Lipid Chemistry PDF

Biochemistry of dietary, functional and structural lipids Dr/ Marwa Abdel Naeem Gaber Assistant prof. of Medical Biochemistry Assiut university  Objectives Classification & Biological importance of fat Discuss structure and function of fatty...

Biochemistry of dietary, functional and structural lipids Dr/ Marwa Abdel Naeem Gaber Assistant prof. of Medical Biochemistry Assiut university  Objectives Classification & Biological importance of fat Discuss structure and function of fatty acids, fatty alcohols, and their derivatives Discuss Structure and function of phospholipids, glycolipids with special emphasis on cell membrane  Definition of lipids Lipids (or fats) are organic compounds formed mainly from alcohol and fatty acids combined together by ester linkage. Lipids include fats, oils, waxes and related compounds.  Fatty Acids Def in ition: Fatty acids are aliphatic mono-carboxylic acids that are mostly obtained from the hydrolysis of natural fats and oils. They have the general formula R-(CH ) -COOH and 2 n mostly have straight chain (with few exceptions). and "n" is mostly an even number of carbon atoms (2-34) with a few exceptions. Fatty acids can be classified as follows:


I) Saturated, i.e., they contain no double bonds. They are solid at room temperature except if they are short chained. The structure formulae of some common saturated FA: Acetic (2C) : CH3-COOH Propionic (3C): CH3-CH2-COOH Palmitic acid (16C): CH3-(CH2)14-COOH Stearic acid (18C): CH3-(CH2)16-COOH  II) Unsaturated, i.e., they contain one (monounsaturated) or more double bonds (polyunsaturated). They are liquid in RT. Unsaturated FA are more chemical reactive than saturated FA. Polyunsaturated fatty acids (Essential fatty acids polyenoic or polyethenoid):
Def in ition: They are essential fatty acids that can not be synthesized in the human body (due to lack of enzymes that can form m ore than one double bond) and m ust be taken in adequate amounts in the diet. They are required for normal growth and metabolism. They are liquids at room temperature.  Function 1. They are useful in the treatment of atherosclerosis by help the transport of blood cholesterol and lowering it. 2. The hormone-like eicosanoids are synthesized from them. 3. They participate in the structure of all cellular and subcellular membranes. 4. T he y a re e sse nt i a l f o r sk i n i nt e g ri t y , no rm a l g ro w t h a nd reproduction. 5. They have an important role in blood clotting. 6. Important in preventing and treating fatty liver. 7. They have an important role in the health of the retina and vision. 8. They can be oxidized for energy production.  Deficiency: n the diet leads to nutritional deficiency disease. poor growth and health with susceptibility to infections, dermatitis, decreased capacity to reproduce, impaired transport of lipids, fatty liver, lowered resistance to stress. Source: vegetable oils such as corn oil, linseed oil, peanut oil, olive oil, cottonseed oil, soybean oil and many other plant oils, cod liver oil and animal fats. Examples: Include: Linoleic: C18:2. 9, 12 CH -(CH ) -CH=CH-CH -CH=CH-(CH ) -COOH 3 2 4 2 2 7 -Linolenic acid: C18:3 , in corn, linseed, peanut, olive, cottonseed and 9, 12, 15 soybean oils. CH3-CH2-CH=CH-CH2-CH=CH-CH2-CH=CH-(CH2)7-COOH Arachidonic acid: C20:4. It is an important component of phospholipids 5, 8, 11, 14 in animal and in peanut oil. CH3-(CH2)4-CH=CH-CH2-CH=CH-CH2-CH=CH-CH2-CH=CH-(CH)3-COOH  Fatty Acids Saturated Unsaturated Short chain Long chain Monounsaturated Polyunsaturated 2-10 c More than 10 c Linoleic Linoleni c acid Volatile Non-volatile Arachidonic acid  Alcohols Alcohols associated with lipids include: glycerol, cholesterol sphingosine and higher alcohols (e.g., cetyl alcohol, mericyl alcohol…etc.) Glycerol: It is a colorless viscous fluid and it has a sweetish taste. It is a trihydric alcohol (contains three OH groups). On esterification with one molecule of fatty acid, it gives monoacylglycerol, with two molecules of fatty acids, it gives diacylglycerol and with three, it gives triacylglycerol (TAG). It is the alcohol present in neutral fat. 
Sphingosine It is the alcohol present in sphingolipids and glycolipid. Cholesterol It is the most important sterol in animal tissues, it is present as free alcohol or in an esterified form (with fatty acids). It is the precursor of steroid hormones, bile salts and vitamin D. It is the alcohol present in wax like fats. Other alcohols: alcohol with higher molecular weight than glycerol Examples are cetyl and mericyl alcohol that enter in structure of waxes) Classification of lipids  I- Simple lipids:
They are esters of fatty acids with alcohol. They are called neutral lipids (i.e., uncharged). They are classif ied according to the alcohol present into: A. Neutral fats: are esters of fatty acids with glycerol (triacylglycerols, or triglycerides or TAG). Fats are saturated fatty acids with glycerol. They are solid in nature. Oils are unsaturated fatty acids with glycerol. They are liquid in nature.  B. Waxes: Esters of fatty acids with long-chain alcohols other than glycerol that is mostly monohydric. Cholesterol esters with any fatty acid are waxes. Ester bond Long-chain alcohol Fatty acid  Rancidity
Definition: It is a physico-chemical change in the properties of neutral fat leading to development of unpleasant odor or taste or abnormal color. Types: 1- Hydrolytic rancidity: It results from slight hydrolysis of the fat by lipase from bacterial contamination at high temperature and moisture leading to the liberation of glycerol and volatile short-chain fatty acids that have bad odor. 2- Oxidative Rancidity: It is due to oxidation of fat by exposure to oxygen, light and/or heat leading to the production of peroxides, aldehydes, ketones and that are toxic and have bad odor. 3- Ketonic Rancidity: It is due to fungal contamination as with Asperigillus Niger at high moisture.  Hazards of Rancid Fats: 1. The products of rancidity are toxic, i.e., causes food poisoning and cancer. 2. Rancidity destroys the fat-soluble vitamins (vitamins A, D, K and E). 3. Rancidity destroys the polyunsaturated essential fatty acids. 4. Rancidity causes economical loss because rancid fat is inedible. Prevention of rancidity is achieved by: 1. Avoidance of the causes (exposure to light, oxygen, moisture, high temperature and bacteria or fungal contamination) by keeping fats or oils in well-closed containers in cold, dark and dry place (i.e., good storage conditions). 2. Addition of anti-oxidants. The most common natural antioxidant is vitamin E that is important both in vitro and in vivo.  II- Compound or conjugated lipids:
They are esters of fatty acids and alcohols in addition to other groups. They include the following types: Phospholipids (phosphatides): Compound lipids containing fatty acids, alcohol, phosphoric acid and often a nitrogenous base. They are further classified according to the alcohol moiety into: (a) Glycerophospholipids, where the alcohol is glycerol. (b) Sphingophospholipids, where the alcohol is sphingosine.  Glycolipids: Compound lipids containing sphingosine, fatty acids, and carbohydrate, but not phosphoric acid nor glycerol. Lip op roteins: Com p ound lip ids c om p osed of a lip id p art associating by secondary bonds with proteins as plasma and membranous lipoproteins. Other compound lipids: include: Sulfolipids: containing sulfur. Aminolipids: containing amino acids.  a) Phospholipids Structure: Phospholipids are compound lipids, which contain alcohol, fatty acids, nitrogenous base and phosphoric acid group. There are two classes of phospholipids according to the type of the alcohol present.  Glycerophospholipids (contain glycerol as alcohol) - Phosphatidate- Lecithins - Cephalins - Cardiolipin - Inositides - Plasmalogens  Sphingophospholipids (contain sphingosine as alcohol) - Sphingomyelins  Functions: 1. form the structural components of cell membranes and regulate membrane permeability. 2.The subcellular organelles (endoplasmic reticulum, Golgi apparatus, mitochondria, nucleus, and lysosomes) are composed mainly of phospholipids. 3.They are important components of the lipoprotein coat help its transport and prevent fatty liver. 4.Myelin sheath of nerves is rich with phospholipids. 5.They have important functions in blood clottingand platelet aggregation (cephalin). 6.Dipalmitoyl Lecithin (major component of lung surfactant) Deficiency of lung surfactant in a newly born baby (preterm) will lead to collapse of alveoli causing Respiratory distress syndrome, that may cause death of the baby. 7. They have an important role in signal transduction across the cell membrane (second messenger) (Phosphatidyl Inositol). 8. They are source of polyunsaturated fatty acids for synthesis of eicosanoids  b) Glycolipids structure: They contain carbohydrate residues with sphingosine as the alcohol and a very long-chain fatty acid (24 carbon series). They are present in cerebral tissue, therefore are called cerebrosides. They are also referred as sphingogalactolipids or galactosides  Classif ication: According to the number and nature of the carbohydrate residue(s) present in the glycolipids, the following types exist: Cerebrosides. They have one galactose molecule (galactoside) or it may have one glucose (glucoside) or one lactose (lactoside) + ceramide. Sulfatides. They are cerebrosides with sulfate on the sugar (sulfated cerebrosides) + ceramide Gangliosides. They have several sugar and aminosuger residues + ceramide + sialic acid.  Functions: Are an essential part of cell membranes Glycolipids determine the blood group of the individuals. Gangliosides transfer biogenic amines across the cell membrane and act as a cell membrane receptor for cholera and tetanus toxins. This helps immune system to destroy and clear pathogen from body.  C- Lipoproteins
Definition: spherical macromolecular complexes of lipids and specific protein called apo-protein functions of lipoproteins: 1- They keep water insoluble lipids in solution of aqueous plasma. 2- Provide mechanism for transporting lipid components to and from the tissue. Name the classes of lipoproteins? 1- Chylomicrons 2- Very low density lipoproteins (VLDL) 3- Low density lipoprotein (LDL or -lipoprotein) 4-High density lipoprotein (HDL or - Lipoprotein) 5-Free Fatty Acid – Albumin complex a) Chylomicrons: They contain 1-2% protein only and 98-99% fat. Function: They carry dietary triglyceride from the intestine to the peripheral tissues (adipose tissues where it can be stored as fat or to muscles where the constituent fatty acids can be used for energy. b) Very low-density lipoproteins (VLDL) or pre--lipoproteins: They contain about 7-10% protein and 90-93% lipid. Function: Transport endogenously synthesised TAG to the extra hepatic tissues where it can be stored as fat or to muscles where the constituent fatty acids can be used for energy.. c) Low-density lipoproteins (LDL) or -lipoproteins: They contain 10-20% proteins in the form of apolipoprotein B.
Their lipid content varies from 80-90%. They contain about 60% of total blood cholesterol Function: It carries cholesterol to various tissues. A s t he i r p e rc e nt a g e i nc re a se s i n t he b l o o d , t he l i a b i l i t y to atherosclerosis increases. d) High-density lipoproteins (HDL) or -Lipoproteins: They contain 40% proteins in the form of apolipoprotein A. and 60% lipids They contain about (40% of total blood content of cholesterol) They act as cholesterol scavengers; they deliver cholesterol from peripheral tissue to the liver for elimination. As their percentage increases, the liability to atherosclerosis decreases e) Albumin-free fatty acids complex: It is a proteolipid carried on plasma albumin with 99% protein content associated with long-chain free fatty acids (1%) for transporting them.  III- Derived Lipids:
They are products of hydrolysis of simple and compound lipids and/or their derivatives that still possess the general characteristics of lipids. They include: Fatty acids, monoglycerides and aldehydes. Alcohols including glycerol. steroids and hormonal derivatives of vitamin D. Eicosanoids (prostaglandins, leukotrienes and thromboxanes). Ketone bodies.  Biological Importance of Lipids They provide more energy/ gram than CHO and proteins (25% of body needs). Supply the essential fatty acids that cannot be synthesized by the body. Supply the body with fat-soluble vitamins (A, D, E and K). They are important constituents of the nervous system. Cholesterol enters in membrane structure and is used for synthesis of adrenal cortical hormones, vitamin D3 and bile acids.  Tissue fat “constant fat” is an essential constituent of cell membrane and nervous system. It is mainly phospholipids in nature that are not affected by starvation.  Stored lipids “depot fat” are stored mainly in adipocytes. It is mainly triglycerides in nature and acts as: - A store of energy. - A pad for the internal organs to protect them from outside shocks. - A subcutaneous thermal insulator against loss of body heat. Thank You