Ee. Rutenber et Rm. Stroud, BINDING OF THE ANTICANCER DRUG ZD1694 TO ESCHERICHIA-COLI THYMIDYLATESYNTHASE - ASSESSING SPECIFICITY AND AFFINITY, Structure, 4(11), 1996, pp. 1317-1324
Background: Thymidylate synthase (TS) catalyzes the reductive methylat
ion of deoxyuridine monophosphate (dUMP) by 5,10-methylenetetrahydrofo
late (CH(2)H(4)folate) to form deoxythymidine monophosphate (dTMP) and
dihydrofolate (H(2)folate). The essential role of TS in the cell life
cycle makes it an attractive target for the development of substrate
and cofactor-based inhibitors that may find efficacy as anticancer and
antiproliferative drugs. Antifolates that compete specifically with t
he binding of CH2H4 folate include the cofactor analog CB3717 (10-prop
argyl-5,8-dideazafolate). However, the development of potent cofactor
analog inhibitors of TS, such as CB3717, as drugs has been slowed by t
heir toxicity, which often becomes apparent as hepatic and renal toxic
ity mediated by the specific chemistry of the compound. Attempts to ab
olish toxicity in human patients while preserving potency against the
target enzyme, have led to the development of ZD1694, which has alread
y shown significant activity against colorectal tumours. Results: The
three dimensional crystallographic structure of ZD1694 in complex with
dUMP and Escherichia coli TS has been determined to a resolution of 2
.2 Angstrom. This was used to evaluate the specific structural determi
nants of ZD1694 potency and to correlate structure/activity relationsh
ips between it and the closely related ligand, CB3717. ZD1694 binds to
TS in the same manner as C83717 and H(2)folate, but a methyl group on
its quinazoline ring, its thiophene ring and the methyl group at N10
are compensated for by plastic accommodation of the enzyme active site
coupled with specific rearrangement in the solvent structure, A speci
fic hydrogen bond between the protein and the inhibitor CB3717 is abse
nt in the case of ZD1694 whose monoglutamate tail is reoriented and mo
re well ordered. Conclusions: The binding mode of ZD1694 to thymidylat
e synthase has been determined at atomic resolution. ZD1694 forms a te
rnary complex with dUMP and participates in the multi-step TS reaction
through the covalent bond formation between dUMP and Cys146 thereby c
ompeting with CH(2)H(4)folate at the active site. Analysis of this inh
ibitor ternary complex structure and comparison with that of CB3717 re
veals that the enzyme accommodates the differences between the two inh
ibitors with small shifts in the positions of key active site residues
and by repositioning an active site water molecule, thereby preservin
g a general binding mode of these inhibitors. (C) Current Biology Ltd