lipid2

• Lipids


  Organic compounds that are esters of fatty acids or substances associated with them in nature, most are insoluble in water but soluble in fat solvents
• Fatty acids


  Monocarboxylic acids containing an even number of carbon atoms, can be saturated or unsaturated
• Saturated fatty acids


  • Short chain (C2-C6), medium chain (C8-C10), long chain (C12-C22), very long chain (≥C24)
• Saturated fatty acids


  • Acetic acid, Butyric acid, Caproic acid, Capric acid, Palmitic acid, Stearic acid, Arachidic acid, Lignoceric acid
• Palmitic acid is the commonest fatty acid in human tissues
• Increased intake of saturated fatty acids may lead to increase in plasma cholesterol levels and incidence of coronary heart disease
• Unsaturated fatty acids


  Contain one or more double bonds, most are cis type and liquid at room temperature, trans type are solid at room temperature
• Unsaturated fatty acids


  • Monoenoic acids (one double bond), Polyenoic acids (more than one double bond, including ω3 and ω6 PUFA)
• Monoenoic acids


  • Palmitoleic acid, Oleic acid, Nervonic acid
• ω3 PUFA


  • α-Linolenic acid, Timnodonic acid
• ω6 PUFA


  • Linoleic acid, Arachidonic acid
• Essential fatty acids


  Not synthesized in our body, must be obtained from diet, include α-linolenic and linoleic acids
• Deficiency of essential fatty acids produces dermatitis in infants, fatty liver, growth retardation, defective sperms, brain and retina
• Non-essential fatty acids


  Formed in our body in good amounts mainly from carbohydrates, not essential to take them in diet
• Fatty acid solubility


  Up to 6 carbons are soluble in water, longer chain are insoluble in water but soluble in fat solvents
• Fatty acid physical state

  Lower members are liquid and volatile, higher than 10 carbons are solids, unsaturated long chain are liquids
• Fatty acid reactions


  Ester formation, Salt formation, Reduction, Addition of hydrogen, Addition of oxygen
• Eicosanoids


  Synthesized from PUFA with 20 carbon atoms, have 20 carbon atoms, produced by most mammalian tissues, have physiological and pharmacological actions, are hormone-like molecules, are autocrine and paracrine regulators
• Addition of oxygen


  1. Double bond makes fatty acids sensitive to oxidation

  2. Double bonds react with oxygen to form corresponding peroxides

  3. Peroxide has a bad taste

  4. Further oxidation produces splitting of fatty acid chain and formation of lower chain aldehydes and acids

  5. Oxidation of USFA by hydrogen peroxide produces destruction of lipoproteins of plasma and cell membranes (lipid peroxidation)
• Eicosanoids


  • Characterized by: 1) Synthesized from PUFA with 20 carbon atoms mainly arachidonic acid
  • 2) Have 20 carbon atoms
  • 3) Produced by most mammalian tissues
  • 4) Have physiological and pharmacological actions
  • 5) Are hormone-like molecules, autocrine and paracrine regulators
  • 6) Subscript number indicates number of double bonds in molecule (e.g. PGE1, PGE2, PGE3)
• Classification of eicosanoids


  • A- Cyclic compounds (prostanoids)
  • B- Acyclic compounds: 1- Prostaglandins (PG), 2- Prostacyclins (PGI), 3- Thromboxanes (TX), 1- Leukotrienes (LT), 2- Lipoxins (LX)
• Prostaglandins


  • Derivatives of the C20 hypothetical compound prostanoic acid
  • First discovered in prostatic gland secretion
  • Have a cyclopentane ring in the middle of molecule (from C8-C12)
  • Important types: PGA, PGB, PGE, PGF, PGG & PGH
• PGE


  Ether soluble and contain a ketone group at C9
• PGF


  Soluble in phosphate buffer and contain a hydroxyl group at C9
• Thromboxanes (TX)


  Characterized by presence of an oxane ring (containing 2 oxygen atoms), e.g. TXA2, formed by platelets
• Prostacyclins (PGI)


  Contain an additional ring in their structure
• Leukotrienes (LT)


  Acyclic compounds characterized by presence of three conjugated double bonds, secreted from leukocytes, platelets and mast cells
• Lipoxins (LX)


  Acyclic compounds containing four conjugated double bonds but more oxygen than LT, secreted from arterial walls
• Importance and Functions of Eicosanoids (see lipid metabolism)
• Simple Lipids


  Esters of fatty acids with alcohols
• Sub-groups of simple lipids

  • Neutral fats or triacylglycerol (TAG)
  • Waxes
• Triacylglycerol (TAG)


  Esters of three fatty acids with glycerol
• Waxes


  Esters of one fatty acid with long chain monohydroxyalcohol higher than glycerol, examples: true wax (bee's wax), cholesteryl esters, vitamin A/D esters
• Glycerol


  Colorless, odorless, sweet taste, liquid and soluble in water
• Fatty acids present in TAG are usually of different types (mixed TAG)
• Hydrolysis of TAG


  1. Acid hydrolysis

  2. Enzymatic hydrolysis

  3. Alkaline hydrolysis (saponification)
• Rancidity


  Development of bad odor and taste (bad flavor) of fats and oils due to exposure to high temperature, oxygen, moisture or humidity, light and metals (act as catalysts)
• Types of Rancidity


  • Hydrolytic rancidity: Hydrolysis of TAG by lipase enzyme, especially in presence of high temperature and moisture, releasing short chain fatty acids with bad odor
  • Oxidative rancidity: Oxidation of USFA produces peroxides, lower chain fatty acids, fatty aldehydes and ketones with bad odor and bitter taste
• Protection against rancidity


  Addition of antioxidants e.g. vitamin E, phenols and quinones helps protect USFA against oxidation and decrease rate of rancidity development
• Compound Lipids


  Contain fatty acids, alcohols and other groups