Lipids are essential components of membranes and precursors of biomolecules , such as steroid hormones and other molecules used in signal transduction . Lipids are important energy-carrying components of food (dosage-dependent).
They also form important compounds with carbohydrates (glycolipids) and phosphate groups (phospholipids).Lipids are classified into hydrolyzable (able to undergo hydrolytic cleavage) and nonhydrolyzable.[/CENTER]
A. Fatty acids
A fatty acid is composed of an unbranched hydrocarbon chain of 4 –24 carbon atoms with a terminal carboxylic acid group. A fatty acid is polar ,with a hydrophilic (—COOH)and a hydrophobic end (—CH3).Saturated fatty acids without a double bond and unsaturated fatty acids with one or more double bonds are distinguished. Linoleic acid and arachidonic acid are essential in human nutrition. A double bond causes a kink in the chain and makes it relatively rigid .The free carboxyl group (—COOH)of a fatty acid is ionized (—COO –).
Fatty acids can combine with other groups of molecules to form various types of lipids. As water-insoluble (hydrophobic) molecules, they are soluble only in organic solvents. The carboxyl group can enter into an ester or an amide bond. Triglycerides are compounds of fatty acids with glycerol.
Glycolipids (lipids with sugar residues)and phospholipids (lipids with a phosphate group attached to an alcohol derivative)are the structural bases of important macromolecules. Their intracellular degradation requires the presence of numerous enzymes, disorders of which have a genetic basis and lead to diseases.
Sphingolipids are an important group of molecules in biological membranes. Here, sphingosine, instead of glycerol, is the fatty acid-binding molecule. Sphingomyelin and gangliosides contain sphingosine.Gangliosides make up 6% of the central nervous systemlipids.They are degraded by a series of enzymes.
C. Lipid aggregates
Owing to their bipolar properties,fatty acids can form lipid aggregates in water. The hydrophilic ends are attracted to their aqueous surroundings;the hydrophobic ends protrude from the surface of the water and form a surface film. If completely under the surface, they may form a micelle,compact and dry within. Phospholipids and glycolipids can form two-layered membranes (lipid membrane bilayer). These are the basic structural elements of cell membranes.
D. Other lipids : steroids
Steroids are small molecules consisting of four different rings of carbon atoms. Cholesterol is the precursor of five major classes of steroid hormones : prostagens, glucocorticoids, mineralocorticoids, androgens, and estrogens. Each of these hormone classes is responsible for important biological functions such as maintenance of pregnancy,fat and protein metabolism,
maintenance of blood volume and blood pressure, and sexual development.
Several groups of disorders of lipoprotein and lipid metabolism exist. Important examples are familial hypercholesterolemia (MIM14389), hyperlipoproteinemia (MIM 238600), dysbetalipoproteinemia (MIM 107741), and high-density lipoprotein binding protein (MIM 142695).
Genetically determined disorders of ganglioside catabolism lead to severe diseases ,e.g., Tay –Sachs disease (MIM 272800)due to defective degradation of ganglioside GM2(deficiency of -N –acetylhexosaminidase) , several types of gangliosidoses (MIM 230500,305650),Sandhoff disease (MIM 268800),and others.