Biology Reference
In-Depth Information
partial hydrogenation of plant lipids, a commonly employed procedure in manufacturing
processed foods. Catalytic hydrogenation results in double bond reduction, migration up
and down the chain and partial conversion of normal cis to deleterious trans double bonds.
Increased incidence of heart disease
[18]
associated with lipid hydrogenation has resulted in
banning trans fats frommany parts of the world. The question remains why cis fatty acids are
essential for human health while trans fatty acids are harmful. A clue may be found in the T
m
s
reported in
Table 4.6
. Trans fatty acids have a much higher melting point than do the homol-
ogous cis fatty acids. For example, the trans monoenoic fatty acid elaidic acid T
m
is 43.7
o
C
while the homologous cis oleic acid T
m
is 16.2
o
C. In comparison, the saturated 18-C stearic
acid has a T
m
of 69.6
o
C(
Table 4.3
). Therefore, at least by T
m
analysis, although both elaidic
and oleic acid have 18 carbons and a
9 double bond, elaidic acid is more similar to satu-
rated stearic acid than it is to oleic acid. From the T
m
s it can be concluded that cis double
bonds have a larger effect on lipid packing than do trans double bonds that exhibit consider-
able saturated fatty acid properties (Chapter 9).
6
Branched Chain (Isoprenoid) Fatty Acids
The primary function of double bonds is to disrupt tight lipid packing in membranes, thus
increasing the packing free volume ('breathing space') required for normal membrane
protein function (Chapter 9)
[19]
. However, nature has found other ways to duplicate the
function of double bonds. In lower organisms bulky methyl branches often replace double
bonds. Higher organisms also contain repeating methyl branches in long hydrophobic struc-
tures called isoprenoids. These long hydrocarbon chains appear to be polymers of a 5-carbon
branched compound called isoprene (see
Figure 4.4
). However, biological isoprenoids are not
actually made from isoprene. Isoprene is a structural component of natural rubber and ~95%
of industrial isoprene is used to produce cis-1,4-polyisoprene, a synthetic rubber used in the
production of diving suits.
In plants the hydrophobic side chain of chlorophyll is phytol, a 20-carbon (4 isoprene
units) isoprenoid. Many other important biochemicals including the family of retinoids
(vitamin A) and ubiquinone (coenzyme Q) are anchored to membranes through long chain
isoprenoids (Chapter 5).
In 1984 Schmidt, Schneider, and Glomset
[20]
reported that many proteins are anchored
to membranes via long chain isoprenoids. This family of proteins has since been termed
'prenylated proteins' and has been shown to facilitate cellular protein
protein interactions
and membrane-associated protein trafficking. The largest group of prenylated proteins is
the plasma membrane GTP-binding proteins that transduce extracellular signals into
e
CH
3
CH
2
H
2
C
FIGURE 4.4
Isoprene.
