lipids

• Lipids


  A heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties
• Lipids


  • Insoluble in water, but soluble in organic solvents including diethyl ether, dichloromethane, and acetone
• Lipids include


  • Fatty acids
  • Triacylglycerols (triglycerides)
  • Sphingolipids
  • Phosphoacylglycerols
  • Glycolipids
  • Lipid-soluble vitamins
  • Prostaglandins, leukotrienes, and thromboxanes
  • Cholesterol, steroid hormones, and bile acids

  
• Triglyceride


  A triester of glycerol with three fatty acids

  
• In most triglycerides, two or three different fatty acid components are present

  
• Hydrophobic character of triglycerides


  Caused by the long nonpolar hydrocarbon chains of the fatty acid components

  
• The ester groups, although polar, are buried within a nonpolar environment, which makes triglycerides insoluble in water

  
• In triglycerides, all three hydroxyl groups of glycerol are esterified with fatty acids

  
• Characteristics of fatty acid components of triglycerides


  • Practically all are unbranched carboxylic acids
  • They range in size from about 10 to 20 carbons
  • They contain an even number of carbon atoms
  • 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
  • In most fatty acids that have carbon-carbon double bonds, the cis isomers predominate

  
• Melting points of fatty acids


  Increase as the number of carbons in the hydrocarbon chains increases and as the number of double bonds decreases
• Triglycerides rich in unsaturated fatty acids


  Generally liquid at room temperature and are called oils

  
• Triglycerides rich in saturated fatty acids


  Generally semisolids or solids at room temperature and are called fats

  
• Hardening


  1. The reduction of some or all of the carbon-carbon double bonds of an unsaturated triglyceride using H2/transition metal catalyst, which converts a liquid triglyceride to a semisolid

  2. The degree of hardening is carefully controlled to produce fats of a desired consistency

  3. The resulting fats are sold for kitchen use (Crisco, Spry, Dexo, and others)

  4. Margarine and other butter substitutes are produced by partial hydrogenation of polyunsaturated oils derived from corn, cottonseed, peanut, and soybean oils

  5. The hardening process is the source of trans fatty acids
• Saponification


  The base-promoted hydrolysis of fats and oils in aqueous NaOH produces glycerol and a mixture of fatty acid sodium salts called soaps
• Phospholipids


  • 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 form the membranes around cells and around small structures within cells
• Lipid bilayer


  • In aqueous solution, complex lipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers
  • Polar (hydrophilic) head groups are in contact with the aqueous environment
  • Nonpolar (hydrophobic) tails are buried within the bilayer and shielded from the aqueous environment
  • The major force driving force for 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)

  
• Membranes typically consist of 40% -50% phosphoacylglycerols and 50% - 60% proteins
• 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)
• The fatty acid on carbon 2 of glycerol is always unsaturated

  
• Phosphatidylcholine (common name lecithin)


  A glycerophospholipid where the other alcohol is choline

  
• Cephalins are another group of glycerophospholipids in which the additional phosphate ester is provided by either ethanolamine or serine
• Sphingolipids


  • Contain the long-chain aminoalcohol, sphingosine, from which this class of compounds in named
  • The sphingolipid myelin is found in the coatings of nerve axons
• Glycolipid


  • A complex lipid that contains carbohydrates and ceramides
  • The carbohydrate is either glucose or galactose
  • The cerebrosides are ceramide mono- or oligosaccharides

  
• Steroids


  A group of plant and animal lipids that contains this tetracyclic ring structure

  
• Cholesterol


  • The most abundant steroid in the human body, and also the most important
  • It is a component in plasma membranes in all animal cells
  • It is the precursor of all steroid hormones and bile acids
• Cholesterol is first converted to progesterone and then to both sex hormones and adrenocorticoid hormones
• Lipoproteins


  • Carriers of cholesterol
  • Most lipoproteins contain a core of hydrophobic lipid molecules surrounded by a shell of hydrophilic molecules such as proteins and phospholipids
  • There are four kinds of lipoproteins: HDL, LDL, VLDL, and chylomicrons

  
• HDL (high-density lipoprotein)


  Consists of about 33% protein and 30% cholesterol

  
• LDL (low-density lipoprotein)


  Contains about 25% protein and 50% cholesterol

  
• VLDL (very-low density lipoprotein)


  Carries triglycerides (fats) synthesized by the liver

  
• Chylomicrons


  Carry dietary lipids synthesized in the intestines
• Cholesterol transport


  1. Transport of cholesterol from the liver starts out as a large VLDL particle

  2. VLDL is carried in the serum

  3. As fat is removed, its density increases and it becomes LDL; LDL stays in the plasma for about 2.5 days

  4. LDL carries cholesterol to cells, where specific LDL receptors bind it

  5. After binding, LDL is taken into cells where enzymes liberate free cholesterol and cholesteryl esters

  6. HDL transports cholesterol from peripheral tissues to the liver and also transfers cholesterol to LDL

  7. While in the serum, free cholesterol in HDL is converted to cholesteryl esters

  8. In the liver, HDL binds to the liver cell surface and transfers its cholesteryl esters to the cell

  9. These esters are used for the synthesis of steroid hormones and bile acids

  10. After HDL has delivered its cholesteryl esters to liver cells, it reenters circulation
• Normal plasma levels of cholesterol

  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
• Familial hypercholesterolemia


  A disease where there are not enough LDL receptors and plasma levels of cholesterol may rise as high as 680 mg/100 mL