Lipids

• Lipids


  A diverse group of naturally occurring molecules that are soluble in non-polar organic solvents (insoluble or poorly soluble in water)
• Main features/variables in a fatty acid

  • Long-chain carboxylic acid (n = 10,12,14,16, etc.)
  • Amphipathic = two different chemical characteristics
• Saturated fatty acid

  Fatty acid with no C=C bonds
• Monounsaturated fatty acid


  Fatty acid with only one C=C bond
• Polyunsaturated fatty acid


  Fatty acid with more than one C=C bond
• Saturation of common fatty acids

  • Stearic acid (Saturated)
  • Palmitic acid (Saturated)
  • Oleic acid (Monounsaturated)
  • Linoleic acid (Polyunsaturated)
  • Linolenic acid (Polyunsaturated)
  • Arachidonic acid (Polyunsaturated)
• Fatty acid nomenclature

  • Fatty acids have both common (trivial) names and systematic names
  • Systematic name based on parent hydrocarbon, with suffixes indicating saturation/unsaturation
  • Position and type of double bonds also specified
• Fatty acid nomenclature (i)


  • Trivial name: Linoleic acid

  Systematic name: cis,cis-9,12-octadecadienoic acid
• Fatty acid nomenclature (ii)

  • Trivial name: Oleic acid

  Systematic name: cis-9-octadecenoic acid

  Stearic acid (saturated form) = octadecanoic acid
• Essential fatty acids (EFAs)


  Unsaturated fatty acids that animals cannot synthesize but which are needed to synthesize other fatty acids

  Animals lack enzymes to insert C=C into the hydrocarbon chain beyond carbon 9

  Two classes: omega-3 (alpha-linolenic acid) and omega-6 (linoleic acid)
• Plants are a source of (short-chain) EFAs in the diet because they possess the enzymes needed for their synthesis (e.g. delta 12 and delta 15 desaturases)
• Fish oils are a good source of longer chain omega 3 fatty acids – fish accumulate by eating microalgae (or eating fish that have eaten microalgae)
• Triacylglycerols

  Carboxylic acid triesters of glycerol (a 3C trialcohol)

  The main dietary source of fatty acids (in fats and oils)

  The storage form of fatty acids

  An efficient energy store (much better than carbohydrates)

  A major source of biochemical energy
• Variation in fatty acids (i)

  • The three fatty acids of any specific triacylglycerol are not necessarily the same, e.g. olive oil typically has two oleic acids and one palmitic acid

  A lipid with one stearic and two linoleic acid chains
• Variation in fatty acids (ii)

  The properties of the triacylglycerol (e.g. melting point) depend on its fatty acid composition:

  Vegetable oils: almost entirely unsaturated fatty acids

  Animal fats: larger percentage of saturated fatty acids

  Bend in tail of unsaturated fatty acid prevents molecules packing tightly together – increasing fluidity – lowering melting temperature (e.g. sunflower oil v lard)
• Fats versus oils

  Fats: Mainly saturated fatty acids, tightly packed molecules, high melting point

  Oils: Mainly unsaturated fatty acids, loosely packed molecules, low melting point
• Hydrogenation


  Industrial processing of unsaturated oils and fats to make them saturated (converts liquid oils to solid or semi-solid fats)

  Why? Partially hydrogenated vegetable oils are cheaper than animal source fats, available in a wide range of consistencies, and have desirable characteristics (e.g. longer shelf life)

  But! Incomplete hydrogenation can isomerize cis double bonds into trans double bonds, to produce 'trans fats' implicated in circulatory diseases
• Cis and trans double bonds

  Cis C=C causes a bend but a trans C=C bond does not

  Fats containing fatty acids with cis double bonds are less solid than ones containing trans double bonds

  Naturally occurring unsaturated fatty acids are usually cis
• Types of phosphoglycerides

  • Phosphatidic acid

  Phosphatidylcholine

  Phosphatidylethanolamine

  Phosphatidylserine

  Phosphatidylinositol
• Phospholipids and membranes


  Phospholipids have two ends with different affinities for water (amphipathic)

  In water, phospholipids spontaneously form lipid bilayers

  These structures cluster the hydrophobic regions toward the inside and leave the hydrophilic regions exposed to the water environment
• Uses of fatty acids:

  • simple lipids
  • building blocks for complex lipids
  • source of energy
  • important for human diet (essential fatty acids)
• General structure of fatty acids:

  
• Omega-3 fatty acid: double bond on carbon 3 from methyl end
• Types of complex lipids:

  • neutral lipids
  • polar lipids
• Type of neutral lipid

  • triacylglycerols
• Types of polar lipids:

  • sphingolipids
  • glycerolipids
• Types of glycerolipids:

  • phosphoglycerides
  • Glycosylglycerides