Chapter 24 Lipids

Lipids Lipids: a class of naturally occurring organic compounds which are: insoluble in water, but soluble in organic solvents Lipids are generally put into three classes Storage Lipids Structural Lipids Lipids as Signals, Cofactors, and Pigments

Lipids Storage Lipids (80% of the mass of a fat cell) Fatty acids Long chain alkane or alkene with carboxylic acid at one end. Triacylglycerols Glycerol triester of three fatty acids Waxes Fatty acid ester with long chain alcohol

Lipids Structural Lipids (5% to 10% of dry mass of most cells) Phosphoacylglycerols (glycerophospholipids) Galactolipids (glycolipids) Sulfolipids Sphingolipids Cholesterol (and Bile acids)

Lipids Signals, Cofactors, and Pigments Steroid hormones Lipid-soluble vitamins Eicosanoids (not transported in the blood) Prostaglandins Thromboxanes Leukotrienes

Triglyceride Triglyceride: an ester of glycerol with three fatty acids

Fatty Acids Fatty acid: an unbranched-chain carboxylic acid derived from hydrolysis of animal fats, vegetable oils, or membrane phospholipids nearly all have an even number of carbon atoms, most between 12 and 20, in an unbranched chain the three most abundant are palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1) in most unsaturated fatty acids, the cis isomer predominates; the trans isomer is rare unsaturated fatty acids have lower melting points than their saturated counterparts; the greater the degree of unsaturation, the lower the melting point

Fatty Acids

Triglycerides Physical properties depend on the fatty acid components melting point increases as the number of carbons in the hydrocarbon chains increases and as the number of double bonds decreases triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats

Triglycerides The lower melting points of triglycerides rich in unsaturated fatty acids are related to differences in their three-dimensional shape hydrocarbon chains of saturated fatty acids can lie parallel with strong London dispersion forces between their chains; they pack into well-ordered, compact crystalline forms and melt above room temperature because of the cis configuration of the double bonds in unsaturated fatty acids, their hydrocarbon chains have a less ordered structure and London dispersion forces between them are weaker; these triglycerides have melting points below room temperature

Hydrogenation Hardening: reduction of some or all of the carbon-carbon double bonds of an unsaturated triglyceride using H2/catalyst in practice, the degree of hardening is carefully controlled to produce fats of a desired consistency the resulting fats are sold for kitchen use (Crisco, Spry, Dexo, and others) margarine and other butter substitutes are produced by partial hydrogenation of polyunsaturated oils derived from corn, cottonseed, peanut, and soybean oils

Soaps Natural soaps are prepared by boiling lard or other animal fat with NaOH, in a reaction called saponification (Latin, sapo, soap)

Soaps Soaps clean by acting as emulsifying agents their long hydrophobic hydrocarbon chains cluster so as to minimize their contact with water their polar hydrophilic carboxylate groups remain in contact with the surrounding water molecules driven by these two forces, soap molecules spontaneously cluster into micelles

Soaps soaps form water-insoluble salts when used in water containing Ca(II), Mg(II), and Fe(III) ions (hard water)

Complex Lipids Phospholipids Glycolipids contain an alcohol, two fatty acids, and a phosphate ester in glycerophospholipids, the alcohol is glycerol in sphingolipids, the alcohol is sphingosine Glycolipids complex lipids that contain a carbohydrate

Complex Lipids Sphingosine – amino alcohol Glycerol

Membranes Complex lipids form the membranes around cells and small structures within cells In aqueous solution, complex lipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers polar head groups are in contact with the aqueous environment nonpolar tails are buried within the bilayer the major force driving the formation of lipid bilayers is hydrophobic interaction the arrangement of hydrocarbon tails in the interior can be rigid (if rich in saturated fatty acids) or fluid (if rich in unsaturated fatty acids)

Fluid Mosaic Model

Glycerophospholipids Glycerophospholipids are the second most abundant group of naturally occurring lipids they are also called phosphoglycerides found almost exclusively in plant and animal membranes, which typically consist of 40% -50% phosphoacylglycerols and 50% - 60% proteins the most abundant phosphoacylglycerols are derived from phosphatidic acid, a molecule in which glycerol is esterified with two molecules of fatty acid and one of phosphoric acid the three most abundant fatty acids in phosphatidic acids are palmitic (16:0), stearic (18:0), and oleic (18:1)

Glycerophospholipids A phosphatidic acid the fatty acid on carbon 2 is always unsaturated further esterification with a low-molecular-weight alcohol gives a glycerophospholipid

Glycerophospholipids More often phosphatidyl____________

Glycerophospholipids a lecithin (phosphotidylcholine)

Sphingolipids Found in the coatings of nerve axons (myelin) contain the long-chain aminoalcohol, sphingosine,

Glycolipids Glycolipid: a complex lipid that contains a carbohydrate the carbohydrate is either glucose or galactose the cerebrosides are ceramide mono- or oligosaccharides

Steroids Steroids: a group of plant and animal lipids that have this tetracyclic ring structure

Cholesterol Cholesterol is the most abundant steroid in the human body in plasma membranes in all animal cells precursor of all steroid hormones and bile acids

Lipoproteins Cholesterol, along with fats, are transported by lipoproteins

Lipoproteins Schematic of a low-density lipoprotein

Cholesterol Transport transport of cholesterol from the liver starts with VLDL 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

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 LDL has delivered its cholesteryl esters to liver cells, it reenters circulation

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 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 hypercholesterolemia, there are not enough LDL receptors and plasma levels of cholesterol may be as high as 680 mg/100 mL

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

Steroid Hormones Androgens: male sex hormones synthesized in the testes responsible for the development of male secondary sex characteristics

Steroid Hormones Among the synthetic anabolic steroids are

Steroid Hormones Estrogens: female sex hormones synthesized in the ovaries responsible for the development of female secondary sex characteristics and control of the menstrual cycle

Steroid Hormones Progesterone-like analogs are used in oral contraceptives

Steroid Hormones Glucorticoid hormones synthesized in the adrenal cortex regulate metabolism of carbohydrates decrease inflammation involved in the reaction to stress

Bile Salts Bile salts, the oxidation products of cholesterol synthesized in the liver, stored in the gallbladder, and secreted into the intestine where they emulsify dietary fats

Prostaglandins Prostaglandins: a family of compounds that have the 20-carbon skeleton of prostanoic acid

Prostaglandins Prostaglandins are not stored in tissues as such, but are synthesized from membrane-bound 20-carbon polyunsaturated fatty acids in response to specific physiological triggers one such polyunsaturated fatty acid is arachidonic acid

Prostaglandins

COX Enzymes The COX 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

Thromboxanes Thromboxanes are also derived from arachidonic acid thromboxane A2 induces platelet aggregation and vasoconstriction aspirin and other NSAIDs inhibit the synthesis of thromboxanes by inhibiting the COX enzyme

Leukotrienes Leukotrienes are also synthesized from arachidonic acid they occur mainly in leukocytes they produce muscle contractions, especially in the lungs … can cause asthma-like attacks they are 100 times more potent than histamine anti-asthma drugs inhibit leukotrienes synthesis

Lipids End Chapter 20