LIPID CHEMISTRY

LIPID OF BIOLOGICAL IMPORTANCE Definition: Lipids are organic compounds, which have the following common properties: 1- They are esters of fatty acids or substances associated with them in nature. 2- Most of them are insoluble in water but soluble in fat solvents (non polar solvents) e.g.: benzene, chloroform, acetone and ether.

Classification: Lipids are classified into three main groups: Derived Lipids Simple Lipids Compound lipids Esters of fatty acids with various types of alcohols in addition to a prosthetic group They are produced by hydrolysis of the first two groups Esters of fatty acids with various types of alcohols

Esters of three fatty acids with glycerol.= TAG Simple lipids: WAXES NUTERAL FAT Esters of fatty acids with higher molecular weight monohydric alcohols. Esters of three fatty acids with glycerol.= TAG

Complex lipids (Compound or Conjugated lipids) Phospholipids: Lipids containing, in addition to fatty acids and an alcohol, a phosphoric acid residue. Glycolipids (glycosphingoLipids:) containing a fatty acid, sphingosine, and carbohydrate. Proteolipids: fatty acid, alcohol and protein

Fatty Acids Fatty acids that occur in natural fats are usually monocarboxylic acids containing an even number of carbon atoms. The chain may be saturated (containing no double bonds) or unsaturated (containing one or more double bonds).

I- Saturated Fatty Acids (SFA): They contain no double bonds. They are either short chain (from C2 to C10) or long chain (from C12 to C24) All have the following general formula: CH₃- (CH₂)n- COOH Where n = Total number of carbons – 2

I- Saturated Fatty Acids (SFA): A- Short chain fatty acids include: Acetic acid (C2) Butyric acid (C4) B- Long chain fatty acid, the most common include mainly: Palmitic acid (C16) Stearic acid (C18) Arachidic acid (C20)

II- Unsaturated Fatty Acids (USFA): They contain one or more double bonds. Unsaturated fatty acids are classified according to the number of double bonds in their chains into two main groups: 1- Monoethenoid: one double bond: Palmitoleic acid Oleic acid

II- Unsaturated Fatty Acids (USFA): Polyethenoid They have more than one double bond in their structure, termed polyunsaturated fatty acids (PUFA). Linoleic acid Linolenic acid Arachidonic acid

Nutritional classification of fatty acids: a)- Essential Fatty Acids: They are not synthesized in our body, so it is essential to take them in diet. They include Linolenic and Linoleic acids. Deficiency of essential fatty acids produces: Fatty liver and sterility in adults. Impaired growth and dermatitis in infants. Sources of PUFA: They are present mainly in fish and vegetable oils e.g.: maize, cottonseed, linseed, olive, sun flower and soya been oils. b)- Non Essential Fatty Acids: They include all other fatty acids because they are formed in our body in good amounts mainly from carbohydrates. It is not essential to take them in diets.  

Physical Properties of Fatty Acids 1. Solubility in water: fatty acids are insoluble in water but soluble in fat solvents. 2. Physical state at room temperature: The saturated fatty acids are solid at room temperature Unsaturated long chain fatty acids are liquids due to the presence of cis double bonds.

Chemical Reactions of Fatty Acids 1. Ester Formation: 2. Salt formation: 3. Reduction: 4. Rancidity: It is the development of bad odor and taste (bad flavor) of fats and oils. Rancidity is due to exposure of fats or oils to high temperature, oxygen, moisture or humidity, light and metals (act as catalysts).

I- Simple Lipids They are esters of fatty acids with alcohols, according to the types of alcohols there are two main sub-groups: 1. Neutral fats or Triacylglycerol (TAG): They are esters of three fatty acids with glycerol. 2. Waxes: They are esters of one fatty acid with long chain monohydroxyalcohol higher than glycerol.  

I- Simple Lipids

II- Conjugated Lipids (Compound Lipids) They contain fatty acids, alcohols and other (prosthetic) groups. According to the type of prosthetic group they are classified into: 1- Phospholipids: Containing phosphate radicals. 2- Glycolipids: Containing carbohydrate radicals.

A-Glycerophospholipids HO – CH2 – CH – COOH NH2 Serine CO2 HO – CH2 – CH2 – NH2 Ethanolamine 3 CH3 HO – CH2 – CH2 – N (CH3)3 Choline OH CH2 – O – CH = CH – R1 R2 – C – O – C – H CH2 – O – P – O – R Plasmalogens OH O Inositol 3 4 6 5 1 2 OH H HO CH2 – O – C – R1 CH2 – O – P – OH Phosphatidic acid OH A-Glycerophospholipids

Importance and functions of phospholipids: 1- Amphipathic molecules that contain non-polar groups (fatty acid side chains) and polar groups (phosphate, serine, ethanolamine, glycerol, choline and inositol) they form micelles in water. 2- They are good emulsifying factors, important for digestion and absorption of dietary fats.

Importance and functions of phospholipids: 3-They are good hydrotropic substances, they prevent deposition of cholesterol as cholesterol stones (biliary calculi). 4- They are important constituents of lipid bilayer in cell membranes. 5- They are important constituents of plasma lipoproteins.

Importance and functions of phospholipids: 6- Lung surfactant is formed mainly of dipalmitoyl-lecithin, the lack of which is responsible for respiratory distress syndrome in premature infants. 7- They provide arachidonic acid for synthesis of eicosanoids. 8- They are essential for blood clotting, as they provide the platelet activating factor (PAF)

B- Sphingomyelin: This type is present in cell membranes specially of the lungs and brain mainly in the myelin sheath. It contains sphingosine (sphingol) which is 18 carbon amino alcohol fatty acids are linked to sphingosine by an amide bond to form ceramide, which is connected to phoshocholine to form sphingomyelin.

Sphingosine containing lipids Phosphocholine FA Sphingomyline Ceramide Mixture of carbohydrates Glucose or Galactose CEREBROCIDES GANGLIOSIDES Sulphates SULPHOLIPIDS

Importance of glycolipids: They are found mainly in the myelin sheath and cell membrane of RBCs. They act as cell membrane receptors for hormones and external stimuli also they provide recognition properties.

III- Derived Lipids They are produced by hydrolysis of either simple or conjugated lipids. They include the following : 1. Fatty acids. 2. Alcohols. 3. Steroids. 4. Carotenoids. 5. Fat soluble vitamins: as vitamins A, D, E & K.  

Steroids

Classification of steroids Steroid hormones. Sterols. Bile acids.  

Cholesterol It is the most important animal sterol. It is present either free (non esterified) or esterified with fatty acid to form cholesteryl ester. Free cholesterol contains 27 carbon atoms. Distribution of cholesterol: It is widely distributed in all tissues but higher concentrations are present in brain, nervous tissue, liver, adrenals, gonads, skin and adipose tissue.

Cholesterol Precursor of cholesterol: It is formed from active acetate (acetyl-CoA). Blood level of cholesterol: Normally it is present in the plasma in concentrations ranging from 100 to 200 mg/dL (30% as free cholesterol and 70% as cholesteryl esters). .

Cholesterol Cholesteryl esters

Importance and derivatives of cholesterol: It is important constituent of cell membranes. It is converted into bile acids and bile salts in the liver. It is the precursor of all steroid hormones. It can be oxidized in the liver into 7-dehydro cholesterol which can be converted under the skin into vitamin D3 by ultra violet

Bile Acids 1. Primary bile acids: they are formed in the liver from cholesterol and they include: - Cholic acid - Chenodeoxy cholic acid 2- Secondary bile acids: they are formed of primary bile acid in large intestine. - Deoxycholic acid - Lithocholic acid

Bile salts: They are formed by conjugation of cholic acid with glycine (80%) or taurine (20%) then they are excreted by liver in bile as sodium glycocholate or sodium taurocholate. Bile salts pass to the intestine where they are reabsorbed and return back to the liver to be excreted again in bile (entero-hepatic circulation).  

Importance of bile salts: Conversion of cholesterol to bile salt is an important mechanism for removal of excess cholesterol from blood. They are good emulsifying factors important for digestion and absorption of fats. They prevent precipitation of cholesterol in the bile as cholesterol stones. They stimulate liver cells to secrete more bile (choleretic effect).  

Classification of Steroid Hormones Sex Hormones Corticoids (C21) Male sex hormones (C19) Female sex hormones Estrogen (C18) Progesterone (C21) Glucocorticoids Mineralocorticoids

Testosterone