Organic Chemistry

1064 CHAPTER 25 The Organic Mechanisms of the Coenzymes • Metabolism

PROBLEM 18

Ethanolamine ammonia lyase, a coenzyme –requiring enzyme, catalyzes the following

reaction:

Propose a mechanism for this reaction.

PROBLEM 19

A fatty acid with an even number of carbon atoms is metabolized to acetyl-CoA, which can

enter the citric acid cycle. A fatty acid with an odd number of carbon atoms is metabolized

to acetyl-CoA and one equivalent of propionyl-CoA. Two coenzyme-requiring enzymes

are needed to convert propionyl-CoA into succinyl-CoA, a citric acid cycle intermediate.

Write the two enzyme-catalyzed reactions and indicate the required coenzymes.

25.8 Tetrahydrofolate: Folic Acid

Tetrahydrofolate (THF)is the coenzyme used by enzymes catalyzing reactions that

transfer a group containing a single carbon to their substrates. The one-carbon group

can be a methyl group a methylene group or a formyl group

Tetrahydrofolate results from the reduction of two double bonds of folic acid (folate),

its precursor vitamin. Bacteria synthesize folate, but mammals cannot.

There are six different THF-coenzymes. transfers a methyl group

transfers a methylene group and the others

transfer a formyl group

+

N^5 -methyl-THF

H 2 N

HN

NHR

N

H

N

N

O CH 3

N^5 ,N^10 -methylene-THF

H 2 N

HN

NR

N

H

N

N

O H 2 C

N^5 ,N^10 -methenyl-THF

H 2 N

HN

NR

N

H

N

N

O HC

(HC“O).

(CH 3 ),N^5 ,N^10 -methylene-THF (CH 2 ),

N^5 -Methyl-THF

(CH 3 ), (CH 2 ), (HC“O).

HOCH 2 CH 2 NH 2 CH 3 CH +

O

NH 3

ethanolamine

ammonia lyase

B 12

Tetrahydrofolate (THF) is the coenzyme
required by enzymes that catalyze the
transfer of a group containing one
carbon to their substrates.

H 2 N

HN

HN

C O

COO−

O

NN

N

NHCHCH 2 CH 2 CO−

O

H 2 N

HN

HN

C O

COO−

O

N

H

N

N

H

NHCHCH 2 CH 2 CO−

O

2-amino-4-oxo-

6-methylpteridine

p-aminobenzoic

acid

glutamate

folic acid (folate)

tetrahydrofolate

5

10

THF

2

(^34)
5
10
18
7
6
9