Lipids

• Lipids


  Organic compounds found in living organisms that are insoluble (or only sparingly soluble) in water but soluble in non-polar organic solvents
• Lipids do not have a common structural feature that serves as the basis for defining such compounds

  
• Classification of lipids


  • Based on solubility, not on any functional groups
  • Insoluble or sparingly soluble in water
  • Soluble in non-polar organic solvents
• Lipids


  • Exhibit structural diversity
  • Some are esters, some are amides, and some are alcohols (acyclic and cyclic) and some are polycyclic

  
• Five categories of lipids

  • Energy-storage lipids - triacylglycerols
  • Membrane lipids - phospholipids, sphingoglycolipids, and cholesterol
  • Emulsification lipids - bile acids
  • Chemical messenger lipids - steroid hormones and eicosanoids
  • Protective-coating lipids - biological waxes

  
• Fatty acids


  • Structural components of all the lipids
  • Carboxylic acids with linear (unbranched) carbon chain
  • Naturally occurring monocarboxylic acids

  
• Types of fatty acids based on carbon chain length


  • Long chain fatty acids: c12 - c26
  • Medium chain fatty acids: c6 - c11
  • Short-chain fatty acids: c4 - c5

  
• Types of fatty acids based on saturation


  • Saturated - all c-c bonds are single bonds
  • Monounsaturated - one c=c bond
  • Polyunsaturated - 2 or more c=c bonds present - up to six double bonds

  
• Saturated fatty acids (SFAs)


  Fatty acid with a c chain in which all carbon-carbon bonds are single bond

  
• Monounsaturated fatty acids (MUFAs)


  Fatty acid with a carbon chain in which one carbon–carbon double bond is present

  
• Polyunsaturated fatty acids (PUFAs)


  Fatty acid with a carbon chain in which two or more carbon–carbon double bonds are present

  
• Types of unsaturated fatty acids


  • Omega (ω)-3 fatty acids - unsaturated fatty acid with its endmost double bond three carbon atoms away from its methyl end
  • Omega(ω)-6 fatty acid - unsaturated fatty acid with its endmost double bond six carbon atoms away from its methyl end

  
• Unsaturated fatty acids and double bond position


  • Numbering starts from the other end of cooh
  • Structural notation indicates number of c atoms and the number of c-c double bonds present
  • To specify double-bond positioning, add greek capital letter delta (∆) followed by one or more superscript numbers

  
• Fatty acids of biological importance


  • Omega-3 fatty acids: linolenic acid
  • Omega-6 fatty acids: linoleic acid

  
• Linoleic acid deficiency


  Skin redness, infections and dehydration, liver abnormalities, children need it the most, human milk has more than cow's milk
• American diet is sufficient in omega 6 fatty acids but deficient in omega 3 fatty acids
• Fish is a good source for omega 3 fatty acids
• High rate of heart disease may be due to imbalance in omega 3 and 6 fatty acids
• Ideal ratio of omega 6 to omega 3 fatty acids is 4 - 10 g: 1g

  
• Physical properties of fatty acids


  • Water solubility is a direct function of carbon chain length, decreasing as chain increases
  • Short chain fatty acids have some solubility whereas long chain fatty acids are insoluble in water
  • Short chain fatty acids are sparingly soluble because of carboxylic acid polar group
  • Melting point increases as carbon chain increases and decreases as degree of unsaturation increases

  
• Melting point of fatty acids


  Depends on length of carbon chain and degree of unsaturation (number of double bonds)

  
• Space-filling molecules


  Number of bends in a fatty acid chain increase as the number of double bonds increase, resulting in less packing, lower melting point, and tendency to be liquids at room temperature

  
• Energy-storage materials


  • Carbohydrate glycogen is the most widespread energy storage material, present in small amounts
  • Triacylglycerols are the most concentrated energy storage material, primarily stored in adipocytes
• Types of triacylglycerols


  • Simple triacylglycerols - three identical fatty acids esterified
  • Mixed triacylglycerols - triester formed from esterification of glycerol with more than one kind of fatty acid

  
• In nature, mostly mixed triacylglycerols are found and are different even from the same source depending on the feed

  
• Difference between fats and oils


  • Fats are predominantly saturated and solids/semisolids at room temperature, from animal sources
  • Oils are predominantly unsaturated and liquids at room temperature, from plant and fish sources

  
• Nations with high dietary intakes of fats and oils tend to have higher incidences of heart disease and certain types of cancers
• Typical American diet contains too much fat, so Americans are being asked to reduce their total dietary fat intake
• Good fats vs bad fats


  • Recommended amounts: 15% monounsaturated, 10% polyunsaturated, 10% saturated
  • Saturated fats are considered "bad fats"
  • Monounsaturated fats are considered "good fats"
  • Trans-monounsaturated fats are considered "bad fats"
  • Polyunsaturated fats can be both "good fats" and "bad fats"
  • Omega 3 and 6 are important "good fats"
• Cold water fish are a good source of omega-3 fatty acids
• Essential fatty acids


  • Fatty acids that must be obtained from dietary sources, not synthesized within the body
  • The two most important are linoleic acid (omega 6) and linolenic acid (omega 3)
  • Needed for proper membrane structure and as starting materials for longer-chain omega-6 and omega-3 fatty acids
  • Deficiencies may result in skin redness, infections, dehydration, and liver abnormalities

  
• Eating nuts can have a strong protective effect against coronary heart disease due to low amounts of saturated fatty acids and presence of antioxidant vitamins, minerals, and plant fiber protein
• Chemical reactions of triacylglycerols


  • Hydrolysis
  • Saponification
  • Hydrogenation
  • Oxidation

  
• Formation of a triacylglycerol


  Esterification of glycerol with three fatty acids

  
• Complete and partial hydrolysis of a triacylglycerol


  Breaking of 1-2 ester bonds to give rise to mono- or diacylglycerol and fatty acid(s)

  
• Saponification


  Hydrolysis in basic solution to produce salt of fatty acid and glycerol
• Oxidation of triacylglycerols present in skin perspiration can lead to trans fatty acid formation

  
• Membrane lipids: Phospholipids


  • Up to 80% of the mass of a cell membrane can be lipid materials, dominated by phospholipids
  • A phospholipid contains one or more fatty acids, a phosphate group, a platform molecule (glycerol or sphingosine), and an alcohol attached to the phosphate group

  
• Glycerophospholipids


  • Lipid that contains two fatty acids and a phosphate group esterified to a glycerol molecule and an alcohol esterified to the phosphate group
  • All attachments are ester linkages, with four ester linkages compared to three in triacylglycerols
  • Function as components of cell membranes, unlike triacylglycerols which serve as energy storage

  
• Sphingophospholipids


  Structures based on the 18-carbon monounsaturated amino alcohol sphingosine, containing one fatty acid and one phosphate group attached to a sphingosine molecule and an alcohol attached to the phosphate group