Classification

The term "lipids" refers to a group of substances that are insoluble in water and soluble in organic solvents. The easiest way to classify lipids is to divide them into classes according to structural characteristics (A). Chemically, these classes differ considerably.

The polyprenyls class is based on the isoprene building block. They include steroids (e. g., cholesterol), fat soluble vitamins (vitamins A, D, E, K), and other terpenes (e. g., menthol).

The largest branch of lipids is derived from fatty acids (see p. 86). Waxes have the simplest structure, resulting from esterification of one fatty acid with a monovalent alcohol. Fatty acids from foods are metabolized to eicosanoid precursors by chain extension and insertion of double bonds. Among them are prostaglandins, leukotrienes, and thromboxanes, which have regulatory functions on the cellular level. Quantitatively, triacylglycerols (or triglycerides) are predominant. Since they are used for energy storage in animals they are also termed storage lipids (B). They are formed by esterification of three fatty acids with one glycerol (glycerin). The resulting molecules are nonpolar, hence the name, neutral fats.

The phospholipids class contains an additional phosphate group. If glycerin is their base component, they are called glycerophospholipids. The phosphate group can be esterified with various alcohols like phosphatidylethanola-mine, -serine, and -choline (lecithin), to name a few important representatives.

Sphingosine, an amino alcohol with a long side chain, represents another basic building block. When sphingosine bonds to a fatty acid with an amide bond, it forms ceramide. Sphingolipids result from linkage with additional R-groups. If that linkage occurs via a phosphate group, the resulting compounds are called sphingophospholipids, of which large amounts are found in the brain and nervous system. Their most important representative is sphingo-myelin, in which choline is attached to the phosphate group.

To build biological membranes (e. g., cell and organelle membranes), polar lipids are needed. In biological membranes, the polar group (alcohol, choline, glucose, etc.) is always oriented outwards, whereas the long, nonpolar fatty acid chains make up the core of the membrane bilayer. Membrane lipids are classified by their base components (glycerol or sphingosine), as well as their polar groups.

In glycolipids, the phosphate group linked to sphingosine is replaced by a sugar molecule. Cerebrosides are simple representatives of glycolipids: their sugars are either glucose or galactose. Sulfatides have an additional sulfate R-group attached to the sugar. Ganglio-sides contain a complex oligosaccha-ride, which in turn contains N-acetyl neuraminic acid. Glycolipids are found in all tissues on the outside of cell membranes. Some have receptor functions.

Classification 85

A. Classification

Lipids

Fatty acids

Eicosanoids

Steroids

Fat-soluble vitamins

Other terpenes

Polyprenyl compounds

Triacylglycer°ls Waxes Ceramides

(fats and oils)

Glycero-phospholipids

Sphingolipids

Plasmalogens r~

Phosphatidates

Sphingomyelin Cerebrosides

Gangliosides

Phosphatidyl- Phosphatidyl- Phosphatidyl- Other ethanolamine serine choline phospholipids

Phospholipids

Other glycolipids

Glycolipids

B. Storage and Membrane Lipids-

Storage lipids (neutral)

Triacylglycerol

_ Fatty acid Fatty acid Fatty acid

Membrane lipids (polar)

G lycero-phospholipids

G lycero-phospholipids

_ Fatty acid Fatty acid Phosphate £ Alcohol

Sphingolipids

Fatty acid Phosphate ^

Fatty acid

Choline

Fatty acid

Glucose or galactose

Glucose or galactose

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