Biology Reference
In-Depth Information
D. STEROLS
Sterols
[29,30]
are major components of many animal, plant, and fungal membranes, but
are absent in prokaryotes. In bacterial membranes sterol function is replaced by hopanoids,
sterol-like lipids. In animals the major sterol is the much maligned cholesterol, in fungi
ergosterol and in plants
-sitosterol (
Figure 5.22
). It is evident from their structures that
these three sterols are very similar, differing only by slight modifications to their floppy tails
(
Figure 5.22
).
Sterols are very water insoluble and so readily partition into membranes. Membrane
sterols can be thought of as having three distinct parts, each with a different function
(
Figure 5.23
). Sterols are anchored to the aqueous interface via a polar
e
OH group. The
rest of the molecule is hydrophobic. The 4 rigid sterol rings are responsible for the sterol's
major function, controlling membrane 'fluidity' and packing free volume (breathing space
for protein function). The function of the floppy tail is far less obvious and is uncertain.
b
CHOLESTEROL
HO
ERGOSTEROL
HO
β
-SITOSTEROL
C
2
H
5
HO
FIGURE 5.22
Structure of the sterols cholesterol (animal), ergosterol (fungi), and
b
-sitosterol (plant).
