LIPOLYSIS LIPOLYSIS
OVERVIEW
CATABOLISM OF FREE FATTY ACIDS Nonesterified fatty acids Source:- (a) breakdown of TAG in adipose tissue (b) action of Lipoprotein lipase on plasma TAG Combined with Albumin soluble Enter cell by binding to membrane fatty acid transport protein and then to intracellular fatty acid binding protein Highest energy yield :- 9Kcal/gm (Carbohydrate/ protein 4Kcal/gm )
MOBILIZATION OF STORED FAT Major fuel store of body:- TAG in White adipose tissue (highly reduced and anhydrous capable of Reducing equivalents) Fatty acids are derived from :- (a) TAG from adipose tissue and (b) Lipoproteins Fatty acids yield 9 Kcal/g energy : Protein/Carbohydrate yield 4 Kcal/g
RELEASE OF FATTY ACIDS FROM TAG 3 Lipases act on the TAG to release FFA 1. ATGL ( Adipose Triacyl Glycerol Lipase):- TAG DAG + FFA 2. HSL {Hormone sensitive Lipase} :- DAG MAG + FFA 3. MAG { Lipases for MAG} :- MAG Glycerol + FFA (Fatty acids + Glycerol)
TAG FATTY ACIDS + GLYCEROL
HORMONE SENSITIVE LIPASE Epinephrine / Glucagon binds to receptor on cell membrane of adipocyte Adenylyl cyclase activated ATP changes to 3 5 cyclic AMP + PPi camp activates camp dependent Protein Kinase (1)Activates Hormone sensitive Lipase (by phosphorylation) (2) Phosphorylation of Perilipin coat on fat droplets allows access of HSL Acetyl CoA carboxylase is inhibited by camp cascade Fatty acid synthesis is turned off when TAG are degraded Insulin/ Glucose dephosphorylate and inactivate HSL
HORMONAL REGULATION OF HSL
FATE OF GLYCEROL Adipocytes do not have Glycerol Kinase Glycerol is transported by blood to liver Phosphorylated to Glycerol 3.P TAG synthesis in liver / DHAP for glycolysis/gluconeogenesis
FORMATION OF GLYCEROL P
FATE OF FATTY ACIDS Free Fatty acids leave by crossing the cell membrane of adipocyte bind to plasma albumin FFA /UFA transported to tissues Enter cells bind to fatty acid binding protein Activated to CoA derivative Oxidized for energy / re esterified to Glycerol3P TAG Free fatty acids cannot be used for energy by:- (a) RBC (no mitochondria) and (b) Brain
FATE OF FREE FATTY ACIDS
β OXIDATION OF FATTY ACIDS Major pathway of Fatty acid catabolism Mitochondrial 2C fragments are successively removed from Carboxyl end of Fatty acyl CoA Acetyl CoA, NADH and FADH2 17 ATP / cycle
OVERVIEW OF β OXIDATION
TRANSPORT OF LONG CHAIN FATTY ACIDS INTO MITOCHONDRIA LCFA enters cell converted in cytosol by LCFACoA synthetase /Thiokinase in outer mitochondrial membrane CoA derivative Carnitine shuttle:- transports the LCFA groups from cytosol across the (impermeable to CoA) Inner mitochondrial membrane Matrix (This is rate limiting process) β oxidation
LCFA TRANSLOCATION CARNITINE PALMITOYL TRANSFERASE I (in the outer mitochondrial membrane):- transfers the acyl group from CoA to Carnitine Acyl Carnitine + free CoA AcylCarnitine transported into mitochondrial matrix in exchange for free Carnitine by CARNITINE ACYLCARNITINE TRANSLOCASE CARNITINE PALMITOYL TRANSFERASE II (in the inner mitochondrial membrane) transfers the acyl group from Carnitine to CoA Free Carnitine regenerated
CARNITINE SHUTTLE
CARNITINE SHUTTLE
INHIBITORS OF CARNITINE SHUTTLE Malonyl CoA inhibits CPT-1 prevents entry of LC acyl groups into mitochondrial matrix (Malonyl CoA indicates fatty acid synthesis Fatty acid synthesis and degradation cannot happen together) Acetyl CoA : CoA ratio Thiolase reaction
SOURCES OF CARNITINE Carnitine is β hydroxy- γ trimethylammonium butyrate ( CH3)3N+-CH2-CH(OH)-CH2-COO (1) Diet:- Meat (skeletal muscle contain 97%) (2) Endogenous Synthesis :- from amino acids Lysine and Methionine in Liver and Kidney Skeletal muscle / Myocardium cannot synthesize it so depend on diet / endogenous synthesis
CARNITINE DEFICIENCIES PRIMARY CAUSES Congenital deficiency of CPT LCFA cannot be used for energy (a) CPT-I liver is unable to use LCFA for fuel ability to synthesize glucose (due to Glycerol3P)during fast severe hypoglycemia, coma death (b) CPT-II Cardiac and skeletal muscle cardiomyopathy, muscle weakness, myoglobinemia following exercise avoid prolonged fast, Carbohydrate in diet, LCFA diet, MCFA + Carnitine supplement (c) Acquired Carnitine deficiency:- (i) FA oxidation Acyl Carnitine accumulation Carnitine excreted in urine Carnitine (ii) Liver disease Carnitine synthesis (iii) Drugs(e.g. Anticonvulsant Valproate) Renal reabsorption of Carnitine
CARNITINE DEFICIENCIES OTHER SECONDARY CAUSES Liver disease synthesis of Carnitine Malnutrition / Vegans requirement of Carnitine:- pregnancy, infections, burns, trauma, hemodialysis
ENTRY OF SCFA and MCFA INTO MITOCHONDRIA Fatty acids shorter than C12 (e.g. milk fat) cross the inner mitochondrial membrane without Carnitine/CPT Inside mitochondria activated to CoA derivatives oxidized Not inhibited by Malonyl CoA because they don t require CPT-1
β-oxidation 4 reactions on βc / C3 i.e. chain broken between α and β C atoms Fatty acid chain shortened by 2C every time. Steps:-repeated for even No.,saturated FA (1) oxidation FADH2 (+ δ2 trans enoyl CoA) (2) hydration (forms δ3 hydroxy acyl CoA) (3) oxidation NADH (forms 3 keto acyl CoA) (4) Thiolytic cleavage at the 2,3 position Acetyl CoA Each cycle 1 FADH2+1 NADH+1 Acetyl group Final thiolytic cleavage 2 acetyl groups Each enzyme is chain length specific Cycle is repeated N/2 1 times for saturated, even number C fatty acids Acetyl CoA induces Pyruvate Carboxylase (Fatty acid oxidation and gluconeogenesis are linked) Acetyl CoA can be (1) oxidized in TCA 12 ATP/ mole (2) used for hepatic ketogenesis
ENERGY YIELD FROM β OXIDATION Each cycle yields :- 1 FADH2 = 1x 2 = 2ATP ( in ETC) 1 NADH = 1x 3 = 3 ATP 1 Acetyl CoA = 1x 12= 12ATP Total = 17 ATP/cycle Oxidation of 1 molecule of Palmitoyl CoA (16C) CO2 + H2O 7 FADH2 + 7 NADH + 8 Acetyl CoA 7x17 = 119 + 12 = 131 ATP - 2 ATP (activation of FA) = 129 ATP
MEDIUM CHAIN FATTY ACYL COA DEHYDROGENASE DEFICIENCY (MCAD deficiency) Mitochondria have 4 Fatty acyl CoA Dehydrogenase each for SCFA, MCFA, LCFA, VLCFA MCAD deficiency is an autosomal recessive disorder Most common inborn error of metabolism(specially in Europeans) ability to oxidize 6-10 C fatty acids in blood in urine Severe hypoglycemia because tissues rely on glucose consumption( since FA available for energy) Treatment = avoid fasting
OXIDATION OF ODD NUMBER CARBON FATTY ACID β oxidation final 3 Carbons(Propionyl CoA) 3 step pathway (1) Carboxylation D methyl malonyl CoA (Propionyl CoA carboxylase requires Biotin) (2) D isomer L isomer by Methylmalonyl CoA racemase (3) Rearrangement of C Succinyl CoA TCA (glucogenic) (Methylmalonyl CoA mutase requires coenzyme Vit B12) In B12 deficiency Propionate + Methylmalonate excreted in urine metabolic acidosis + retarded development
METABOLISM OF PROPIONYL COA
OXIDATION OF UNSATURATED FATTY ACIDS Provides less energy (less reduced reducing equivalents) Monounsaturated fatty acids:- require 1 more enzyme (3,2, enoyl coa isomerase converts 3-trans to 2-trans derivative) substrate for enoyl CoA hydratase of β oxidation Polyunsaturated fatty acids:- also require NADPH dependent 2,4 dienoyl CoA reductase to reduce the double bonds
BRANCHED CHAIN FATTY ACID OXIDATION (e.g. 20 C Phytanic acid)/ α OXIDATION Β Oxidation of VLCFA fist occurs in peroxisomes no ATP generated, rather H2O2 (Catalase) H2O α oxidation also takes place in peroxisomes Acyl CoA dehydrogenase cannot act on it because of the methyl group on β Carbon It is hydroxylated at α Carbon by Phytanoyl CoA α hydroxylase C1 CO2 + 19C Pristanic acid CoA derivative β oxidation Refsum disease:- autosomal recessive disorder Phytanoyl CoA α hydroxylase Phytanic acid in plasma + tissues neurologic symptoms Treatment = dietary restriction
PHYTANIC ACID (20C BRANCHED CHAIN FATTY ACID)