Crystal structure of thymidylate synthase A from Bacillus subtilis

Thymidylate synthase (TS) converts dUMP to dTMP by reductive methylation, w here 5,10-methylenetetrahydrofolate is the source of both the methylene gro up and reducing equivalents. The mechanism of this reaction has been extens ively studied, mainly using the enzyme from Escherichia coli. Bacillus subt ilis contains two genes for TSs, ThyA and ThyB. The ThyB enzyme is very sim ilar to other bacterial TSs, but the ThyA enzyme is quite different, birth in sequence and activity. In ThyA TS, the active site histidine is replaced by valine. In addition, the B. subtilis enzyme has a 2.4-fold greater k(ca t) than the E. coli enzyme. The structure of B. subtilis thymidylate syntha se in a ternary complex with 5-fluoro-dUMP and 5,10-methylenetetrahydrofola te has been determined to 2.5 Angstrom resolution. Overall, the structure o f B. subtilis TS (ThyA) is similar to that of the E. coli enzyme. However, there are significant differences in the structures of two loops, the dimer interface and the details of the active site. The effects of the replaceme nt of histidine by valine and a serine to glutamine substitution in the act ive site area, and the addition of a loop over the carboxy terminus may acc ount for the differences in k(cat) found between the two enzymes.