Biology Reference
In-Depth Information
membranes, primarily differing by the components attached to the C-1 alcohol. A variety
of fatty acids are also esterified to the nitrogen attached at C-2 forming a saponifiable
amide linkage. The first three carbons in sphingosine are structurally analogous to the
glycerol in phospholipids. Therefore, like phospholipids, sphingolipids have a polar
head group that interacts with water and two hydrophobic tails. However,
while both phospholipid acyl chains are variable and can be hydrolyzed, one hydro-
phobic chain in sphingolipids is neither variable nor hydrolyzable (it is sphingosine)
and the other, the variable chain, is attached via a nitrogen ester (amide), not an oxygen
ester. The sphingosine chain also contains a permanent
double bond between
carbons 4 and 5 while the variable acyl chain can be either saturated or contain
trans
cis
double bonds.
Awide variety of polar head groups may replace the simple proton attached to the
e
OH at
C-1 resulting in the different sphingolipid families listed in
Table 5.3
.
Most abundant of the human sphingolipids is sphingomyelin (SM,
Figure 5.21
) where
the head group is phosphocholine (or less commonly phosphoethanolamine). SM is rarely
found in plants and bacteria. The variable chain in most SMs is longer and more saturated
than most phospholipid acyl chains. For example, the most common acyl chains in myelin
SM are lignoceric acid (24:0) and nervonic acid (24:1). The general structure of SM therefore
resembles PC (or PE). Ceramide with a simple
e
OH polar head is structurally similar to
diacylglycerol. Cerebrosides contain a single sugar as its head group. Cerebrosides with
galactose are found in the plasma membrane of neuronal cells while those with glucose
are in the plasma membrane of non-neuronal cells. Globosides have 2 or more sugars,
usually D-glucose, D-galactose or N-acetyl-D-galactosamine. Gangliosides have several
sugars (they are oligosaccharides) and have at least one unusual, and characteristic,
9-carbon anionic sugar, sialic acid (N-acetylneuraminic acid). Sialic acid is discussed
with the other membrane sugars in Chapter 7. Gangliosides are further subdivided into
families or series that are characterized by the number of sialic acid residues on the
sphingolipid:
1 sialic acid
GM series
2 sialic acids
GD series
3 sialic acids
GT series
4 sialic acids
GQ series
TABLE 5.3
Head Groups Attached to the C-1 Position of Sphingosine.
Name
Attached group
Ceramide
e
H
Sphingomyelin
phosphocholine, phosphoethanolamine
Cerebroside
glucose or galactose
Globoside
di, tri, or tetra saccharide
Ganglioside
complex oligosaccharide (always with at
least one sialic acid)
