Oxidation of Fatty Acids Fatty acids are an important source of energy Fatty acids energy Oxidation is the process where energy is produced by degradation of fatty acids There are several types of fatty acids oxidation. (1) β- oxidation of fatty acid (2) α- oxidation of fatty acids (3) ω- oxidation of fatty acids

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β- oxidation of fatty acid Beta-oxidation is the process by which fatty acids, in the form of Acyl-CoA molecules, are broken down in mitochondria and/or in peroxisomes to generate Acetyl-CoA, the entry molecule for the Citric Acid cycle.fatty acidsAcyl-CoAmitochondriaperoxisomes Acetyl-CoACitric Acid cycle It occurs in many tissues including liver kidney and heart. Fatty acids oxidation doesn't occur in the brain, as fatty acid can't be taken up by that organ.

The beta oxidation of fatty acids involve three stages: 1. Activation of fatty acids in the cytosol 2. Transport of activated fatty acids into mitochondria ( carnitine shuttle ) carnitine shuttle 3. Beta oxidation proper in the mitochondrial matrix

1) Activation of FA: This proceeds by FA thiokinase (acyl COA synthetase) present in cytosol Thiokinase requires ATP, COA SH, Mg ++. The product of this reaction is FA acyl COA and water.

2- Transport of fatty acyl CoA from cytosol into mitochondria: ( rate-limiting step) Long chain acyl CoA traverses the inner mitochondria membrane with a special transport mechanism called Carnitine shuttle. The matrix The cytosol

2-Transport of acyl CoA into the mitochondria (rate-limiting step) 1. Acyl groups from acyl COA is transferred to carnitine to form acyl carnitine catalyzed by carnitine acyltransferase I, in the outer mitochondrial membrane. 2. Acylcarnitine is then shuttled across the inner mitochondrial membrane by a translocase enzyme. 3. The acyl group is transferred back to CoA in matrix by carnitine acyl transferase II. 4. Finally, carnitine is returned to the cytosolic side by translocase, in exchange for an incoming acyl carnitine.

3. Proper of β – oxidation in the mitochondrial matrix There are 4 steps in β C– oxidation Step IFAD linked dehydrogenase Step I – Oxidation by FAD linked dehydrogenase Step IIHydratase Step II – Hydration by Hydratase Step IIINAD linked dehydrogenase Step III – Oxidation by NAD linked dehydrogenase Step IVThiolase Step IV – Thiolytic clevage Thiolase

The first reaction is the oxidation of acyl CoA by an acyl CoA dehyrogenase to give α-β unsaturarted acyl CoA (enoyl CoA). FAD is the hydrogen acceptor.

The second reaction is the hydration of the double bond to β-hydroxyacyl CoA (p- hydroxyacyl CoA).

The third reaction is the oxidation of β- hydroxyacyl CoA to produce β-Ketoacyl CoA a NAD-dependent reaction.

The fourth reaction is cleavage of the two carbon fragment by splitting the bond between α and β carbons By thiolase enzyme.

The release of acetyl CoA leaves an acyl CoA molecule shortened by 2 carbons. This acyl CoA molecule is the substrate for the next round of oxidation starting with acyl CoA dehydrogenase. Repetition continues until all the carbons of the original fatty acyl CoA are converted to acetyl CoA. In the last round a four carbon acyl CoA (butyryl CoA) is cleaved to 2 acetyl CoA.

Energetics of FA oxidation e.g. Palmitic (16C): 1. β-oxidation of palmitic acid will be repeated 7 cycles producing 8 molecules of acetyl COA. 2. In each cycle FADH2 and NADH+H + is produced and will be transported to the respiratory chain. FADH 2 2 ATP NADH + H + 3 ATP So 7 cycles 5x7 = 35 ATP

3. Each acetyl COA which is oxidized in citric cycle gives 12 ATP (8 x 12 = 96 ATP) 4. 2 ATP are utilized in the activation of fatty acid (It occurs once). Energy gain = Energy produced - Energy utilized = 35 ATP + 96 ATP - 2 ATP = 129 ATP

Non-covalent assemblies of lipids and proteins LP core  Triglycerides  Cholesterol esters LP surface  Phospholipids  Proteins  cholesterol Plasma Lipoproteins (Structure) Function as transport vehicles for triacylglycerols and cholesterol in the blood All the lipids contained in plasma, including fat, phosphalipids, cholesterol, cholesterol ester and fatty acid, exist and transport in the form of lipoprotein

CM VLDL LDL HDL Lipoprotein Nomenclature, Composition and separation Major apoB 48 apoB 100 apoB 100 apoA-I Protein Major TG TG CE CE Lipid 1.Electrophoresis method: CM (chylomicron) slow  -Lipoprotein pre  -Lipoprotein fast  - Lipoprotein 2. Ultra centrifugation method ： CM (chylomicron ) slow very low density lipoprotein ( VLDL) low density lipoprotein ( LDL) high density lipoprotein (HDL) high

Apolipoproteins (apoproteins) and functions They are protein components of lipoproteins consisting 60% of some lipoproteins (HDL) and 1% of some lipoproteins (chylomicrons) 1. To combine and transport lipids. 2. To recognize the lipoprotein receptors Apo-B 100 is the ligand for LDL-receptors Apo-B48 for chylomicron remmenant Apo-A 1 is the ligand for HDL receptor. 3. Activators for certain enzymes involved in lipoprotein metabolism Apo C II activates lipoprotein lipase and apo-A 1 activates LCAT (Lecithin Cholesterol Acyltransferase, formation of cholesterol esters in lipoproteins).

Lipids are Transported as Lipoproteins All lipids in plasma are transported in the form of lipoproteins. Transport dietary lipids from intestine to liver (exogenous ) Chylomicrons Transport lipids from liver to peripheral tissues (endogenous) VLDL (very low density lipoproteins)

 Synthesized in small intestine  Transport dietary lipids (exogenous TG)  98% lipid, large sized, lowest density  Apo B-48  Receptor binding  Apo C-II  Lipoprotein lipase activator  Apo E  Remnant receptor binding Chylomicrons Nascent chylomicron are formed in the intestinal and consists of rich in dietary TG + minimal amount of dietary cholesterol + Apo (B-48) Mature chylomicron after Nascent chylomicron passage to blood, addition of apoC and apoE from HDL Lipoprotein lipase hydrolyzes TG present in chylomicrons Chylomicron remnant taken up by the liver through endocytosis. Apo C removed and returns to HDL

 Synthesized in liver  Transport endogenous triglycerides  90% lipid, 10% protein  Apo B-100  Receptor binding  Apo C-II  LPL activator  Apo E  Remnant receptor binding The major fraction of VLDL remnant further loses TG, so as to be converted to LDL Very Low Density Lipoprotein (VLDL) Nascent VLDL are formed in the liver and consists of endogenous TG + 17 % cholesterol + Apo (B-100) Mature VLDL after Nascent VLDL passage to blood, addition of apoC, apoE and cholesterol esters from HDL Lipoprotein lipase (LPL) hydrolyzes TG present in VLDL VLDL remnant containing less of TG and more of cholesterol and taken up by the liver through endocytosis. Apo C removed and returns to HDL

Blood Cholesterol  Cholesterol is the most important animal sterols which is the precursor of all other steroid in the body e.g. corticosteroids, sex hormones, bile acids and vitamin D. Cholesterol biosynthesis:  All tissues containing nucleated cells are capable of synthesizing cholesterol.  Cholesterol is derived about equally from the diet ( exogenous) or manufactured de novo (endogenous) in cells of humans especially in liver, intestine, and adrenal cortex.  Acetyl CoA is the source of all carbon atoms in cholesterol.  The liver is the main source of plasma cholesterol but intestine also participates.  The liver is the principle organ which removes cholesterol from blood.

 The enzymes involved in cholesterol biosynthesis are present in cytosol and microcosms of the cell.  Total cholesterol in plasma is normally between mg/dl.  Cholesterol esters are continually hydrolysed in liver and resynthesized in plasma.  Cholesterol is present in all the lipoproteins but in fasting more than 60% is carried in (LDL).

Formation site: from VLDL in blood, but a small part is directly released from liver Function: transport cholesterol from liver to the peripheral tissues. Carries aprox. 50% of blood cholesterol. containing only apo B-100. LDL concentration in blood has positive correlation with incidence of cardiovascular diseases. Expulsion cholesterol from the cell, and transported by HDL and finally excreted through liver. Low Density Lipoproteins (LDL - Bad)

Formation site: liver and intestine Function: transport cholesterol from peripheral tissues to liver (reverse cholesterol transport) converted cholesterol to bile acids and excreted Reservoir of apoproteins Contain  protein,  Cholesterol  Apo A  Apo C Activates LPL  Apo E Remnant receptor binding Protects against heart disease High Density Lipoproteins (HDL – Good)