Biology Reference
In-Depth Information
FIGURE 5.5
A phospholipid is
formed from the condensation (dehy-
dration) of one glycerol, two fatty
acids, one phosphate, and one alcohol.
O
R
1
fatty acid R
1
C
H
HO
O
H
C
OH
R
2
C
H
C
OH
fatty acid R
2
HO
H
C
OH
O
H
glycerol
HO-alcohol
HO
P
OH
OH
phosphate
O
H
R
1
C
O
H
C
O
R
2
C
O
H
C
O
H
P
Alcohol
C
O
O
H
OH
PHOSPHOLIPID
shown in
Figure 5.7
. The structure has an oval head (glycerol, phosphate, and the alcohol)
and the two acyl chains. The acyl chains are fatty acids esterified to the glycerol.
Fatty Acids
The fatty acids are esterified to the glycerol at carbon positions C-1 (called the
sn
-1 posi-
tion) and at C-2 (the
sn
-2 position). As a general rule the
sn
-1 chain is saturated while the
sn
-2 chain is unsaturated. The phosphate is esterified to the glycerol C-3 position. The types
and structures of membrane fatty acids were discussed in Chapter 4.
Glycerol
Although the molecule glycerol is not optically active (it does not have an asymmetric
carbon with 4 different groups attached), once the other phospholipid components are
attached, the
-2 position of glycerol becomes asymmetric (it now has 4 different groups
attached) and hence is optically active. Consider the case for glycerol-3-phosphate. It comes
in two optical isomers, D and L, differing only by the orientation of the same groups about
the
sn
-2 carbon (
Figure 5.8
). The biologically correct form is the D isomer. While generally
unappreciated, biological phospholipids are similar to amino acids and sugars in being opti-
cally active.
sn
