Organic Chemistry

Section 26.8 Biosynthesis of Terpenes 017

absorbs visible light, it isomerizes to the trans isomer. This change in molecular geom-
etry causes an electrical signal to be sent to the brain, where it is perceived as a visual
image. The trans isomer of rhodopsin is not stable and is hydrolyzed to (11E)-retinal
and opsin in a reaction referred to as bleaching of the visual pigment. (11E)-Retinal is
then converted back to (11Z)-retinal to complete the vision cycle.

The details of how the foregoing sequence of reactions creates a visual image are
not clearly understood. The fact that a simple change in configuration can be responsi-
ble for initiating a process as complicated as vision, though, is remarkable.

26.8 Biosynthesis of Terpenes

Biosynthesis of Isopentenyl Pyrophosphate
The five-carbon compound used for the biosynthesis of terpenes is 3-methyl-
3-butenylpyrophosphate, loosely called isopentenyl pyrophosphate by biochemists.
Each step in its biosynthesis is catalyzed by a different enzyme. The first step is the
same Claisen condensation that occurs in the first step of the biosynthesis of fatty
acids, except that the acetyl and malonyl groups remain attached to coenzyme A rather
than being transferred to the acyl carrier protein (Section 19.21). The Claisen conden-
sation is followed by an aldol addition with a second molecule of malonyl-CoA. The
resulting thioester is reduced with two equivalents of NADPH to form mevalonic acid
(Section 25.2). A pyrophosphate group is added by means of two successive phospho-
rylations with ATP. Decarboxylation and loss of the OH group result in isopentenyl
pyrophosphate.

9

10

11

121314

11

12

15

retinol

vitamin A

CH 2 OH

oxidation

(isomerization)

H O

(11Z)-retinal

H 2 N opsin

the chemistry of vision

11

12

activated rhodopsin

visible light

(isomerization)

11

12

H N

H+H 2 O

H

N

11

12

(11E)-retinal

H

O + H 2 N

rhodopsin

opsin

opsin

opsin