Organic Chemistry

Section 25.8 Tetrahydrofolate: Folic Acid 1067

A common anticancer drug that inhibits thymidylate synthase is 5-fluorouracil.
5-Fluorouracil and uracil react with thymidylate synthase in the same way. However,
the fluorine at the 5-position cannot be removed by the base in the third step of the re-
action because fluorine is too electronegative to come off as Because fluorine can-
not be removed, the reaction stops at this point, leaving the enzyme permanently
attached to the substrate. The active site of the enzyme is now blocked with 5-fluoro-
uracil, so the enzyme cannot bind uracil, which means that thymidine can no longer be
synthesized. Consequently, the synthesis of DNA is also stopped. Unfortunately, most
anticancer drugs cannot discriminate between diseased and normal cells. As a result,
cancer chemotherapy is accompanied by terrible side effects. However, cancer cells
undergo uncontrolled cell division so, because they are dividing more rapidly than
normal cells, they are harder hit by cancer-fighting chemotherapeutic agents.

5-Fluorouracil is a mechanism-based inhibitor—it inactivates the enzyme by
taking part in the normal catalytic mechanism. It is also called a suicide inhibitor
because when the enzyme reacts with it, the enzyme “commits suicide.”The use of
5-fluorouracil illustrates the importance of knowing the mechanism for an enzyme-
catalyzed reaction. If you know the mechanism, you may be able to design an inhibitor
to turn the reaction off at a certain step.
Aminopterin and methotrexate are anticancer drugs that are inhibitors of dihydrofo-
late reductase. Because their structures are similar to that of dihydrofolate, they com-
pete with it for binding at the active site of the enzyme. Since they bind 1000 times
more tightly to the enzyme than does dihydrofolate, they inhibit the enzyme. These
two compounds are examples of competitive inhibitors.

Because aminopterin and methotrexate inhibit the synthesis of THF, they interfere
with the synthesis of any compound that requires a THF-coenzyme in one of the steps
of its synthesis. Thus, not only do they prevent the synthesis of thymidine but they also
inhibit the synthesis of adenine and guanine—other heterocyclic compounds needed

C O

COO−

HNCHCH 2 CH 2 CO−

O

N

NN

N N

NH 2

H 2 N

R

R = H

R = CH 3

N

N

NH 2

H 2 N

CH 2 OCH 3

OCH 3

OCH 3

aminopterin

methotrexate

trimethoprim

5-fluorouracil

5-FU

O

O

HN

N

H

F

the enzyme has become

irreversibly attached to the substrate

O O

O

HN

N

R

CH 2

H 2 N N

N

H

N

HNR

F

S

B

E

HN

F+.