Lipids

Lipids

1. Definition

   • - Heterogeneous group of compounds which are soluble in organic solvents and insoluble in water.
   • - Not polymers.
   • - Related to fatty acids.

2. Function

   • - Important dietary constituents, because of their high energy value.
     • - Fate soluble vitamins D,E,K,A. (DEKA).
       • - Lipids act as thermal insulator in subcutaneous tissues and around certain organs.
         • - Act as electrical insulator around myelinated nerves.
           • - Cell membrane structure.

3. Classification

   • Simple Lipids
     • Ester of (fatty acids + alcohol)
       • Tag
         • Oils or fat (Triacylglycerol)
           • - They are Ester of ( fatty acids + Glycerol )
           • Simple Triacylglycerol
             • If the three fatty acids are the same
           • Mixed Triacylglycerol
             • If the three fatty acids are not the same
           • Function
             • - Main storage form of fatty acids.
               • They provide sufficient energy for survival for several weeks of starvation.
                 • - Can be stored in large amount as it is hydrophobic
                   • - acts as insulator for heat
                     • - supports internal viscera
           • Solid or liquid ?
             • Solid Form
               • If the fatty acid chain is long or saturated (fat)
             • Liquid form
               • If the fatty acid chain is short or unsaturated (oils)
             • ( In room temp )
       • Wax
         • - Ester of ( fatty acid + high molecular monohydric alcohol)
           • (alcohol has 16~30 carbon)
             • As : Bee wax, Lanoline
   • Compound lipids
     • Esters of fatty acids with alcohols containing additional groups
     • Lipoproteins
       • contain protein
     • Sulfolipids
       • contain sulphate
     • Glycolipids
       • contain carbohydrates
         • - Contain ( Ceramide + carbohydrate )
           • Contain Ceramide+ two or more monosaccharides+ one or more sialic acid.
             • The principle sialic acid is gangliosides is N-acetyl neuraminic acid (NANA).
               • They are present in ganglion cells of nervous system.
                 • They act as receptor or for cell- cell recognition.
                   • Act as receptor e.g. for Cholera toxin in human intestine.
                     • *All glycolipids and sphingomyelin contain Sphingosine alcohol and are referred as Sphingolipids*
       • Cerebrosides
         • ( Ceramide monohexosides )
           • Contain Ceramide that is attached to monosaccharide unit by β-glycosidic bond
             • Galactocerebrosides
               • .Present in brain & nervous tissue
             • Glucocerebrosides
               • .Present in extra neural tissue
       • Sulfatides
         • Contain galactocerebroside + sulfate
       • Gangliosides
     • Phospholipids
       • contain phosphate
       • Glycerophospholipids
         • ( phosphoglycerides )
           • - They are polar, ionic lipids
             • Acidic phospholipids:
               • They are phospholipids with (-1) net charge or more at physiological PH (7.4).
                 • Example: Phosphatidyl serine, inositol, glycerol and cardiolipin
                   • They play a role in blood coagulation.
                     • Antibodies against acidic phospholipids lead to recurrent thrombosis and recurrent fetal loss (antiphospholipids syndrome)
           • Phosphatidic acid
             • - An Intermediate in the synthesis of TAG, phospholipid (parent compounds).
               • It isn’t common constituents in tissue.
           • Phosphatidyl choline
             • ( lecithin )
               • ( Dipalmityl lecithin )
                 • ( lung surfactant )
                   • - It contains palmitic acid at C1,2
                     • - It decreases the surface tension in the lung alveoli.
                       • - It is synthesized around time of birth.
                         • - Deficiency of lung surfactant in premature babies causes respiratory distress syndrome (RDS).
           • Phosphatidyl serine
           • phosphatidyl ethanolamine
             • - They have role in blood coagulation.
             • - They are found in the cell membrane.
             • (cephalin)
           • Phosphatidyl inositol
             • Phosphatidyl inositol 4,5 bisphosphate acts as second messenger in signal transduction and present in cell membrane.
               • *inositol is a cyclic hexahydric alcohol*
           • Phosphatidyl glycerol
             • Present in mitochondrial membrane and is the precursor of cardiolipin.
           • Cardiolipin (Diphosphatidyl glycerol)
             • - Present in mitochondrial membrane -It has some immunological properties.
           • Lysophospholipid
             • (lysophosphatidyl choline) (lysolecithin)
               • - This type of phospholipid with free OH in C2.
                 • - They are important intermediates in metabolism and phospholipid interconversions.
           • Plasmalogens
             • - Resemble phospholipids, except: they have ether link in C1 not ester link.
               • - Represent 10-30% of phospholipids in brain and heart.
                 • - Example: Ethanolamine plasmalogen.
       • Sphingophospholipid
         • ( Sphingomyelin )
         • Ceramide
           • fatty acid
           • Sphingosine
             • Serine
               • ( amino acid )
             • (palmitate)
               • ( fatty acid )
         • Phosphocholine
       • Functions of phospholipids
         • 1- They serve as structural component of membranes
           • They are important for mobilization of triacylglycerol from liver preventing the development of fatty liver
             • 2- They form the surface layer of lipoprotein structure, which help in lipid transport in blood.
             • 3- They solubilize cholesterol in bile, and prevent the formation of cholesterol stones
         • Important notes on phospholipids:
           • • Most phospholipids contain more than one kind of fatty acids.
             • • Saturated fatty acid is attached to C1. While, unsaturated fatty acid is attached to C2 except in Dipalmityl lecithin
               • • Phospholipase A2 enzyme attacks C2 position leads to removal of unsaturated fatty acid at C2.
   • Derived lipids
     • These are produced from the hydrolysis or digestion of simple or compound lipids.
       • Fatty acids
         • properties
           • They are mono carboxylic (only one –COOH-), which is completely ionized in plasma.
             • Fatty acids are amphipathic as it contain hydrophilic part and hydrophobic part.
               • They have long straight chains Except, phytanic acid is branched.
                 • - Up to five carbon : short chain fatty acid
                   • - 5~10 : medium fatty acid
                     • - More than 10 : long chain
                       • Fatty acids in human have even number of carbon atoms. plants have odd number.
                         • Some fatty acids contain a hydroxyl group as ( cerebronic acid)
                         • Saturated
                         • Unsaturated
                           • Mono-unsaturated
                           • Poly-unsaturated
                             • Essential fatty acids
                               • polyunsaturated fatty acids
                                 • Can't be synthesized inside the body.
                                   • They should be taken in diet for synthesis of other members of ω3,ω6 family.
                                     • Linoleic acid
                                       • (18:2 Δ9,12 ω6)
                                         • Constitutes about 60-80% of total fatty acid in dietary oils as corn oil, sunflower oil, and soyabean oil.
                                     • Linolenic acid
                                       • (18:3 Δ9,12 ,15 ω3)
                                         • Present in small amount in plant oil in association with linoleic acid, particularly present in linseed oil
                                     • Arachidonic acid
                                       • - Present in (arachis oil) Peanut oil. Present in small amount in plant oil in association with linoleic acid
                                         • - It may not be essential to take Arachidonic acid in diet if there is an adequate supply of linoleic acid.
                                           • - It can be synthesized inside the body from linoleic.
                                             • - It contains 20 carbons contains 4 double bonds, ( so it's more kinked )
                                               • - Its U shape is very important in fluidity of biological membrane
                               • The importance of essential fatty acids
                                 • - Are used for synthesis of prostaglandins and thromboxanes (Eicosanoids)
                                   • - Enter in cell membrane structure.
                                     • - Lower the cholesterol level in plasma.
                                       • - Role in growth and development
                                         • - Role of Linolenic acid in the formation of docosahexaenoic (DHA) acid (22:6 3), DHA is needed for normal brain & retina.
                             • other ω-3 fatty acids
                               • Eicosapentaenoic acid (20:5)
                               • Docosahexaenoic acid (22:6)
                               • Both are present in fish oil (Salmon) They protect from cardiovascular diseases
                         • Notes
                           • The double bound are always separated from each other by methylene group (-CH2-)
                             • They are neither adjacent nor conjugated ( not alternating with single bonds )
                               • The presence of double bound produces ( Cis-trans ) isomerism
                                 • The melting point of fatty acid decreases if the fatty acids are shorter and have more double bonds
                                   • Short fatty acids (up to 5C) and medium chain fatty acids (6-10C) are liquid at room temp
                                     • Longer chain fatty acids (more than 10C) are solid at room temp.
                                       • Parent fatty acids :
                                         • Fatty acid numbering : اسمع المحاضرة التانية على الدرايف
                               • Cis
                                 • When the two bulky groups are at the same side of the bond
                                   • - Less thermal stable, have lower melting temp
                               • Trans
                                 • When the two bulky groups are at the opposite side of the bond
                                   • - More thermal stable, need high melting temp.
                                     • - They are nearer to in properties to the corresponding saturated fatty acid.
                                       • - They are metabolized like saturated fatty acids. So, they tend to increase cholesterol in blood.
       • glycerol
       • sphingosine
   • Polyprenoid Compounds
     • A group of compounds which contains isoprene units
       • Steroids
         • • They are compounds that have steroid nucleus.
           • Steroid nucleus is composed of 4 rings identified by A-BC- D, and formed of 19 carbons
             • The OH at C3 is often esterified with unsaturated fatty acid (cholesterol ester)
               • Cholesterol ester is the storage form inside the cell. Cholesterol and cholesterol ester present in animal fats not in plant fats.
                 • Blood cholesterol is either in free form (1/3 of total) or in ester form (2/3 of total).
                   • Free cholesterol is an amphipathic as it contains the OH group at C3 (hydrophilic).
                     • Cholesterol is an important constituent in cell membrane
                       • Plant sterol is ergosterol, the precursor of vitamin D2
                         • Bacteria do not contain sterol
         • Cholesterol
           • precursor of
             • Vitamin D
         • Fat soluble Vitamins (EKA)
         • Bile acids
         • sex hormones
         • adrenocortical hormones
       • Terpenes
         • in plants
           • As essential oils in plants that have characteristics flavors . Example : limonene (from lemon oil)

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