Lipogenesis & Lipolysis

Lipogenesis Triacylglycerol synthesis Definition: Lipogenesis is the synthesis of TAG from fatty acids and glycerol. Site: Sub-cellular site: cytoplasm Organ (tissue) site: liver (primary site), adipose tissue, lactating mammary gland, kidney Esters of glycerol with three fatty acids Functions of Triglyceride Main stored from of energy in adipose cells. Energy: 9kcal/gram

Steps of Lipogenesis Synthesis of glycerol phosphate (activation of glycerol):

Synthesis of acyl-CoA (activation of fatty acids): Thiokinase CoA ATP AMP+Pi Fatty acid ------------------------------------------> acyl CoA Acyl transferases catalyes the transfer acyl groups Fates of the formed TAG In liver: TAG VLDL tissues In adipose tissue: TAG stored as depot fat Regulation of lipogenesis Insulin stimulates lipogenesis Adipocytes can take glucose only in the presence of insulin Insulin stimulate glycolysis which supplies glycerol phosphate Anti-insulin hormones inhibit lipogenesis

Lipolysis is the hydrolysis or breakdown of triacylglycerol in adipose tissue to glycerol and 3 fatty acids It requires enzymes called lipases which are present in the adipose tissue. Site:Sub-cellular site: cytoplasm The hydrolysis of triacylglycerol is initiated by hormone sensitive lipase. HSL is Hormonally regulated

Regulation of lipolysis Hormone-sensitive lipase (HSL) is activated when phosphorylated by cyclic AMP-dependent protein kinase Epinephrine & other anti-insulin hormones stimulate lipolysis during fasting, stress & hypoglycaemia by activation of adenylate cyclase forming cAMP Insulin inhibits lipolysis by Inhibiting adenylate cyclase enzyme Stimulating phosphodiesterase Stimulating phosphatase enzyme

Lipid Metabolism in Fat Cells: Fed State Starved State Insulin Stimulates up-take of glucose by Adipocytes stimulates glycolysis increased glycerol phosphate synthesis increases esterification inactivates HSL net effect: TG storage Glucagon, epinephrine activates HSL net effect: TG mobilization and increased FFA inhibit lipogenesis

FATTY ACIDS SYNTHESIS Fatty acid synthase multienzyme complex. Definition: Synthesis of saturated fatty acids Product: palmitate (16 c) Site: Sub-cellular site: cytoplasm Organ (tissue) site: liver, lactating mammary gland, adipose tissue & kidney Main requirements: Acetyl CoA (Active Acetate) Acetyl CoA carboxylase enzyme ATP for activation & fixation of CO2 in the synthesis of malonyl CoA from acetyl CoA NADPH Fatty acid synthase multienzyme complex. It is a dimer. Each unit contains 7 enzymes and a protein (acyl carrier protein) ACP Fatty Acid Synthase β-Ketoacyl synthase β-Ketoacyl synthase Fatty Acid Synthase ACP SH

Steps of Fatty acid synthesis Translocation of acetyl CoA from mitochondria to cytoplasm (Citrate shuttle) B. Cytoplasmic pathway for fatty acid synthesis: Carboxylation of acetyl CoA (2 carbon atoms) to malonyl CoA by acetyl CoA carboxylase (ACC) Formation of palmitate (16 C) by Fatty acid synthase multienzyme complex.

Cytoplasmic pathway for fatty acid synthesis: 1- Carboxylation of acetyl CoA by Acetyl CoA Carboxylase ‘first reaction’ of fatty acid synthesis This is the irreversible regulatory step in fatty acid synthesis malonyl-CoA serves as activated donor of acetyl groups in FA synthesis

Thioesterase 2- Formation of palmitate (16 C): All the next steps are catalyzed by Fatty acid synthase Priming reactions (Transacetylases) (1) Condensation Rxn (2) Reduction Rxn (3) Dehydration Rxn (4) Reduction Rxn Thioesterase The overall reaction for palmitate synthesis: 1 Acetyl CoA+7 Malonyl CoA+14(NADPH+H†)+7ATP---------Palmitate(16C)+8CoA+14NADP†+7(ADP+Pi)+7H2O

REGULATION OF FATTY ACID SYNTHESIS Acetyl CoA Carboxylase Citrate Malonyl CoA Palmitoyl CoA Insuline Glucagon Epinephrine - I- short-term regulation of acetyl CoA carboxylase High-calorie, high- carbohydrate diets Low-calorie diet or fasting + II- long-term regulation of acetyl CoA carboxylase Acetyl CoA---------------------------> Malonyl CoA Allosteric Hormonal

Catabolic Pathway Of FA Beta Oxidation Of Fatty Acids

Beta Oxidation of Fatty acids Beta Oxidation is the process where energy is produced by degradation of fatty acids to acetyl-CoA units The process of fatty acid oxidation is termed b oxidation since it occurs through the sequential removal of 2-carbon units (as acetyl-CoA) by oxidation at the b-carbon position of the fatty acyl-CoA molecule. Importance of b-oxidation Energy production Acetyl-COA production

β -oxidation takes place in Mitochondria 1 β -oxidation takes place in Mitochondria → Fatty acids which are participating in β-oxidation undergo activation to form Fatty acyl CoA → Activation of fatty acid takes place in CYTOPLASM, requires 2 high energy bonds and enzyme is Thiokinase or fatty acyl Co A synthetase.

N(CH3)3 CH2 H-C-OH COO- Carnitine FA~CoA HS-CoA Carnitine FA~Carnitine Transport of acyl CoA into the mitochondria ( rate-limiting step) The activated fatty acids which are present in CYTOPLASM enters into MITOCHONDRIA with the help of CARNITINE CARRIER. N(CH3)3 CH2 H-C-OH COO- + Carnitine FA~CoA Acyl transferase I HS-CoA Carnitine FA~Carnitine CYTOPLASM Translocase FA~Carnitine Carnitine THE MATRIX FA~CoA HS-CoA Acyl transferase II

β – oxidation proper There are 4 steps in β C– oxidation Step I – Oxidation by FAD linked dehydrogenase Step II – Hydration by Hydratase Step III – Oxidation by NAD linked dehydrogenase Step IV – Thiolytic clevage Thiolase

Beta Oxidation

Beta Oxidation

Beta Oxidation Palmitic (16 c)

-Oxidation and ATP Activation of a fatty acid requires: 2 ATP One cycle of oxidation of a fatty acid produces: 1 NADH 3 ATP 1 FADH2 2 ATP Acetyl CoA entering the citric acid cycle produces: 1 Acetyl CoA 12 ATP Palmitic acid (16 C) needs 7 cycles of beta-oxidation, which gives rise to 8 acetyl Co A x 12 = 96 ATP 7 FADH2 x 2 = 14 ATP 7 NADH x 3 = 21 ATP Total 131 ATP In initial activation Palmic acid → Palmitoyl Co A requires 2 high energy phosphates, so net is 131 – 2 = 129 ATP

β- oxidation of odd number fatty acids Odd number fatty acids are oxidized by β-oxidation until the final 3 carbons propionyl CoA is produced. So, β- oxidation of odd number fatty acid gives: (n) acetyl CoA + 1 propionyl CoA Propionyl CoA is converted to succinyl CoA which can enter the TCA cycle

Ketone Bodies A special source of fuel and energy for certain tissues Some of the acetyl-CoA produced by fatty acid oxidation in liver mitochondria is converted to acetone, acetoacetate and -hydroxybutyrate These are called "ketone bodies" Source of fuel for brain, heart and muscle Ketogenesis increases in fasting , diabetes and starvation 19

Oxidation of Ketone Bodies for Energy Most tissues can oxidize ketone bodies -requires mitochondria Conditions that increase KB synthesis and oxidation Length of fasting (> 3 days) Low carb diets Untreated Type 1 diabetes Diabetic ketoacidosis Liver synthesizes KB but can not oxidize them: liver lacks CoA transferase ENERGY PRODUCTION FROM OXIDATION OF -HYDROXYBUTYRATE 4/29/11

Lipid transport

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

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

Lipids (Triacylglycerols) are Transported as Lipoproteins Intestinal Mucosa (Exogenous Lipids) Chylomicrons Liver (Endogenous Lipids) VLDL (very low density lipoproteins)

Chylomicrons 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 Nascent chylomicron (B-48) Mature chylomicron (+apo C & apo E) Lipoprotein lipase Chylomicron remnant Apo C removed Removed in liver

Very Low Density Lipoprotein (VLDL) 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 Nascent VLDL (B-100) + HDL (apo C & E) = VLDL LPL hydrolyzes TG forming IDL 75% of IDL removed by liver 25% of IDL converted to LDL by hepatic lipase

Blood Cholesterol Is both consumed (exogenous) and produced by the liver (endogenous). Endogenous production increases with high ingestion of Saturated fat Found ONLY in animal products

Acetyl CoA is the source of all carbon atoms in cholesterol Acetoacetyl CoA CoA Acetyl CoA Inhibition β -hydroxy- β- methylglutaryl CoA HMG-CoA reductase Mevalonate Squalene Cyclization Farmesyl pyrophosphate

Low Density Lipoproteins (LDL - Bad) 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. Fates of cholesterol in the cells 1. Incorporated into cell membranes. 2. Metabolized to steroid hormones. 3. Re-esterified and stored. 4. Expulsion of cholesterol from the cell, and transported by HDL and finally excreted through liver.

High Density Lipoproteins (HDL – Good) Formation site: liver and intestine Function: transport cholesterol from peripheral tissues to liver (reverse cholesterol transport) Reservoir of apoproteins Contain  protein,  Cholesterol Protects against heart disease Apo A Activates lecithin- cholesterol acyltransferase (LCAT) Apo C Activates LPL Apo E Remnant receptor binding Uptake of cholesterol from peripheral tissues Esterification of HDL-C by LCAT Transfer of CE to (IDL and CR) removal of CE-rich remnants by liver converted to bile acids and excreted

Functions of apolipoproteins a . To combine and transport lipids. b . To regulate lipoprotein metabolism. apo A II activates hepatic lipase（HL） apo A I activates LCAT apo C II activates lipoprotein lipase（LPL） c. To recognize the lipoprotein receptors.

Apolipoproteins