Page 59 - Color Atlas of Biochemistry

P. 59

50 Biomolecules

Phospholipids and glycolipids Some phospholipids carry additional
charges, in addition to the negative charge
at the phosphate residue. In phosphatidylcho-
A. Structure of phospholipids and
line and phosphatidylethanolamine, the
glycolipids
N-atom of the amino alcohol is positively
Fats (triacylglycerol, 1) are esters of glycerol charged. As a whole, these two phosphatides
with three fatty acids (see p. 48). Within the therefore appear to be neutral. In contrast,
cell, they mainly occur as fat droplets. In the phosphatidylserine—with one additional pos-
blood, they are transported in the hydropho- itive charge and one additional negative
bic interior of lipoproteins (see p. 278). charge in the serine residue—and phosphati-
Phospholipids (2)are the mainconsti- dylinositol (with no additional charge) have a
tuents of biological membranes (see negative net charge, due to the phosphate
pp. 214–217). Their common feature is a phos- residue.
phate residue that is esterified with the hy- Sphingolipids (3), whichare found inlarge
droxyl group at C-3 of glycerol. Due to this quantities in the membranes of nerve cells in
residue, phospholipids have at least one neg- the brain and in neural tissues, have a slightly
ativechargeat a neutral pH. different structure from the other membrane
Phosphatidates (anions of the phosphatidic lipids discussed so far. In sphingolipids, sphin-
acids), the simplest phospholipids, are phos- gosine, an amino alcohol with an unsaturated
phate esters of diacylglycerol. They are impor- alkyl side chain, replaces glycerol and one of
tant intermediates in the biosynthesis of fats the acyl residues. When sphingosine forms an
and phospholipids (see p.170). Phosphati- amidebondto a fatty acid, the compoundis
dates can also be released from phospholipids called ceramide (3). This is the precursor of
by phospholipases. the sphingolipids. Sphingomyelin (2)—the
The other phospholipids can be derived most important sphingolipid—has an addi-
from phosphatidates (residue = phospha- tional phosphate residue with a choline group
tidyl). Their phosphate residues are esterified attached to it on the sphingosine, in addition
with the hydroxyl group of an amino alcohol to the fatty acid.
(choline, ethanolamine,or serine)or with the Glycolipids (3)are present in all tissues on
cyclohexane derivative myo-inositol. Phos- theouter surfaceofthe plasma membrane.
phatidylcholine is shown here as an example They consist of sphingosine, a fatty acid, and
of this type of compound. When two phos- an oligosaccharide residue, which can some-
phatidyl residues are linked with one glyc- times be quite large. The phosphate residue
erol, the result is cardiolipin (not shown), a typical of phospholipids is absent. Galacto-
phospholipid that is characteristic of the inner sylceramide and glucosylceramide (known as
mitochondrial membrane. Lysophospholipids cerebroside) are simple representatives of
arise from phospholipids by enzymatic cleav- this group. Cerebrosides in which the sugar
age of an acyl residue. The hemolytic effect of is esterified with sulfuric acid are known as
beeand snakevenoms is due in part to this sulfatides. Gangliosides are the most complex
reaction. glycolipids. They constitute a large family of
Phosphatidylcholine (lecithin) is the most membrane lipids with receptor functions that
abundant phospholipid in membranes. are as yet largely unknown. A characteristic
Phosphatidylethanolamine (cephalin) has an component of many gangliosides is N-acetyl-
ethanolamine residue instead of choline, and neuraminic acid (sialic acid; see p. 38).
phosphatidylserine has a serine residue. In
phosphatidylinositol, phosphatidate is esteri-
fied with the sugarlike cyclic polyalcohol
myo-inositol. A doubly phosphorylated deriv-
ative of this phospholipid, phosphatidylinosi-
tol 4,5-bisphosphate, is a special component
of membranes, which, by enzymatic cleavage,
can giveriseto two second messengers,diacyl-
glycerol (DAG) and inositol 1,4,5trisphosphate
(InsP 3 ; see p. 386).

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