title: Thymidylate synthetase catalyzed exchange of tritiumfrom [5-3H]-2'-deoxyuridylate for protons of water.
abstract: Thymidylate synthetase catalyzes an exchange of tritium of [5-3H]dUMP for protons of water in the absence of CH2-H4folate. The turnover number for this reaction is some 45,000-fold lower than that of dTMP formation and Km is 1.2 X 10(-5) M, similar to the dissociation constant of the enzyme-dUMP complex determined by equilibrium dialysis. The presence of 4 mM folate has no effect on Vmax but results in a decrease in the Km of dUMP to a value close to that in the normal enzymic reaction. The exchange reaction provides definitive evidence that the enzymic reaction involves attack of a nucleophile of the enzyme on the 6 position of dUMP to provide a 5,6-dihydro-dUMP intermediate which is covalently bound to the enzyme. Stereochemical considerations of the exchange reaction require proposal of a partial reaction which is not completely sterospecific or a complex reaction in which protons of water are handled with complete stereospecificity in a fashion similar to the one carbon unit of the normal enzymic reaction.
mesh_terms: D007553:Isotope Labeling / Q000379:methods; D007700:Kinetics; D007780:Lacticaseibacillus casei / Q000201:enzymology; D008780:Methyltransferases / Q000378:metabolism*; D013940:Thymidylate Synthase / Q000378:metabolism*; D014316:Tritium
chemical_list: D014316:Tritium; D008780:Methyltransferases; D013940:Thymidylate Synthase