lipids digestion

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بسم الله الرحمن الرحيم

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Welcome...

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What is Lipid ? How it’s classified ? Fatty acids Neutral fats and oils Waxes Phospholipid Sterols Lipoprotein Role of lipids

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The term Lipid applies to a class of compounds that are soluble in organic solvent (Non-polar) and in soluble in water (Polar solvents) Chemically: Esters of Fatty acids with alcohol (Lipids are organic compounds formed mainly from alcohol and fatty acids combined together by ester linkage) LIPIDS

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Fatty Acids

DIGESTION of lipids :

DIGESTION of lipids

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DIGESTION OF LIPIDS DR MUHAMMAD MUSTANSAR

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Lipids: Fats & Oils

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Dietary lipids TAGs & Phospholipids: - Major ones - Since hydrophobic , they need to be hydrolyzed & emulsified to “ micelles ” before absorption. Fat-soluble Vits (A, D, E, & K) & cholesterol absorbed dissolved in lipid micelles

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Lipids energy storage

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DEFINITION Lipids may be regarded as organic substances relatively insoluble in water, soluble in organic solvents i.e. alcohol, ether etc, related to fatty acids and utilized by living cells Unlike the polysaccharides, proteins and nucleic acids, lipids are not polymers They are mostly small molecules.

Lipids :

Lipids Lipids are composed of C, H, O long hydrocarbon chain Diverse group fats phospholipids steroids Do not form polymers big molecules made of smaller subunits not a continuing chain fat

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Most membrane lipids are amphipathic , having a non-polar end and a polar end. Fatty acids consist of a hydrocarbon chain with a carboxylic acid at one end.

Building Fats:

Building Fats Triacylglycerol 3 fatty acids linked to glycerol ester linkage = between OH & COOH

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B. Structure of fats

Saturated fats:

Saturated fats All C bonded to H No C=C double bonds long, straight chain most animal fats solid at room temp. contributes to cardiovascular disease (atherosclerosis) = plaque deposits

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Digestion of Lipids The major dietary lipids are TRIACYL GLRCEROL CHOLESTEROL PHOSPHOLIPIDS

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DIETARY FAT : - 90% is normally triglycerides. - Cholesterol, Cholesteryl esters, Phospholipids, and Unesterified fatty acids. Average Normal Pakistani Diet: 20-30 gm/day Western Diet: 2 or 3 times of this

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Digestion in Mouth Very little amount of lipid digestion takes place in mouth because of the presence of Lingual lipase . Lingual lipase action continues in stomach also. It acts mainly on short chain Tryglycerides.

Digestion & Absorption of TAGs:

Digestion & Absorption of TAGs Initiated by lingual & gastric lipases attacking ester bond, forming 1,2- DAGs & FFAs. In intestine, further hydrolysis is by pancreatic lipase which: Needs colipase , for activity. Specific for -1 & -3 ester bonds → 2-monoacylglycerols – major end-products of TAG digestion – & FFAs. N.B : <25% of ingested TAG is completely hydrolyzed to glycerol & FFAs. H Lipase H 2 O FFA +

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Digestion in stomach Lipids are emulsified by peristaltic movements Gastric lipase is an important lipid digesting enzyme in stomach. Around 30% digestion of TAG occurs in stomach

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IN STOMACH Lingual lipase: Active at low ph (pH 2.5 – 5) Short chain TGS. In milk, butter and ghee. Gastric lipase: - Up to 30% of TGS

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IN SMALL INTESTINE 1.Emulisification: Dispersion of lipids into small droplets - Bile salts ( detergent action ) Function to transport cholesterol in the digestive system - Peristalsis ( mechanical mixing ) - phospholipids

Digestion in Intestine:

Digestion in Intestine Bile Bile Salts Pancreatic Juice – Pancreatic Lipase Co Lipase Cholesterol esterase Phospholipases. All these lipases target primary ester linkage . Emulsification Neutralization of acid

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Pancreatic juice: - Pancreatic lipase - Cholesterol estarase - Phospholipase A2 - Colipase Lipolytic enzymes colipase lipase TG particle

Bile Salts:

Bile Salts Bile salts are synthesized from cholesterol in the liver - they are stored in the gall bladder and released into the upper small intestine to help break down fats and oils (like soaps) - too much accumulated cholesterol in the gall bladder can lead to gallstones; if a gallstone passes into the bile duct, severe pain results and the gallbladder often has to be removed Cholic acid, a bile acid Glycine, an amino acid Sodium glycocholate, a bile salt

Structure of a Glycerophospholipid:

Structure of a Glycerophospholipid

Steroids:

Steroids Steroids are lipids containing a steroid nucleus (core structure) The steroid nucleus is a fused ring system consisting of three cyclohexane rings and one cyclopentane ring The rings are designated A, B, C and D Attachment of different groups to the core steroid structure leads to a wide variety of steroid compounds, including cholesterol, bile salts and steroid hormones

Physiologically important lipases:

Physiologically important lipases Lipase Site of action Preferred substrate Product(s) Lingual / acid stable lipase Mouth , stomach TAG S with med chain FA S FFA+DAG Pancreatic lipase + co-lipase Small intestine TAG S with long chain FA S FFA+2MAG Intestinal lipase with bile acids Small intestine TAG S with med chain FA S 2FFA+glycerol Phospholipase A 2 + bile acids Small intestine PLs with unsat. FA on position 2 Unsat FFA lysolecithin Lipoprotien lipase insulin (+) Capillary walls TAGs in chylomicron or VLDL FFA+glycerol Hormone sensitive lipase Adipose cell TAG stored in adipose cells FFA+glycerol

Hydrolysis of lipids:

Hydrolysis of lipids Triglycerides (TG) TG + H2O → Diglyceride + fatty acid (FA) Diglyceride + H2O → Monoglyceride (MG) + FA + 2 H +

Absorption of lipids:

Absorption of lipids The end products of lipid digestion are Fatty acids Glycerol 2-Monoacylglycerol 1- Mono acylglycerol Cholesterol Lysophonpholipid

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2-monoacylglycerols, Fat soluble Vits, cholesterol & phospholipids are emulsified by bile salts into micelles and liposomes. Water soluble micelles are transported through the intestinal lumen and come into close contact with the brush border of jejunal mucosal cells, allowing cellular uptake of enclosed lipids.

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The released bile salts pass on to the ileum, where absorbed into the enterohepatic circulation returning to the liver

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Glycerol released in the intestinal lumen is not reutilized but passes via portal vein to the liver; where it is used for esterification of FAs synthesized in lipogenesis. Glycerol released within the epithelium is reutilized for TAG synthesis via the normal phosphatidic acid pathway.

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All long-chain fatty acids absorbed are converted to TAG in the mucosal cells and, together with the other products of lipid digestion, secreted as chylomicrons into the lymphatics, entering the blood stream via the thoracic duct.

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Bile Salts Acyl 1 Acyl 2 Acyl 3 TAG 100% Intestinal Lumen Intestinal Cell Lymphatics Pan. Lipase FA FA Acyl 1 Acyl 2 OH 1,2-DAG OH Acyl 2 OH 2-MAG * Acyl 1 OH OH 1-MAG FA Pan. Lipase Isomerase OH OH OH Glycerol FA Pan. Lipase OH Acyl 2 OH Acyl-CoA Acyl 1 Acyl 2 Acyl 3 Acyl 1 Acyl 2 Acyl 3 Acyl 1 Acyl 2 Acyl 3 Acyl 1 OH OH OH OH OH Glycerol FA Int. Lipase Glycerol Kinase ATP ATP Acyl CoA Synthetase OH OH P Glycerol-3 phosphate Enterohepatic circulation Liver Glycolysis Monoacylglycerol pathway Phosphatidic acid pathway Portal Vein Glycerol 6% 72% Acyl CoA Synthetase + Fat-soluble Vits & Cholesterol Chylomicrones ATP To ileum 22% 28% TAG TAG 1-MAG Thoracic Duct 2-MAG

STEATORRHEA:

STEATORRHEA Defined as an increase in stool fat excretion of >6% of dietary fat intake due to one or more defects in the digest

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Steatorrhea

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LIPID METABOLSIM

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