There are two classes of phospholipids: those that have glycerol (from glucose) as a backbone and those that have sphingosine (from serine and palmitate).
STRUCTURE OF PHOSPHOLIPIDS
There are two classes
of phospholipids: those that have glycerol (from glucose) as a backbone and
those that have sphingosine (from serine and palmitate). Both classes are found
as structural components of membranes, and both play a role in the generation
of lipid-signaling molecules.
Figure 17.1 A. Structures of some glycerophospholipids. B. Phosphatidic acid. P = phosphate (an anion).
Phospholipids that
contain glycerol are called glycerophospholipids (or phosphoglycerides).
Glycerophospholipids constitute the major class of phospholipids and are the
predominant lipids in membranes. All contain (or are derivatives of)
phosphatidic acid (PA), which is DAG with a phosphate group on carbon 3 (Figure
17.1B). PA is the simplest phosphoglyceride and is the precursor of the other
members of this group.
1. Glycerophospholipids from phosphatidic acid and
an alcohol: The
phosphate group on PA can be esterified to another compound containing an
alcohol group (see Figure 17.1). For example:
Serine + PA →
phosphatidylserine (PS)
Ethanolamine + PA →
phosphatidylethanolamine (PE) (cephalin)
Choline + PA →
phosphatidylcholine (PC) (lecithin)
Inositol + PA →
phosphatidylinositol (PI)
Glycerol + PA → phosphatidylglycerol (PG)
2. Cardiolipin: Two molecules of PA esterified through their
phosphate groups to an additional molecule of glycerol is called cardiolipin,
or diphosphatidylglycerol (Figure 17.2). Cardiolipin is found in membranes in
bacteria and eukaryotes. In eukaryotes, cardiolipin is virtually exclusive to
the inner mitochondrial membrane, where it maintains the structure and function
of certain respiratory complexes of the electron transport chain. [Note:
Cardiolipin is antigenic and is recognized by antibodies raised against
Treponema pallidum, the bacterium that causes syphilis.]
Figure 17.2 Structure of cardiolipin (diphosphatidylglycerol). P = phosphate.
3. Plasmalogens: When the fatty acid at carbon 1 of a
glycerophospholipid is replaced by an unsaturated alkyl group attached by an
ether (rather than by an ester) linkage to the core glycerol molecule, an ether
phosphoglyceride known as a plasmalogen is produced. For example,
phosphatidalethanolamine, which is abundant in nerve tissue (Figure 17.3A ), is
the plasmalogen that is similar in structure to phosphatidylethanolamine.
Phosphatidalcholine (abundant in heart muscle) is the other quantitatively
significant ether lipid in mammals. [Note: Plasmalogens use “al” rather than
“yl” in their names.]
4. Platelet-activating factor: A second example of an ether
glycerophospholipid is platelet-activating factor (PAF), which has a saturated
alkyl group in an ether link to carbon 1 and an acetyl residue (rather than a
fatty acid) at carbon 2 of the glycerol backbone (Figure 17.3B ). PAF is
synthesized and released by a variety of cell types. It binds to surface
receptors, triggering potent thrombotic and acute inflammatory events. For
example, PAF activates inflammatory cells and mediates hypersensitivity, acute
inflammatory, and anaphylactic reactions. It causes platelets to aggregate and
activate, and neutrophils and alveolar macrophages to generate superoxide
radicals to kill bacteria. It also lowers blood pressure. [Note: PAF is one of
the most potent bioactive molecules known, causing effects at concentrations as
low as 10-11 mol/l.]
Figure 17.3 The ether glycerophospholipids. A. The plasmalogen phosphatidalethanolamine. B. Platelet-activating factor. ( is a long, hydrophobic hydrocarbon chain.).
The backbone of sphingomyelin is the amino alcohol sphingosine, rather than glycerol (Figure 17.4 ). A long-chain fatty acid is attached to the amino group of sphingosine through an amide linkage, producing a ceramide, which can also serve as a precursor of glycolipids. The alcohol group at carbon 1 of sphingosine is esterified to phosphorylcholine, producing sphingomyelin, the only significant sphingophospholipid in humans. Sphingomyelin is an important constituent of the myelin sheath of nerve fibers. [Note: The myelin sheath is a layered, membranous structure that insulates and protects neuronal fibers of the central nervous system (CNS).]
Figure 17.4 Structure of
sphingomyelin, showing sphingosine (in green box) and ceramide components (in
dashed box). P = phosphate.
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