Structure of Phospholipids

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Chapter: Biochemistry : Phospholipid, Glycosphingolipid, and Eicosanoid Metabolism

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).


 

A. Glycerophospholipids

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).  = 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.).

 

B. Sphingophospholipids: sphingomyelin

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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