lipids

Lipids consist of two parts1. Glycerol 2. Fatty Acids

a. Saturated fatty acids b. Unsaturated fatty acids .Monounsaturated . Polyunsaturated

Figure 3 : Formation of TG

Table 1 : Saturated fatty acids

Table 2 : Unsaturated fatty acids

Lipid metabolism

• Almost all the fats of the diet except short chain fatty acids are absorbed from the intestine in to the lymph.

• During digestion most triglycerides are split in to monoglycerides & fatty acids.

• Once they reached the intestinal tract they are re synthesized as triglycicleserides.

• From the intestinal tract they are transported in to the blood in the form of chylomicrons.

• Lipo protein lipase is enzyme present in the blood which hydrolysis triglycerides & phospholipids in to fatty acid & Glycerol for its storage in adipose & liver cells.

• Lipo proteins• They are small particles much smaller than

chylomicrons but similar in composition (TAG,cholesterol,phospholipids,&prtn)

• 1/3rd or 1/4th protein & remaining lipids

Major classes of Lipo Protein• Very low density lipo protein• Contains high conc of triglycerides & moderate

conc of cholesterol & TAG.• Intermediate density lipoprotein• Large share of TAG removed so Conc of

cholesterol & phospholipids ↑ses.• Low density lipoprtn• All the triglycerides are removed so high conc

of cholesterol & moderate amount of phopholipids are seen.

• High density lipoprtn• Which contain a high concentration of

protein but smaller conc of cholesterol & Phospo lipids seen.

Phospholipids

• Phospholipids – contains one or more fatty acid molecules & one phosphoric acid radical they usually they contain a nitrogenous base.

• The major types of phospholipids are lecithin, cephalins & sphingo myelins.

Lipids are classified as simple or complex

1. Simple lipid a. Fatty acidsb. neutral fats (monoglyceride, diglycerde, and triglyceride)c. waxes

2. Complex lipidsa. Phospholipidsb. Glycolipidsc. Lipoproteins

3. Derived lipids

Figure 1 : Triglyceride

Figure 2 : Phosphatidic acid

Omega Fatty acid

Cis- Trans Fatty acid

Function of Fat

. Energy

. Essential nutrient

. Flavor and Satisfaction

. Adipose Tissue . Cell membrane Structure

Fat in Food

Fat in Body

Food Sources of Fat

. Meat Fat (bacon, sausage….)

. Dairy Fats and products (cream, butter, cheese..)

. Egg yolk

. Monounsaturated , polyunsaturated Fatty acid . Vegetable oil (safflower, corn, soybean,

cottonseed, olive oil)

Animal Fats

Plant Fat

Characteristics Of Food Fat Sources

. Butter, margarin, salad oils and dressing,shirteninig fat meat

. Chees, cream portion of homogenised milk, egg yolk, nuts, seeds, olives…..

Visible Fat

Invisible Fat

Digestion of lipids

1. In mouthmechanical digestion Lingual Lipase

2. In stomach mechanical digestion

Gastric lipase

3. In small intestine Bile acid, Pancreatic Lipase, Cholesterol esterase, Phospholipase

Triglycerides

2 fatty acids + Mono glycerides3 fatty acids + glycerolTriglycerid

es

Triglycerides

1 fatty acids + Diglycerides

Absorption of lipids

Figure 4 : Absorption of fat.

Transportation of lipids

Source Destination Major lipids Functions

Chylomicrons

Intestine Many organs Triglycerides, other lipids

Deliver lipids of dietary origin to

body cell. VLDLs Liver Many organs Triglycerides,

Cholesterol Deliver

endogenously produced

triglycerides to body cells.

LDLs Intraviscular removal of triglycerides from VLDL

Blood vessels, Liver

Cholesterol Deliver endogenously produced cholesterol to various organs.

HDLs Liver and intestine

Liver and steroid-hormone-producing glands

Cholesterol Remove and degrade

Cholesterol.

Table 3 Lipids are transported in the plasma as lipoproteins.

Lipids Metabolism . Liver and Adipose tissue play a central role in lipid metabolism.

. Adipose tissue is the main store of triglyceride in the body.

. Lipid metabolism either lipolysis or lipogenesis.

. Fatty acids are both oxidized to acetyl CoA and synthesized from acetyl CoA.

Lipolysis . Complete hydorlysis of triglyceride yeild gelycerol and 3 fatty acids.

. Fatty acids oxidiation take place in the mitochonderia.

. Fatty acids are activated before being catabolized (oxidized).

Fatty acid + ATP + CoA Acyl CoA + AMP + PPi

Long chain fatty acids penetrate the inner -Mitochinderial membrane as carnitine derivatives.

Figure 5 : Role of carnitine in the transport of long chain fatty acids.

β - oxidation pathway of fatty acids . Many tissues are capable of oxidizing fatty acids in the mitochondria by β - oxidation pathway.

. The chain is broken between the α (2) and β (3) carbon atoms.

Figure 6 : Overview of β - oxidation pathway of fatty acids

β - oxidation of fatty acids (Figure 7)

The products of oxidation of fatty acids Oxidation of fatty acids produces a large quantity

of ATP,

.example : Palmitic acid (C:16 ) 7 cycles from 7 NADH + 7 FADH2

(7 x5 = 35 ATP)7 cycles from 8 Acetyl CoA (8 x 12 = 96 ATP) 2 ATP for the initial activation

of fatty acid ( 35 +96 – 2 = 129 molecule of ATP)

Figure 8 : acetyl CoA product

Oxidation of unsaturated fatty acids occurs by amodified β - oxidation pathway (figure9)

KetonebodiesKetogenesis occurs when there is a high rate of

fatty acid oxidation in the Liver (figure 10).

Aceto acetyl CoA

Acetyl CoA

Acetoacetate

Acetone β -hydroxy butyrate

Acetyl CoA

CoA

CoA

Lipogenesis

. The body is capable of synthesizing fatty acids in the cytoplast of the cell from starter ( acetyl CoA + malonyl CoA).

. Malonyl CoA is formed from Acetyl CoA

Figure 11 : Biosynthesis of Malonyl CoA

. Acetyl CoA transfer to the cytoplast across the mitochondrial membrane in the form of citrate

Figure 12 : Conversion the citrate to Oxaloacetate and Acetyl CoA.

. The enzymes involved in fatty acid synthesis are arranged in a complex called the fatty acid synthase system.

The steps in the synthesis of fatty acid ( Figure 13)

Cholesterol

Figure 14 : Cholesterol

. Cholesterol is not an energy producing nutrient.

. It is an essential structural component of membrane of the outer layer of

plasma lipoprotien. • Bile acids and steriod hormones are formed from

cholesterol.

. Cholesteryl ester is a storage form of cholesterol which is found in most tissues.

Acetyl CoA is the source of all carbon atoms in cholesterol

Squalene

β -hydroxy- β- methylglutaryl CoA

Mevalonate

Farmesyl pyrophosphate

Acetyl CoA

CoA

Acetoacetyl CoA

CoA

Acetyl CoA

HMG-CoA reductase

Cyclization

References

Brene, R., Levy, M., Koeppen, B., Stanton, B., (2004) Physiology, Fifth edition. Mosby. Fox, S., (2004) Human Physiology, Eighth edition. Mc Graw Hill. Hunt, S., Groff, J., (1997) Advanced Nutrition and Human Metabolism, West Puplishing Company. Lehninger, A., Nelson, D., Cox, M.,(1993) Principles of Biochemistry, 2nd edtion. Worth Puplishers. Murray, R., Granner, D., Mayes, P., Rodwell, V., (1996) Haper’s Biochemistry, 24th edition, Prentic-Hall International, Inc.  Seeley, R., Stephens, T., Tate, P., (1992) Anatomy and Physiology,Mosby Yearbook.