Organic Chemistry

Farnesyl pyrophosphate can react with another molecule of isopentenyl pyrophos-

phate to form geranylgeranyl pyrophosphate, a 20-carbon compound. Two geranylger-

anyl pyrophosphates can join to form phytoene, a 40-carbon compound. Phytoene is

the precursor of the carotenoid (tetraterpene) pigments in plants.

PROBLEM 18

In aqueous acidic solution, farnesyl pyrophosphate forms the following sesquiterpene:

Propose a mechanism for this reaction.

PROBLEM 19 SOLVED

If squalene were synthesized in a medium containing acetate whose carbonyl carbon were

labeled with radioactive which carbons in squalene would be labeled?

SOLUTION Acetate reacts with ATP to form acetyl adenylate, which then reacts with

CoASH to form acetyl-CoA (Section 17.20). Because malonyl-CoA is prepared from

acetyl-CoA, the thioester carbonyl carbon of malonyl-CoA will also be labeled. Examin-

ing each step of the mechanism for the biosynthesis of isopentenyl pyrophosphate from

acetyl-CoA and malonyl-CoA allows you to determine the locations of the radioactively

labeled carbons in isopentenyl pyrophosphate. Similarly, the locations of the radioac-

tively labeled carbons in geranyl pyrophosphate can be determined from the mechanism

for its biosynthesis from isopentenyl pyrophosphate. And the locations of the radioac-

tively labeled carbons in farnesyl pyrophosphate can be determined from the mechanism

for its biosynthesis from geranyl pyrophosphate. Knowing that squalene is obtained

from a tail-to-tail linkage of two farnesyl pyrophosphates tells you which carbons in

squalene will be labeled.

14 C,

022 CHAPTER 26 Lipids

O

− + AMP

O

SCoA

O

SCoA

O O

SCoA

CoASH

−O

O

O

adenosine

+ pyrophosphate

P

O−

−OO

O

P

O−

O

O

P

O−

O

adenosine

O

P

O−

CH 3

O

C

CH 3

O

C

acetyl-CoA

acetyl-CoA malonyl-CoA

14

14 14

14 14

farnesyl pyrophosphate

tail to tail

O

O

O

P

O−

O

O−

P

O−

O

O

O

P

O−

O

−O P

O−

squalene synthase

farnesyl pyrophosphate

squalene

+