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Lipid s. Dr. Sooad Al-Daihan Biochemistry department. Lipids. Heterogeneous group of biomolecules. Water insoluble (hydrophobic). Soluble in organic and non-polar solvents acetone, ether, chloroform and benzene. Classes of lipids. Derived lipids. Simple lipids. - PowerPoint PPT Presentation
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LipidsLipids
Heterogeneous group of biomolecules.
Water insoluble (hydrophobic).
Soluble in organic and non-polar solvents acetone, ether, chloroform and benzene.
Classes of lipids
Conjugated (complex) lipids
Derived lipids
- Triglycerides- Waxes -Phospholipids,
- Glycolipids- Sphingo- phospholipids- Lipoproteins.
-Fatty acids-Cholesterol-Ketone bodies
Esters of FA with alcohols
Esters of FA with alcohols and additional group
Obtained on hydrolysis of simple or complex lipids
Simple lipids
Major source of energy source of energy for the body: Fat stored in the adipose tissue is a
direct and potential source of energy.
1 gm lipids 9.3 Kcal
1 gm CHO 4.1 Kcal
Storage form of energy-triglyceridetriglyceride
Serve as structural components structural components of cell membrane (PLs, glycolipids).
Some are hormoneshormones e.g. steroid hormone (cortisol, aldosterone, sex
hormones).
Essential in the diet Essential in the diet to provide the body with some essential FA and of fat-
soluble vitamins (A, D, E,K) which have regulatory or coenzyme function
FunctionsFunctions
An adult man eats about 100-150 g100-150 g of lipids/day
The main lipids in diet are TG which constitute most of fat and oils we eat, but diet contains also some cholesterol and phospholipids
Digestion and Absorption of Lipids
STEP 3STEP 3Liver releases bile acids to solubilize lipid products in mixed micelles
STEP 2STEP 2Pancreas releases:Lipase (+colipase)cholesterol esterasephospholipase A2
Lipids:Lipids:TriacylglycerolsCholesterol estersPhospholipids
pancreaspancreas
liverliver
STEP 4Lipids absorbed from micelles into epithelial cells
STEP 5STEP 5Chylomicrons form and travel through lymphatics
Gastric Lipase
STEP 1STEP 1
small intestinesmall intestine
stomachstomachpHopt ~5, Initiates hydrolysis and acts on TG with short chain FAs
Lingual lipase begins emulsification of lipids and PLs (negligible)
Lipid emulsificationLipid emulsification
Bile acids help emulsifying fat droplets thus increasing their surface area
Pancreatic lipasePancreatic lipase
Pancreatic lipase, also known as pancreatic triacylglycerol lipase, is secreted from the pancreas, and is the primary lipase (enzyme) that hydrolyze dietary fat molecules in the human digestive system.
converting TG to monoacylglycerol and free fatty acid.
Triacylglycerol + 2 H2O 2-monoacylglycerol + 2 fatty acid
Unlike some pancreatic enzymes that are activated by proteolytic cleavage (e.g. trypsinogen), pancreatic lipase is secreted in its final form.
However it only becomes efficient in the presence of colipasecolipase in the duodenum.
Glycerol + two fatty acids + phosphorus
Phosphorus part makes it soluble in water
Fatty acids make it soluble in fat
Therefore can serve as an emulsifier
Key role is in cell membranes
CH2 – O – C - R1
CH2 – O – P – O - X
R2 - C - O - CH
O-
O
O
O
O
PhospholipidPhospholipid
Is secreted by the pancreas into the intestine where it is activated by trypsin and its activity requires the presence of bile salts and calcium ions.
Phospholipase A2
A phospholipase is an enzyme that hydrolyzes Phospholipids into fatty acids and other lipophilic substances. There are four major classes, termed A, B, C and D, distinguished by the type of reaction which they catalyze:
I. Phospholipase A Phospholipase A1-cleaves the SN-1 acylchain. Phospholipase A2- cleaves the SN-2 acylchain.
Phospholipase B - cleaves both SN-1 and SN-2 acylchain. Phospholipase C - cleaves before the phosphate, and a phosphate- releasing diacylglycerol containing head
group. Phospholipase D - cleaves after the phosphate.
ContinueContinue
The end product of lipid digestion are:MGFA (short FA C4-C10; long chain FA C12-C18)GlycerolLysoPLCholesterol
Absorption of LipidsAbsorption of Lipids
Absorption of lipidsAbsorption of lipids
MGFA
LysoPLCholesterol
+Bile SaltsMicelles
Bile Salts
Cholesterol
LysoPL
Chylomicrons
MG
Long chain FATG
Glycerol
Short chain FA
Glycerol
Short chain FA
Protein
LUMENINTESTINAL WALL
Systematic
Circulation
Th
oracic D
uct
HOHO
O
R
HOHO
O
R
HOHO
(No need for bile acids)
PL
Absorption of lipidsAbsorption of lipids
Short chain fatty acids and glycerol are water soluble and pass via the portal system directly to the liver
Other lipid are water insoluble. They combine with bile salts to form a water soluble complex called micelles which enter the mucosal cells
(move down concentration gradient passive diffusion)
Bile salts are reabsorbed to the liver again
Long chain fatty acid are activated in the mucosal cells and combine with monoglycerols again to reform triglycerides
The TG, PL, and cholesterol combine with protein forming chylomicrons which enter the circulation via lacteals and thoracic duct
Body tissues can extract whatever fat they need from chylomicrons
After absorption, lipids are either oxidized mainly in the liver or are stored in the depot (adipose tissue)
Fate of Dietary Lipids
TG in chylomicron are degraded to glycerol + FFA by Lipoprotein lipase in the luminal surface of capillary bed.
Lipoprotein LipaseFFA + Glycerol
Taken up by liver
• Taken up by peripheral tissues (muscles, adipocytes)• May bind to albumin and transported to other cells
Chylomicron remnants
Taken up by liver
TGChylomicron
1. Tissue lipids Included in the structure of the cell e.g. Cell membrane and
mitochondria.
Never be oxidized to give energy.
Appears as a yellow droplets in the cytoplasm of adipose tissue cells
Body lipids are 2 types: Tissue lipids depot fat (adipose tissue)
Continue…
Sites:Sites: Under skin and breast. Around important organs e.g. kidneys. In the omentum and mesentery.
Composition:Composition: Mainly TG which contains saturated and unsaturated. Fas Contains also a little of PLs and cholesterol
Sources:Sources: Absorbed Fat Carbohydrates by lipogenesis
Functions:Functions:
Important source of energySupports some of the internal organs like the kidney.7-dehydrocholesterol present in adipose tissue gives Vit D3 on exposure to UV rays.Protection of bony prominence.Protection against cold,
Continue…
In animal that are in caloric balance, the stored TG in adipose tissue is continuously undergoing lipolysis
and reesterification
Metabolism of adipose tissue
FAFA
Glycerol
Lipolysis
FA esterification
Triglycerides
Glycerol-3P
Glucose, amino acids, lactate, pyruvate
FA
Adipocyte
If the rate of lipolysis is more excessive than the rate of reestirification, FFAs accumulate and diffuse into the plasma, where they bind to serum albumin and
are transported to tissue for oxidation
Metabolism of adipose tissue
FAFA
Glycerol
Lipolysis
FA esterification
FA
Glycerol
Triglycerides
Glycerol-3P
Glucose, amino acids, lactate, pyruvate
FA
Adipocyte
Blood
Tissues
Albumin
This occurs in condition where the need for energy is increased as in
.1Starvation
.2Diabetes mellitus
.3During growth
.4Low carbohydrate diet
.5Certain infectious disease as tuberculosis
Causes of excessive lipolysis
Lipolysis is carried out by a number of lipase enzymes which are present in adipose tissue
.1Hormone Sensitive triacyglycerol Lipase (HSL)
.2Diacylglycerol lipase
.3Monoacylglycerol lipase
Mechanism of lipolysis
TG DAG + FFA HSL
DAG MAG + FFADAG lipase
MAG glycerol + FFA MAG lipase
FAFA
Glycerol
Lipolysis
FA esterification
FA
Glycerol
Triglycerides
Glycerol-3P
Glucose, amino acids, lactate, pyruvate
FA
Tissues
Albumin Glycerol-3P
GlyconeogenesisGlycolysisLipogenesis
oxidation
FA
ATP
Blood
Adipocyte
IntroductionIntroduction Lipid breakdown is the process by which a
molecule of fatty acid is degraded by the sequential removal of 2C units, producing acetyl CoA which can then be oxidized to CO2 and H2O by the TCA cycle.
It occurs in many tissues, especially liver and muscle.
Certain tissues such as brain, RBCs, and adrenal medulla are unable to oxidize fatty acids ( because they lack the necessary enzymes)
The Four Stages of Lipid Breakdown
1. Hydrolysis of TAG by Lipase (Lipolysis)
2. Activation of Fatty Acids3. Transport into Mitochondria4. β-Oxidation
LipolysisIt occurs in the cytosol of adipose cells.
TAG is converted into glycerol and 3 fatty acids in 2 steps:
1.A hormone- sensitive lipase hydrolyses TAG at the C1 and C3 positions to form monoglycerol.
2.A monoglycerol specific lipase removes the remaining fatty acid.
Continue .. Glycerol produced cannot
be metabolized by adipose tissue because it does not contain glycerol kinase.
Glycerol is transported to the liver where it is phosphorylated, either to be used again to make TAG or to be converted to dihydroxyacetone phosphate (DHAP), a glycolytic intermediate.
The free F.As produced are either re-esterified to TAG in the adipose tissue or travel in the blood to be taken up by the cells for oxidation.
Activation of Fatty Acids Fatty acids must be esterified to Coenzyme A before they can
undergo oxidative degradation, be utilized for synthesis of complex lipids, or be attached to proteins as lipid anchors.
Acyl-CoA Synthases (Thiokinases) of ER & outer mitochondrial membranes catalyze activation of long chain fatty acids, esterifying them to coenzyme A.
This process is ATP-dependent.
There are different Acyl-CoA Synthases for fatty acids of different chain lengths.
PPi is cleaved by pyrophosphatase to 2 inorganic phosphate (2Pi)
Transport of Fatty Acyl CoA into Mitochondria
Fatty acyl CoA is impermeable to inner mitochondrial membrane while enzymes for β-oxidation are present in the mitochondria
Therefore after activation, the fatty acid (fatty acyl CoA) interacts with carnitine which helps in its translocation across the inner mitochondrial membrane.
Transfer of the fatty acid moiety across the mitochondrial inner membrane involves carnitine.
Carnitine Palmitoyl Transferases catalyzes transfer of a fatty acid between the thiol of Coenzyme A and the hydroxyl on carnitine
Carnitine-mediated transfer of the fatty acyl moiety into the mitochondrial matrix is a 3-step process: 1. Carnitine Palmitoyl Transferase I, an enzyme on the cytosolic surface of the outer mitochondrial membrane, transfers a fatty acid from CoA to the OH on carnitine. 2.Carnitine acylcarnitine translocase in the inner mitochondrial membrane mediates exchange of carnitine for acylcarnitine. 3.CarnitinePalmitoylTransferaseII,an enzyme within the matrix,transfers the fatty acid from carnitine to CoA. (Carnitine exits the matrix in step 2) The fatty acid is now esterified to CoA in the matrix