Lipids PDF

Lipids Chemistry of lipids  Definition  Biological functions  Classification  Examples Definition: Lipids are heterogeneous group of substances which are insoluble in water, but soluble in non-polar solvents (fat solvents) e.g. ether, chloroform, benzene. They are related to fatty acids este...

Lipids Chemistry of lipids  Definition  Biological functions  Classification  Examples Definition: Lipids are heterogeneous group of substances which are insoluble in water, but soluble in non-polar solvents (fat solvents) e.g. ether, chloroform, benzene. They are related to fatty acids esters. Biological functions: 1) They are important dietary constituent. They are of high energy value which is over double that of carbohydrate. They provide the body with polyunsaturated fatty acid (PUFA) or essential fatty acids. They contain fat soluble vitamins. 2) In the body, the stored fat «Depot fat» acts as: a. Store of energy. b. Insulator against heat loss. c. Protector against trauma. d. Support of internal organs e.g. kidney.  This type of fat is affected by diet e.g. starvation, hence called variable element. 3) Lipids are structures of every cell e.g. cell membrane, mitochondrial membranes … etc. This type of lipids is not affected by starvation, called «constant element» or «tissue fat». Classification 1. Simple lipids. 2. Compounds (conjugated) lipids. 3. Derived lipids (alcohol, cholesterol, FA,…..). Sulfolipid  1- Simple lipids  Are esters of fatty acid with alcohols.  According to the type of alcohols, they are divided into: a) Neutral fats: Esters of fatty acids with alcohol glycerol (T.G or TAG). They are either solid at room temperature (fats) or liquid (oils). b) Waxes: Esters of fatty acids with alcohols higher than glycerol (high molecular weight monohydric alcohols). 1- Neutral fat  They carry no charge.  They are «triacyglycerols = TAG» or «triglycerides= T.G».  They are composed of the trihydric alcohol esterified to 3 fatty acids.  Carbons are named either α, β and γ, or 1, 2 and 3. The 1 and 3 carbons are not identical when viewed in three dimensions. 2- Waxes These are esters of fatty acids with long chain alcohols other than glycerol. They are secreted extra-cellulary in some plants and animals and have protective functions. Examples: 1) Bee wax. 2) Sebacious secretions. 3) Cuticle of leaves. They are solid at room temperature, not digested by human enzymes consequently they are not utilized by the body.  Glycerol:  Glycerol is a triol with a structure of propane substituted at positions 1, 2 and 3 by hydroxyl groups.  It is used in medicine for: 1) Preparation of cosmetics. 2) Formation of nitroglycerine, which is a coronary vasodilator, used to treat angina pectoris. 3) It is a dehydrating agent, hence used to dehydrate the eye e.g. during corneal edema and glaucoma (increased intra-ocular pressure).  Chemically it is tested by boiling with dehydrating agent e.g. acid potassium sulfate or sulfuric acid, producing acrolein, which has characteristic odor.  It can be esterified to one, two and three fatty acids producing mono, di, and triacyglycerols respectively. Fatty acids:  These are water insoluble, long chain hydrocarbons, containing the active carboxylic group COOH.  Most fatty acids present in the body are aliphatic or not branched and contain even number of carbon atoms.  Few acids present in animals and plants are either branched or containing odd number of carbons.  They are divided into: 1. Saturated fatty acid. 2. Unsaturated fatty acid. (1) Saturated fatty acids  They have no double bonds in their structure.  Their general formula CH3 (CH2)n COOH, where n equals the number of CH2-groups between the methyl and carboxylic groups.  They are short chain or long chain fatty acids a. Short chain fatty acids (less than 8C)  e.g. Acetic 2C, Propionic 3C, Butyric 4C, Valeric 5C, Caproic 6C  They are liquid at room temperature.  They are volatile and water soluble. b. Long chain fatty acids  e.g. Palmitic 16C, Stearic 18C  They are solid at room temperature.  They are not volatile and soluble in fat solvents. (2) Unsaturated fatty acids  They have double bonds in their structure.  They are monounsaturated ( 1 =) such as: oleic acid or polyunsaturated (more than =) such as: linoleic acid and arachidonic acid. Note:  Sulfur containing fatty acid These are fatty acids that contain sulfur e.g Lipoic acid (6,8 dithio-octanoic), a member of B-complex vitamins.  Hydroxy containing fatty acids Hydroxyl group is attached to α-carbon e.g Cerebronic acid (Hydroxy lignoceric) Triglycerides (triacyglycerols)  These are three fatty acids esterified to glycerol.  TAG are synthesized in the body, mainly from excess CHO in the diet by a process called lipogenesis.  They act as storage form of energy (1 gm fat = 9 Kcal, while 1 gm of CHO = 4 Kcal).  They are present in a liquid form at the body temperature in the cytoplasm of cells of the adipose tissue.  When needed, they are hydrolyzed by a process called «lipolysis», liberating fatty acids and glycerol which are oxidized. Sources: 1. Animal sources e.g. butter. 2. Plant sources e.g. cotton seed oil, linseed oil, olive, palm oil. These are rich in unsaturated fatty acid. 3. Marine oils e.g. cod liver oil and shark oil. Types: A- Simple T.G: Similar 3 fatty acids are attached to glycerol e.g. tristearin (3 stearic acids).This type is rare. B- Mixed T.G : This is common type, where the three fatty acids are different. 2- Compound (conjugated lipids)  They are composed of fatty acids, alcohols and other group or substance.  According to other substance they are subdivided into: a. Phospholipids, containing phosphoric acid. b. Glycolipids, containing sugar or carbohydrates. c. Sulfolipids, containing sulfur. d. Lipoproteins, containing proteins:  such as high density lipoprotein (HDL) and low density lipoprotein ( LDL). (A) Phospholipids  These are the main constituent of all biological membranes.  They contain phosphoric acid in their structure.  According to the type of alcohol, they are divided into: a) Glycerophospholipids (glycerol) b) Sphingophospholipids (long chain, unsaturated, amino dihydric alcohol sphingosine) A- GLycerophospolipids e.g: 1) Phosphatidic acid 2) Cardiolipin (Diphosphatidyl Glycerol) 3) Lecithin (Phosphatidyl Choline) 4) Cephalins 5) Lysophospholipid 6) Plasmalogens Phosphatidic acid Cardiolipin Lecithin Lysophospholipid  Cephalin Plasmalogen OH Phospatidyl inositol  Type Significance Phosphatidic acid Key building block of phospholipid synthesis and a major 2ry messenger conveying signaling information. Cardiolipin Major lipid of cardiac mitochondrial membrane. Lethicin Enters in the structure of all biological membranes. Plays a role in digestion and absorption of fats from the intestine. Lung surfactant. (Deficiency in premature newborns leads to respiratory distress syndrome) Cephalin Enters in the formation of all biological membrane. Formation of thromboplastin. Phosphoatidyl inositol acts as a precursor of diacylglycerol and IP3 both are internal signals or second messenger mediating some hormonal actions. Lysophospholipid Important in metabolism and inter-conversion of phospholipids. Plasmalogens They constitute 10% of phospholipids present in brain and muscles. B- Sphingophospholipids Sphingomyelin Ceramide Sphingosine + FA attached by an amide Sphingosine + FA attached to NH2 bond to the NH2 of sphingosine (C 2) + by amide bond. Phosphoric acid & choline attached to the OH of C1 of sphingosine. Present in high concentration in brain Form the skin’s barrier and help and nerve tissue. skin to retain moisture. (B) Glycolipids  They are complex lipids containing carbohydrates.  They also contain the alcohol sphingosine, so, glycolipids together with sphingomyelin can be classified as sphingolipids.  E.g. cerebrosides and gangliosides.  Cerebrosides Gangliosides Sphingosine + FA attached to the Called complex glycolipids, because NH2 of C2 + Galactose (rarely they contain in addition to hexoses glucose) attached to OH of C1. one or more sialic acid molecules. Present in many tissues especially They are present in high brain cerebral hemispheres and concentration in brain. myelin of nerve fibers. They act as insulator of nerve Act as receptors on cell membrane. impulse. Types of cerebrosides acc. to type of FA: 1. Nervon (nervonic acid) 2. Oxynervon (oxynervonic acid) 3. Cerebron (cerebronic acid) 4. Kerasin (Contains lignoceric acid) (C) Sulfolipids  Sphingosine + FA in position 2 attached to NH2 + Galactose attached to C1.  The carbon 3 of galactose is sulfated.  They are present in high concentration in liver, testes and muscles.