Chemistry 2100 Chapter 21

Lipids

Fatty Acids

oleic acid (mp 4°C) stearic acid (mp 70°C)

Triacylglycerols

Fatty Acids The fatty acid components of triglycerides have certain things in common: 1. Practically all are unbranched carboxylic acids. 2. They range in size from about 10 to 20 carbons. 3. They contain an even number of carbon atoms. 4. Apart from the –COOR ester groups, triglycerides have no functional groups, except that some have one or more carbon-carbon double bonds in the fatty acid hydrocarbon chains. 5. In most fatty acids that have carbon-carbon double bonds, the cis isomers predominate. 8

Saponification Saponification: the base-promoted hydrolysis of fats and oils in aqueous NaOH and produces glycerol and a mixture of fatty acid salts called soaps.

anionic detergents Synthetic Detergents (syndets)

Hydrogenation Hardening: The reduction of some or all of the carbon-carbon double bonds of an unsaturated triglyceride using H 2 /transition metal catalyst, which converts a liquid triglyceride to a semisolid.

"partially hydrogenated vegetable oil"

Complex Lipids

Membranes: Glycerophospholipids

Fluid Mosaic Model Figure 21.2 The fluid mosaic model of membranes.

Other Lipids: Waxes

Sterols

Steroids

Bile Salts Bile salts Bile salts are oxidation products of cholesterol. Synthesized in the liver, stored in the gallbladder, and secreted into the intestine where they emulsify dietary fats and aid in their absorption and digestion. 23

Low-Density Lipoprotein

Cholesterol Transport –Transport of cholesterol from the liver starts out as a large VLDL particle. –VLDL is carried in the serum. –As fat is removed, its density increases and it becomes LDL; LDL stays in the plasma for about 2.5 days. –LDL carries cholesterol to cells, where specific LDL receptors bind it. –After binding, LDL is taken into cells where enzymes liberate free cholesterol and cholesteryl esters. 25

Cholesterol Transport – High-density lipoproteins (HDL) transport cholesterol from peripheral tissues to the liver and also transfer cholesterol to LDL. – While in the serum, free cholesterol in HDL is converted to cholesteryl esters. – In the liver, HDL binds to the liver cell surface and transfers its cholesteryl esters to the cell. – These esters are used for the synthesis of steroid hormones and bile acids. – After HDL has delivered its cholesteryl esters to liver cells, it reenters circulation. 26

Levels of LDL and HDL –Most of the cholesterol is carried by LDL. –Normal plasma levels are 175 mg/100 mL. –If there are sufficient LDL receptors on the surface of cells, LDL is removed from circulation and its concentration in blood plasma drops. –The number of LDL receptors is controlled by a feedback mechanism. –When the concentration of cholesterol inside cells is high, the synthesis of LDL receptors is suppressed. –In the disease called famelial hypercholesterolemia, there are not enough LDL receptors and plasma levels of cholesterol may rise as high as 680 mg/100 mL. 27

Levels of LDL and HDL – These high levels of cholesterol can cause premature atherosclerosis and heart attacks. – In general, high LDL means high cholesterol content in the plasma because LDL cannot get into cells. – Therefore, high LDL together with low HDL is a symptom of faulty cholesterol transport and a warning of possible atherosclerosis. – The serum cholesterol level controls cholesterol synthesis in the liver. – When serum cholesterol is high, its synthesis in the liver is low, and vice versa. – The commonly used statin drugs inhibit the synthesis of cholesterol by blocking HMG-CoA reductase. 28

Prostaglandins 29

COX Enzymes The COX (cyclooxygenase) enzyme occurs in two forms: COX-1 catalyzes the normal physiological production of prostaglandins. COX-2 is responsible for the production of prostaglandins in inflammation. –When a tissue is injured or damaged, special inflammatory cells invade the injured tissue and interact with resident cells, for example, smooth muscle cells. –This interaction activates COX-2 and prostaglandins are synthesized. 30

Thromboxanes Thromboxanes are also derived from arachidonic acid. –Thromboxane A 2 induces platelet aggregation and vasoconstriction. –Aspirin and other NSAIDs inhibit the synthesis of thromboxanes by inhibiting the COX enzyme. 31

Leukotrienes Leukotrienes are also synthesized from arachidonic acid. –They occur mainly in leukocytes. –They produce muscle contractions, especially in the lungs and thereby can cause asthma-like attacks. They are 100x more potent than histamine. –Several recently-developed anti-asthma drugs inhibit the synthesis of leukotrienes. 32