Understanding Lipids: Types, Functions, and Classification

Lipids Lipids are a heterogeneous group of compounds including fats, oils, steroids, waxes, and related compounds, that are insoluble in water, but soluble in an organic solvent (e.g., ether, benzene, acetone, chloroform) lipid composed mainly from C, H, O The main feature, in all lipids, is the large number of carbon-hydrogen bonds which makes them non-polar.

Function of Lipids 1. Energy Stores Triglycerides act as energy reserves. The majority of triglycerides are stored in adipose tissue, in adipocytes (fat cells) 2. Sources of energy Fatty acids are released from triglycerides are broken down in the mitochondria and used in the production of energy. 3. Insulation Subcutaneous adipose tissue is important in the maintenance of body temperature 3. Absorption Phospholipids help to emulsify fats and increase absorption of fats and fat-soluble nutrients (e.g. sterols, vitamins) 4. Local hormones Membrane phospholipids can be converted into hormone-like substances called eicosanoids which control smooth muscle contraction, blood clotting and immune cell stimulation

Function of Lipids 5. Structural element of cell membrane Phospholipids 6. Metobolic Functions: Cholesterol is metabolised to: Sex Hormones - reproduction Corticosteroids stress response Mineralcorticoids blood pressure regulation 7. Bile Acids digestive health 8. Vitamin D ( the immune modulater ) with other fat-soluble vitamins and essential fatty acids are contained in the fat of natural foods

Lipids Classification Lipids can be classified into: 1. Simple lipids: Esters of fatty acids with various alcohols. 1. Fats. 2. Waxes. 2. Complex lipids: Esters of fatty acids containing groups in addition to an alcohol and a fatty acid. 1. Phospholipids. 2. Glycolipids (glycosphingolipids). 3. Other complex lipids: 1. Sulfolipids 2. Amino lipids. 3. Lipoproteins. 4. Precursor and derived lipids: These include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, ketone bodies , hydrocarbons, lipid-soluble vitamins, and hormones. Note : Because they are uncharged, acylglycerols (glycerides), cholesterol, and cholesteryl esters are termed neutral lipids.

Classification Scheme Lipids Complex Derived Simple 1. Wax esters 2. Triacylglycerol 1. Fatty acids 2. Sterols 3. Diglycerides 4. monoglycerides Phospholipids Glycolipids 1.Cerebrosides 2.Gangliosides Glycerophospholipids 1.Phosphatidylcholine (PC) 2.Phosphatidylethanolamine (PE) 3.Phosphatidylinositol (PI) Sphingolipids 1.Ceramides 2.Sphingomyelin

Biological Classification Of Lipids Based on there Biological functions Lipids can be classified into: Storage Lipids: The principle stored form of energy example : Triglycerols Structural Lipids: The major structural elements of biological Membranes example: Phospholipids and its derivatives

Fatty Acids Common building block for most lipids Long-chain carboxylic acids Made up of carbon, hydrogen, oxygen Mostly carbon & hydrogen atoms Usually have an even number of carbons Fatty acids occur in the body mainly as esters in natural fats and oils, but are found in the unesterified form as free fatty acids, a transport form in the plasma.

Fatty Acid Characteristics A fatty acid may be characterized as saturated, unsaturated, monounsaturated, polyunsaturated or trans fatty acid. This is determined by its chemical bonds and structure

Cis- and Trans-Fatty Acids Compared

Fatty acids may occur as saturated , monounsaturated or polyunsaturated

Fatty Acid Nomenclature 1. Fatty acids could be named using common names (e.g., oleic, stearic, palmitic acid) 1. Fatty Acids also could be named using the Systematic names are based on IUPAC nomenclature : This system is most frequently used, in which the fatty acid named after the hydrocarbon with the same number and arrangement of carbon atoms, with -oic being substituted for the final -e (Genevan system). Thus, saturated acids end in -anoic, for example, octanoic acid, and unsaturated acids with double bonds end in -enoic, for example, octadecenoic acid (oleic acid).

Fatty Acid IUPAC Nomenclature Using this system: The carbon atoms are numbered, beginning with the carboxyl carbon as carbon 1. The number before the colon indicates the number of carbons in the chain, and those after the colon indicate the numbers and positions (relative to the carboxyl end) of double bonds. For example, arachidonic acid, 20:4(5,8,11,14), is 20 carbons long and has 4 double bonds (between carbons 5 6, 8 9, 11 12, and 14 15). Various conventions use for indicating the number and position of the double bonds; for example, D9 indicates a double bond between carbons 9 and 10 of the fatty acid [Note: Carbon 2, the carbon to which the carboxyl group is attached, is also called the -carbon, carbon 3 is the -carbon, and carbon 4 is the -carbon. The carbon of the terminal methyl group is called the -carbon regardless of the chain length.]

Fatty Acid Nomenclature The double bonds in a fatty acid can also be denoted relative to the (or methyl-terminal) end of the chain. Arachidonic acid is referred to as an -6 fatty acid acid (also an n- 6) because the terminal double bond is six bonds in from the end . Another -6 fatty acid is the essential linoleic acid, 18:2(9,12). In contrast, -linolenic acid, 18:3(9,12,15), is an essential -3 fatty acid. O C 1 O 3 4 2 fatty acid with a cis- 9 double bond

Fatty Acid Nomenclature Example the saturated fatty acid : Palmitic acid (palmitate) CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COO C l6:0 or 16:0 CH3 (CH2)14 COOH hexadecanoic acid (Palmitate) IUPAC Name common Name

Systematic names are based on IUPAC nomenclature CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH2-COOH 12 11 10 9 8 7 6 5 4 3 2 1 or 12:0 CH3 (CH2)10 COOH Dodecanoic acid (Lauric Acid) 14:0 tetradecanoic acid ( myristic acid) 16:0 Hexadecanoic acid ( palmitic acid) 20:0 Eicosanoic acid (arachidic acid) 22:0 Docosanoic acid (behenic acid) 24:0 Tetracosanoic acid (lignoceric acid) Common Name IUPAC Name No double bonds for eg in 18:0, Octadecanoic acid If one double bond then acid Octadecenoic acid If two double bonds then Octadecadienoic acid If three double bonds Octadecatrienoic acid. ( stearic acid)

Nomenclature of Fatty Acids Example for unsaturated fatty acid linonenic acid 3 COOH 1 5 17 15 13 11 9 7 14 4 2 18 16 12 10 8 6 Number of Double Bonds Carbon Chain Length Position of Double Bonds 18:3 cis 9,12,15 -linonenic acid

Fatty Acid Nomenclature Example : the fatty acid Oleic acid could be named as

-6 fatty acid -3 fatty acid

Essential fatty acids Two fatty acids are dietary essentials in humans because of our inability to synthesize them: 1. linoleic acid, which is the precursor of -6 arachidonic acid, the substrate for prostaglandin synthesis 2. -linolenic acid, the precursor of other -3 fatty acids important for growth and development. Plants provide us with the essential fatty acids. [Note: Arachidonic acid becomes essential if linoleic acid is deficient in the diet.]

Storage Lipids Storage Lipids include fats and oils, and wax. 1. Fats and oils: Are Esters of fatty acids with glycerol, composed of 3 fatty acids each in ester linkage with a single glycerol (Triacylglycerols) Oils are fats in the liquid state 2. Waxes: Are esters of long-chain(C14-C36) saturated and unsaturated fatty acids with long chain (C16-C30) monohydric alcohols

Triglycerols Triglycerol (Triglyceride) is an ester of glycerol with three fatty acids. Its also called neutral fat. They are stored in adipocytes in animals A mammal contains 5% to 25% or more of its body weight as lipids, 90% are TAG TAGs are non polar, hydrophobic molecules, essentially insoluble in water

Structure of Triacylglyceride

Lipids as structural elements

1- Phospholipids Phospholipids includes glycerophospholipids & Sphingolipids

A- Glycerophospholipids Glycerophospholipids formed from 2 Fatty Acids and a phosphate group esterified with glycerol O Act as biological detergents and stabilizers O H2C O C R2 R1 C O CH O Mainly present with cell membrane H2C O P O X Each glycerophospholipid includes: O glycerophospholipid Apolar region: Glycerol Carbonyl O of fatty acids, Pi The polar head group (X) polar "kink" due to double bond non-polar A non-polar region: The hydrocarbon tails of fatty acids (R1, R2).

General structure of Glycerophospholipid The type of fatty acid that connects to L-glycerol phosphate 3 Phosphate are specific for different organisms, different tissues of the same organisms, and different glycerophospholipids in the same cells and tissues.

Phospholipids are The Main Lipid Constituents of Membranes Phospholipids may be regarded as derivatives of phosphatidic acid in which the phosphate is esterified with the OH of a suitable alcohol. Phosphatidic acid is important as an intermediate in the synthesis of triacylglycerols phosphoglycerols but is not found in any great quantity in tissues. as well as

Sphingolipids are derivatives of the lipid sphingosine, which has a long hydrocarbon tail, and a polar domain that includes an amino group. Sphingosine may be reversibly phosphorylated to produce the signal molecule sphingosine-1-phosphate. Other derivatives of sphingosine are commonly found as constituents of biological membranesand nerveous Sphingomyelin system e.g. Ceramide & O OH OH OH OH CH3 O H2 C H2 C O O P H C H C N+ H3C O P O H2C CH H2C CH O OH CH3 phosphocholine O OH H C NH CH H3 N+ CH H C H2C CH H2C CH sphingosine N+ H3 CH O C H C H C NH CH H C O C H C R CH2 ( )12 CH2 ( )12 fatty acid CH2 ( )12 R CH2 ( )12 ceramide sphingosine CH3 sphingosine-1-P CH3 Sphingomyelin CH3 CH3

2- Glycolipids Glycolipids are widely distributed in every tissue of the body, particularly in nervous tissue such as brain. The major glycolipids glycosphingolipids e.g. cerebroside & gangliosides found in animal tissues are Cerebroside monosaccharide group. is such as glucose or galactose as polar head a sphingolipid (ceramide) with a Ganglioside is a ceramide with a polar head group that is a complex oligosaccharide, including the acidic sugar derivative sialic acid ( neuraminic acid)

Cerebroside Ganglioside

Cholesterol a Type of Sterol Lipids Cholesterol is an essential component of cell membranes modulating their fluidity, and, in specialized tissues, cholesterol is a precursor of bile acids, steroid hormones, and vitamin D. Cholesterol is a very hydrophobic compound. It consists of four fused hydrocarbon rings (A-D) called the steroid nucleus ), and it has an eight-carbon, branched hydrocarbon chain attached to carbon 17 of the D ring. Ring A has a hydroxyl group at carbon 3, and ring B has a double bond between carbon 5 and carbon 6. Found only in animals

cholesterol PDB 1N83 HO Cholesterol