Harper's Illustrated Biochemistry, Volume 1 - Murray R. 1993

Bioenergetics and Metabolism of Carbohydrates and Lipids
Physiologically Important Lipids
Phospholipids

Phospholipids include 1) phosphatidic acid and phosphatidylglycerols, 2) phosphatidylcholine, 3) phosphatidylethanolamine, 4) phosphatidylinositol, 5) phosphatidylserine, 6) lysophospholipids, 7) plasmalogens, and 8) sphingomyelins.

Phosphatidic acid and phosphatidylglycerols

Phosphatidic acid serves as a crucial intermediate in the synthesis of triacylglycerols and phospholipids, although it is present in tissues only in trace amounts (Fig. 15.10).

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Fig. 15.10. Phosphatidic acid.

Cardiolipin is a phospholipid found in mitochondrial membranes. It is synthesized from phosphatidylglycerol (Fig. 15.11).

Phosphatidylcholine (lecithin)

Lecithins, like simple fats, contain glycerol and Fatty acids, but their composition also includes phosphoric acid and Choline. Lecithins are widely distributed in the Cells of various tissues, performing both metabolic and structural Functions within membranes. Dipalmitoyl lecithin is a highly effective surfactant that lowers surface tension, thereby preventing the collapse of the inner surfaces of the Airways in the Lungs. Its absence in the lungs of premature newborns leads to respiratory distress syndrome. Most phospholipids contain a saturated acyl radical at position C₁ and an unsaturated radical at position C₂ (Fig. 15.12).

Fig. 15.11. Diphosphatidylglycerol (cardiolipin).

Fig. 15.12. 3-Phosphatidylcholine.

Phosphatidylethanolamine (cephalin)

Cephalins differ from lecithins only in that choline is replaced by ethanolamine (Fig. 15.13).

Fig. 15.13. 3-Phosphatidylethanolamine.

Phosphatidylinositol

In this compound, Inositol is represented by one of its stereoisomers, myo-inositol (Fig. 15.14). Phosphatidylinositol 4,5-bisphosphate is a vital component of phospholipids in Cell membranes; upon stimulation by an appropriate hormone, it is cleaved into diacylglycerol and inositol trisphosphate—both of which act as intracellular messengers, or second messengers.

Fig. 15.14. 3-Phosphatidylinositol.

Phosphatidylserine

Tissues also contain a phospholipid related to cephalin, which contains a Serine residue instead of ethanolamine (Fig. 15.15). Additionally, phospholipids containing a Threonine residue have been isolated.

Fig. 15.15. 3-Phosphatidylserine. Lysophospholipids

This group of compounds consists of phosphoacylglycerols containing only a single acyl radical. An example is lysolecithin, which plays an important role in Phospholipid METABOLISM (Fig. 15.16).

Fig. 15.16. Lysolecithin.

Plasmalogens

These compounds account for up to 10% of the phospholipids in Brain and Muscle tissue. Structurally, they are related to phosphatidylethanolamine, but contain an ether bond at the C₁ carbon atom rather than an ester bond, which is typical for most other acylglycerols. The alkyl radical in plasmalogens is usually an unsaturated alcohol (Fig. 15.17).

In some cases, ethanolamine is replaced by choline, serine, or inositol.

Fig. 15.17. Plasmalogen (phosphatidylethanolamine). Sphingomyelins

Sphingomyelins are abundant in Nervous Tissue. Upon Hydrolysis, sphingomyelins yield a fatty acid, phosphoric acid, choline, and the complex amino alcohol sphingosine (Fig. 15.18). Glycerol is absent from these compounds. The linkage of sphingosine with a fatty acid is termed a ceramide, which is also found as a component of Glycolipids (see below).

Fig. 15.18. Sphingomyelin.