Organic Chemistry

Section 26.9 Steroids 023

26.9 Steroids

Hormonesare chemical messengers—organic compounds synthesized in glands and
delivered by the bloodstream to target tissues in order to stimulate or inhibit some
process. Many hormones are steroids. Because steroids are nonpolar compounds, they
are lipids. Their nonpolar character allows them to cross cell membranes, so they can
leave the cells in which they are synthesized and enter their target cells.
All steroids contain a tetracyclic ring system. The four rings are designated A, B,
C, and D. A, B, and C are six-membered rings and D is a five-membered ring. The
carbons in the steroid ring system are numbered as shown.
We have seen that rings can be trans fusedor cis fusedand that trans fused rings
are more stable (Section 2.15). In steroids, the B, C, and D rings are all trans fused. In
most naturally occurring steroids, the A and B rings are also trans fused.

2. H 2 O


3. NADPH


4. ATP


5. ATP

O O

SCoA

O O

SCoA

OH

C

O

O

O

O

P

O−

O

O−

P

O−

−O

O−

1.

mevalonyl pyrophosphate

acetoacetyl-CoA

14 14

14 14

14

14

O

O

O

P

O−

O

O−

P

O−

O

O

O

P

O−

O

O−

P

O−

O

O

O

P

O−

O

O−

P

O−

O

O

O

P

O−

O

O−

P

O−

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

14

dimethylallyl pyrophosphate isopentenyl pyrophosphate

geranyl pyrophosphate

squalene

farnesyl pyrophosphate

CH 3 and H

are trans

H

R

H 3 C

H 3 C

CH 3 and H

are cis

angular methyl

groups

H

H

H

H

R

H 3 C

H 3 C

H H

H

H

H

the steroid ring system

AB

1 C

4

5

10

6

2

37

814

13

12

11

9

15

17

D^16