Lipids (structure & function)

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ABID ALIBS-HONS (Health & Medicine)Khyber Medical University Peshawar

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INTRODUCTION:

The word lipid is derived from a greek word “lipos” which means FAT.

These are heterogeneous compounds (solid, liquid, or gaseous).

Lipids are organic compound formed mainly from alcohol (ETHANOL- C2H5OH) and fatty acids combined together by ester linkage.

ESTER→ is an organic compound where the hydrogen in the compound’s carboxyl group is replaced with a hydrogen group. E.g Ethyl acetate is an ester. The hydrogen on the carboxyl group (oxygen atom & hydrogen atom) of acetic acid is replaced with an Ethyl group (RCOOR)

Lipids are insoluble in water, but soluble in fat or organic solvents such as- Chloroform (general anesthetic )- Benzene (Benzene works by causing cells not to work correctly)- Acetone (used as an organic solvent and synthetic reagent)- Alcohol (organic compound formed when a hydroxyl group –OH is changed for a

hydrogen atom in a hydrocarbon). Lipids include fats, oils, waxes, and related compounds. They are widely distributed in nature both in plants and animals. Their primary building blocks are fatty acids, glycerol, sphingosine ( important in the

metabolism of nerve cells) and sterol (unsaturated steroid alcohols). In most cases they can be utilized by the living organism.

BIOLOGICAL IMPORTANCE OF LIPIDS

They are more palatable (acceptable) and storable to unlimited amount compared to carbohydrates.

They have high-energy value (25% of body needs) and they provide more energy per gram than carbohydrates and proteins but carbohydrates are the preferable (more desirable) source of energy.

Supply the essential fatty acid that cannot be synthesized by the body.ABID ALIBS-HONS (Health & Medicine)Khyber Medical University Peshawar

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Supply the body with fat soluble vitamins (A,D,E,K) They are important constituents of the nerves system. Tissue fat is an essential constituent of cell membrane and nervous system. It is mainly

phospholipids in nature that are not affected by starvation. Structural components of biological membranes (lipoprotein, phospholipids,

sphingomyelins). Cholesterol: enters in the membrane structure and is used for synthesis of adrenal

cortical hormones, vit-D3 and bile acids. Lipids provide bases for dealing with disease such as obesity, atherosclerosis (a disease of

the arteries characterized by the deposition of fatty material on their inner walls) , lipid storage disease, essential fatty acid deficiency, respiratory distress syndrome.

CLASSIFICATION OF LIPIDS

There is no single internationally accepted system of classification of lipids. Lipids can be classified into three classes or groups.

1. Simple lipids - fats (Triglycerides)- waxes

2. Complex or compound lipids- Glycolipids- Gangliosides- Phospholipids- Lipoproteins

3. Derived, precursor and associated lipids- Fatty acid- Steroids and sterols

SIMPLE LIPIDS : These consist of the following subgroups.

(I) Fats (Triglycerides) (triacylglycerol)

Triglycerides are a combination of three 3 fatty acid molecules with a glycerol molecule.

The names neutral fat, triglyceride, and fat are used synonymously; the term triacylglycerol is being used now-a-days instead of triglyceride. However due to its common usage in medical literature the term triglyceride rather than the more scientific term triacylglycerol will also be used.

They are called neutral because they are uncharged due to absence of ionizable group in it.


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They are ester of glycerol with various fatty acids. Since the 3 hydroxyl groups of glycerol are esteirfied.

Esterification of glycerol with one molecule of fatty acid gives monoglyceride, and that with 2 molecules gives diglyceride.

Wide spread abundant in nuts seeds and fat depots of animals

Represent storage form of lipids

In mammals the fats are stored in adipose tissue.

Adipose tissue also functions to protect organ from shock and cold.

Glycerol: This is also called glycerin, is an alcohol with three hydroxyl groups. Since there are three hydroxyl groups, three fatty acids can react to form three esters.


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In hydrolysis triacylglycerol split into glycerol and 3 fatty acids. An acid or enzyme catalyst is required.

Waxes

These are esters of fatty acids with alcohols other than glycerol. Waxes are very hydrophobic (water soluble) and are used by plants and animals for protective, water-proof coatings.

For example: Cholesteroyl stearate: it type of wax found in human plasma.

(II) COMPOUND OR COMPLEX LIPIDS

These are ester of fatty acids containing groups in addition to an alcohol and a fatty acid. These are subdivided as follows:

a) Glycolipids (also called Glycosphingolipids)- These contain sphingosine, a fatty acid and a monosaccharide or an oligosaccharide

unit.- They are lipids that 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.

b) Gangliosides- These are very complex molecules. Their hydrolytic products are sphingosine, fatty

acids, sugar unite, N-acythylhexsamine and N-acetylneuaminic acid; the last is one of the sialic acids.

- They are more complex Glycolipids that occur in the gray matter of the brain, ganglion cells, and RBCs. They transfer biogenic amines across the cell membrane and act as a cell membrane receptor.

- Gangliosides contain sialic acid (N-acetylneuraminic acid) , ceramide (sphingosine+ fatty acid carbon atom length), 3 molecules of hexoses (1 glocuse+2 galactose) and hexosamine.


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The most simple type of it the monosialoganglioside,. It works as a receptor for cholera toxin in the human intestine.

c) Phospholipids- Phospholipids or phosphatides are compound lipids, which contain phosphoric acid

group in their structure.

Importance

- They are present in large amounts in liver and brain as well as blood. Every animal and plant cell contains phospholipids.

- The membranes bounding cells and sub cellular organelles are composed mainly of phospholipids, thus the transfer of substances through these membranes is controlled by properties of phospholipids.

- They are important components of the lipoprotein coat essential for secretion and transporting of plasma lipoprotein complexes. Thus, they are lipotropic agent that prevent fatty liver.

- Myelin sheath of nerves is rich with phospholipids.- Important in digestion and absorption of neutral lipids and excretion of cholesterol in the bile.- Important function in blood clotting and platelet aggregation.- Important role in signal transduction across the cell membrane.- They are source of polyunsaturated fatty acids for synthesis eicosanoids.( Eicosanoids are signaling

molecules made by oxidation of 20-carbon fatty acids)

Sources: They are found in all cells (plant and animal), milk and egg-yolk in the form of lecithins.

Structure:

i. fatty acids (a saturated and a unsaturated fatty acid)ii. nitrogenous base (choline, serine, threonine, or ehanolamine)

iii. phosphoric acidiv. fatty alcohols (glycerol, inositol, or sphingosine)


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d) Lipoprotein - Lipid attached with protein- Which allow fats to move through the water inside and outside cell.

- Lipoproteins are lipids combined with proteins in the tissues. The lipid component is phospholipid, cholesterol or triglycerides. The holding bonds are secondary bonds.

They include:

Structural lipoproteins: These are widely distributed in tissues being present in cellular and sub-cellular membranes. In lung tissues acting as a surfactant in a complex of a protein and lecithin. In the eye, rhodospin of rods is a lipoprotein complex.

Transport lipoproteins: These are the forms present in blood plasma. The are composed of a protein called apolipoprotein and different types of lipids, (cholesterol, cholesterol esters, phospholipids and triglycerides). As the lipid content increases, the density of plasma lipoprotein decreases.

Types of lipoproteins:

1. Chylomicrons: They have the largest diameter and the least density. They contain 1-2% protein only and 98-99% fat. The main lipid fraction is triglycerides absorbed from the intestine and they contain small amounts of the absorbed cholesterol and phospholipids.

2. Very low-density lipoproteins (VLDL): Their diameter is smaller than chylomicrons. They contain 7-10% protein and 90-93% lipid. The lipid content is mainly triglycerides formed in the liver. They contain phospholipids and cholesterol more than chylomicrons.

3. Low-dnsity lipoproteins (LDL): They contain 10-20% proteins in the form of apolipoprotein B. Their lipid content varies form 80-90% they contain about 60 % of total blood cholesterol and 40% of total blood phospholipids. As their percentage increases, the liability to atherosclerosis (a disease of the

arteries characterized by the deposition of fatty material on their inner walls) increases.4. High-density lipoproteins (HDL):

They contain 35-55% protein in the form of apolipoprotein A. they contain 45-65% lipids formed of cholesterol (40%of total blood content) and phospholipids (60% of total blood content). They act as cholesterol scavengers, as their percentage increases, the liability to atherosclerosis decrease. They are higher in females than males. Due to their high protein content they possess the highest density.


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Derived, precursor and associated lipids

These are the hydrolytic products of the above-mentioned compound and include diacylglycerols (glycerol+2fatty acid), fatty acids, alcohols, sterols, vitamins D, E, K and carotenoids (color to plant parts).

Ecosanoid compounds including (prostaglandin, prostacyclins, thromboxanes, leukotrienes and lipoxins) and terpenes also belong to this class of lipids.

FATTY ACIDS

Are long-chain carboxylic acids. (an organic acid containing a carboxyl group. COOH)

Typically contain 12-20 carbon atoms. Are insoluble in water. Can be saturated or unsaturated Fatty acids contain a carboxylic acid group This should make them quite polar However, they also contain a long hydrocarbon tail Which overall, makes them nonpolar.

CLASSIFICATIO OF FATTY ACIDS

1. Saturated fatty acids (single bonds)2. Unsaturated fatty acids (one double bonds)3. Polyunsaturated fatty acids (more double bonds)

SATURATED FATTY ACIDS

They contain no double bonds with 2-24 or more carbon. They are solid in room temperature except if they are short chained. Strong attractions between fatty acid chains. They have the following molecular formula (CnH2nO2) A short chain saturated fatty acid (2-10 carbon) Long chain fatty acid (more than 10 carbon)

Important saturated fatty acids with their formulas and symbols.ABID ALIBS-HONS (Health & Medicine)Khyber Medical University Peshawar

nonpolar polar

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Common name No. of carbon atoms formula symbolsAcetic acid 2 CH3COOH 2:0Butyric acid 4 C3H3OOH 4:0Caproic acid 6 C5H11COOH 6:0Palmitic acid 16 C15H31COOH 16:0Stearic acid 18 C17H35COOH 18:0

Arachidic acid 20 C19H39COOH 20:0Lingoceric acid 24 C23H17\COOH 24:0

UNSATURATED FATTY ACIDS

They contain double bond. Do not pack closely. Have few attractions between chains. Have low melting (At the melting point the solid and liquid phase exist in equilibrium) points. Are liquid at room temperature.

Sub types of unsaturated fatty acids

I. Monounsaturated fatty acid- Those having only one double bond are called monounsaturated or (monoenoic) fatty

acids.- Oleic acid and palmitoleic acid are the most abundant.- Monounsaturated i.e. monoenoic fatty acids in man.II. Polyunsaturated fatty acids

- Fatty acids with more than one double bond.- Polyenoic fatty acid with 2,3 and 4 double bonds are called dienoic, trienoic, and

tetraenoic fatty acids respectively.

Important unsaturated fatty acids with their formulas and number of bondsABID ALIBS-HONS (Health & Medicine)Khyber Medical University Peshawar

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Name of acid No. of carbons No. of double bond formulapalmitoleic 16 1 C15H29COOH

Oleic 18 1 C17H33COOHLinoleic 18 2 C17H31COOHLinolenic 18 3 C17H29COOH

arachidonic 20 4 C19H31COOH

Sources:

Vegetable oils, corn oil, linseed oil, peanut oil, olive oil, cottonseed oil, soybean oil and many other plant oils, cod liver oil and animal fats.

Deficiency:

Their deficiency in the diet leads to nutrition deficiency disease.

→ Its symptoms include:- Poor growth and health with susceptibility to infections- Dermatitis- Decreased capacity to reproduce- Impaired transport of lipids- Fatty liver- And lowered resistance to stress

Function of Essential fatty acid:- They are useful in the treatment of artherosclerosis by help transporting blood

cholestrol and lowering and transporting triglycerides.- The hormones are synthesized from them.- They enter in structure of all cellular and subcellular membranes and the trasporting

plasma phospholipids.- They are essential for skin integrity, normal growth and reproduction.- They have an important role in blood clotting (intrinsic factor)- Important in preventing and treating fatty liver.- Important role in health of the retina vision.- They can be oxidized for energy production.


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Signs and symptoms of Essential fatty acids Defeciency

Falky, itchy skin Diarrhea Infections Retarded growth and wound healing Anemia

Chemical properties of fats and oils

1. Hydrolysis:- They are hydrolyzed into their constituents (fatty acid and glycerol) by the action of

super heated steam, acid, alkali or enzyme (e.g.→ lipase of pancrease)- During their enzymatic and acid hydrolysis glyceto and free fatty acid are produce.

2. Saponipication: - Alkalin hydrolysis produces glycero and salts of fatty acids(soaps).- Soaps cause emulsification of oily meterial this helps easy washing of the fatty acie

materials.

3. Halogenation:


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- Neutral fats containing unsaturated fatty acids have the ability of adding halogens (e.g.→hydrogen or hydrogenation and iodine or iodination) at the double bonds.

Steroids

- Steroids are a type of lipid that is not derived from a fatty acid.- Non saponifiable (to convert a fat or oil into soap) form of lipids - steroid consist of 3 six membered and one 5 membered rings. Total 17 carbon atoms.- This structure is called perhydrocyclopentanophenanthrene

Sterol (a sub group of steroids)

- Sterol contain one or more hydroxyl groups.- Some of the natural compounds belonging to steroids are cholesterol, ergosterol, bile

acids, male and female sex hormones and the hormones of the adrenal cortex.- Cholesterol is the steroid that used as the starting point for the synthesis of other

steroids.- Cholesterol is only found in animals- Is the most abundant steroid in the body.- Has methyl CH3- groups, alkyl chain, and -OH attached to the steroid nucleus.


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Cholesterol in the Body

- Is obtained from meats, milk, and eggs. - Is synthesized in the liver.- Is needed for cell membranes, brain and nerve tissue, steroid hormones, and Vit-D.- Clogs arteries when high levels form plaque.

Steroid Hormones

- Are chemical messengers in cells.- Are produced from cholesterol. - Include sex hormones such as androgens (testosterone) in males and estrogens

(estradiol) in females.

Caloric values of fats, proteins and carbohydrates


Fat: 1 gram = 9 calories Protein: 1 gram = 4 calories Carbohydrates: 1 gram = 4 calories

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LIPID BLOOD TESTS

→Total Cholesterol (TC) ( Directly linked to risk of heart and blood vessel disease. )

Goal values: 75-169 mg/dL for those age 20 and younger 100-199 mg/dL for those over age 21

Preparation:This test may be measured any time of the day without fasting. However, if the test is drawn as part of a total lipid profile, it requires a 12-hour fast (no food or drink, except water). For the most accurate results, wait at least two months after a heart attack, surgery, infection, injury or pregnancy to check cholesterol levels.

→ High Density Lipoprotein (HDL) “Good cholesterol”High levels linked to a reduced risk of heart and blood vessel disease. The higher your HDL level, that better.

Goal value: Greater than 40 mg/dL

Preparation:This test may be measured any time of the day without fasting. However, if the test is drawn as part of a total lipid profile, it requires a 12-hour fast (no food or drink, except water). For the most accurate results, wait at least two months after a heart attack, surgery, infection, injury or pregnancy to check HDL levels

→ Low Density Lipoprotein (LDL) “Bad cholesterol”

Goal values: Less than 70 mg/dL for those with heart or blood vessel disease and for other patients at very

high risk of heart disease (those with metabolic syndrome) Less than 100 mg/dL for high risk patients (e.g., some patients who have multiple heart disease

risk factors) Less than 130 mg/dL for individuals who are at low risk for coronary artery disease


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Preparation:Blood should be collected after a 12-hour fast (no food or drink, except water). For the most accurate results, wait at least two months after a heart attack, surgery, infection, injury or pregnancy to check LDL levels.

→ Triglycerides (TG)Elevated in obese or diabetic patients. Level increases from eating simple sugars or drinking alcohol. Associated with heart and blood vessel disease.

Goal value: Less than 150 mg/dl

Preparation:Blood should be collected after a 12-hour fast (no food or drink, except water). For the most accurate results, wait at least two months after a heart attack, surgery, infection, injury or pregnancy to check triglyceride levels.

Which patients should undergo screening for lipid profile?

Screening tests for lipid profile are recommended for adults of 20 years or more. However, the age for starting screening for lipids varies with different recommendations. People less than 20 years of age may be screened if they suffer from the following risk factors for atherosclerosis:-

Diabetes High blood pressure Obesity Premature heart disease or very high cholesterol level in immediate family. In case of smoker
