quinazoline tended to form the complex with the greatest filtration efficiency on nitrocellulose filters, followed by folic acid triglutamate and folic acid, whereas among the nucleotides, the most stable complexes were formed with FdUMP, followed by dUMP and dTMP. A correlation was observed between the apparent stability of the ternary complex and the magnitude of the absorbance change in its difference spectrum. The formation of the various ternary complexes showed three different categories of rate behavior: 1) very rapid formation of the complex; 2) biphasic formation with a rapid phase and a slow phase requiring up to 90 min for completion; and 3) in the case of the ternary complex formed with enzyme, FdUMP, and folic acid, only a slow phase of binding. The slow formation of the latter complex was accompanied by concomitantly slow changes in the difference spectrum. However, in those cases of biphasic formation of the complexes, almost all of the spectral change occurred rapidly, and very little of it corresponded to the slow phase of complex formation. To accommodate these observations, a model is proposed involving a sequential interaction of the two subunits of thymidylate synthetase.