Organic Chemistry

Section 25.8 Tetrahydrofolate: Folic Acid 1065

3-D Molecule:

Tetrahydrofolate (THF)

Homocysteine methyl transferase and glycinamide ribonucleotide (GAR) trans-
formylase are examples of enzymes that require THF-coenzymes.

Thymidylate Synthase: The Enzyme That Converts U’s into T’s
The heterocyclic bases in RNA are adenine, guanine, cytosine, and uracil (A, G, C,
and U); the heterocyclic bases in DNA are adenine, guanine, cytosine, and thymine
(A, G, C, and T). In other words, the heterocyclic bases in RNA and DNA are the
same, except that RNA contains U’s, whereas DNA contains T’s (Sections 27.1 and
27.14). The T’s used for the biosynthesis of DNA are synthesized from U’s by
thymidylate synthase, an enzyme that requires as a coen-
zyme. Even though the only structural difference between a U and a T is a methyl
group, a T is synthesized by first transferring a methylenegroup to a U.

In the first step of the reaction catalyzed by thymidylate synthase, a nucleophilic
cysteine group at the active site of the enzyme attacks the of uridine. (This is
an example of conjugate addition; see Section 18.13.) A subsequent nucleophilic at-
tack by the of uridine on the methylene group of
forms a covalent bond between uridine and the coenzyme. A proton on the of
uridine is removed with the help of a basic group at the active site of the enzyme, elim-
inating the coenzyme. Transfer of a hydride ion from the coenzyme to uridine and
elimination of the enzyme forms thymidine and dihydrofolate (DHF).

a-carbon

a-carbon N^5 ,N^10 -methylene-THF

b-carbon

N^5 ,N^10 -methylene-THF

GAR transformylase

+

CH 2

C

ribose-5-phosphate

O

N^10 -formyl-THF

NH 2

NH

+

CH 2 CH

C O

ribose-5-phosphate

O

THF

NH

NH

N^5 -formyl-THF

H 2 N

HN

NHR

N

H

N

N

CH

O CH

O

O

N^10 -formyl-THF

H 2 N

HN

N CH

NH

N

R

N

H

N

N

O H

N^5 -formimino-THF

H 2 N

HN

NHR

N

H

N

O

+

homocysteine methionine

homocysteine

methyl transferase

HSCH 2 CH 2 CHCO− +

O

N^5 -methyl-THF

NH 3

CH 3 SCH 2 CH 2 CHCO− +

O

THF

+ NH 3

2 ′-deoxyuridine N^5 ,N^10 -methylene-THF
5 ′-monophosphate
dUMP
R′ = 2 ′-deoxyribose-5-phosphate

H 2 N CH 3

HN

HN

NR

N

H

N

N N

R′ H 2 C

O

O

O

thymidylate

synthase

+

2 ′-deoxythymidine dihydrofolate

5 ′-monophosphate

dTMP

DHF

H 2 N

HN

HN

NHR

N

H

N

N N

R′

O

O

O

+