Lipid Metabolism Term4 (14) PDF

Lipid Metabolism Medical Biochemistry Dr RB Khan, ext 4597, [email protected]  Objectives Discuss major pathways of triacylglycerol metabolism – Catabolism  Oxidation – Ketogenesis – Anabolism  Biosynthesis – Storage – Phospholipid synthes...

Lipid Metabolism Medical Biochemistry Dr RB Khan, ext 4597, [email protected]  Objectives Discuss major pathways of triacylglycerol metabolism – Catabolism  Oxidation – Ketogenesis – Anabolism  Biosynthesis – Storage – Phospholipid synthesis Oxidation of fatty acids  Objectives : Oxidation Describe the process of TAG release from adipose tissue Types of oxidation (α-oxidation, β-oxidation, ω-oxidation) Describe pathway for activation and transport of fatty acids to the mitochondrion for oxidation Outline the sequence of reactions involved in oxidation of fatty acids in the mitochondrion Be aware that oxidation can occur in peroxisomes and compare it to mitochondrial oxidation Explain why ketogenesis occurs and know the intermediates and products of this pathway Catabolism of stored TAG Aka hormone sensitive lipase ⊕ glucagon & epinephrine – Adenylyl cyclase  cAMP  protein kinase  HSL-P ⊗ insulin – Protein phosphatase  Types of Oxidation α-oxidation Mitochondrial β- oxidation Peroxisomal β- oxidation ω-oxidation β α ω - end  α Oxidation Branched chain fatty acid (phytanic acid) Methyl group on its third (β) carbon Phytanic acid is activated to CoA derivative Hydroxylated at the α-carbon by fatty acid hydroxylase Product is decarboxylated  β-oxidation Refsum’s disease – rare, autosomal recessive disorder – deficiency of α-hydroxylase  accumulation of phytanic acid in the plasma and tissue – dietary restriction to halt disease progression  Mitochondrial β-Oxidation Transport of fatty acids into the mitochondrion – Mobilisation – Shuttling Reactions of β-oxidation Energy yield  Long chain fatty acids are the major components of storage triglycerides and dietary fats They are activated to their CoA derivatives in the cytoplasm (thiokinase, requires ATP) and are transported into the mitochondria via the carnitine shuttle  Function : to bring the fatty acyl group across the IMM to the site of β-oxidation, the mitochondrial matrix Carnitine parmatoyl transferase I (CPT I) - catalyses the transfer of the fatty acyl group from CoA to carnitine Carnitine acyl carnitine translocase - translocates the acyl group across the IMM in the form of acyl carnitine ester CPTII - catalyses the transfer of the fatty acyl group from carnitine to CoA present in the mitochondrial matrix.  Genetic defects in carnitine shuttle Congenital absence of carnitine acyltransferase in skeletal muscle CPT-I deficiency CPT-II deficiency Myopathic carnitine deficiency Systemic carnitine deficiency 1. Oxidation produces FADH2 – acyl CoA dehydrogenase 2. Hydration 3. Oxidation produces NADH 4. Thiolytic cleavage (thiolase) releases AcCoA and an acyl CoA that is 2 carbons shorter than the one that entered the cycle  14C 7AcCoA 6 NADH 6 FADH2 112 ATP 10C? 6C? C – C – C – C – C – C – C – C – C – C – C – C – C –C – C – C  Regulation Pyruvate dehydrogenase inhibition mediated by – Fatty acyl carnitine – Acetyl CoA Acetyl CoA Glucose  Acetyl CoA  Malonyl CoA β - oxidation inhibition mediated by – Malonyl CoA  Diseases Acyl CoA dehydrogenase deficiency CH3-(CH2)x-COOH – 3 different isozymes Omega oxidation  dicarboxylic acid HOOC-(CH2)x-COOH  Acidaemia & metabolic acidosis  death Medium chain acyl CoA dehydrogenase (MCAD) deficiency – autosomal recessive – 1:10000 live births – Mutation is 985A>G, 583G>A – Inability to carry out the first step of β-oxidation  ↓ oxidation  hypoglycaemia – Associated with SIDS  Metabolism of Propionyl CoA C – C – C – C – C – C – C – C – C – C – C – C – C –C – C – C – C Propionyl CoA Propionyl CoA carboxylase Important vitamins (biotin, CO2, ATP) Methylmalonyl CoA 1. Biotin Methylmalonyl CoA mutase 2. VitB12 (deoxyadenosylcobalamin) Succinyl CoA TCA Cycle  β-oxidation in peroxisomes Very long chain, branched chain, hydroxylated fatty acids No carnitine required 2 oxidation cycles – Acyl CoA oxidase : FAD  FADH2 – FADH2 oxidised by O2 – O2  H2O2  H2O (catalase) – Formation of β-ketoacyl CoA ↓ energy production Different enzymes Genetic defects = lethal  Zellweger syndrome Peroxisome biogenesis disorder Leukodystrophy SKL receptor defective Deficient peroxisomal functions Enzyme deficiencies may occur Ketogenesis  Balance of CHO and fat metabolism FATTY ACIDS KETOGENESIS LACTATE PYRUVATE ALANINE CITRATE  Utilisation of ketone bodies β-HYDROXYBUTYRATE Heart (preferential use β-Hydroxybutyrate of fatty acid for energy dehydrogenase production) ACETOACETATE Succinyl CoA Brain (obligatory use of 3-ketoacyl CoA glucose for energy transferase Succinate production) ACETOACETYL CoA Both organs can Thiolase reconvert ketone bodies to acetyl CoA for 2 X ACETYL CoA energy production  Summary TRIGLYCERIDES FREE FATTY ACIDS FREE KETONE FATTY BODIES ACIDS ACYL CoA FREE FATTY ACIDS ACYLCARNITINE ACYL CoA ACYLCARNITINE ACYL CoA FADH2 NADH KETONE BODIES ACETYL CoA OXIDATIVE PHOSPHORYLATION ATP Lipid synthesis  Objectives : Anabolism Describe the pathway for fatty acid synthesis – Role of acetyl CoA carboxylase – Role of fatty acid synthase Regulation of fatty acid synthesis Concepts of elongation and desaturation Synthesis of triglycerides Transport of triglycerides Production of cytosolic AcCoA Isocitrate Isocitrate dehydrogenase α-ketoglutarate  Carboxylation of AcCoA Regulation Acetyl CoA Citrate ⊕ Biotin CO2 Insulin ⊕ ATP Acetyl CoA carboxylase ADP +Pi High CHO diet ⊕ Malonyl CoA Θ Malonyl CoA Palmitoyl CoA Θ Epinephrine Θ Glucagon Θ  Carboxylation of AcCoA Regulatory/rate-limiting step 5 regulatory subunits of acetyl CoA carboxylase – A biotin carboxylase – A transcarboxylase – A biotin-carboxylase carrier protein – A molecule of biotin – A regulatory allosteric binding site for citrate Reaction takes place in 3 stages – Carboxylation of biotin – requires ATP – Transfer of carboxyl group to acetyl CoA to produce malonyl CoA – Release of the free enzyme-biotin complex  Acetyl CoA carboxylase regulation Inactive as a phosphorylated monomer Dephosphorylated by protein phosphatase Active in polymeric form (3 or more dephosphorylated monomers  is polymerised to increase activity AMP kinase facilitates phosphorylation into inactivity  Acetyl CoA Carboxylase Deficiency Defective metabolic processes: – ⊗ malonyl CoA synthesis – ⊗ fatty acid synthesis – ⊗ elongation Possible causes – Biotin deficiency – High CHO diet  accumulation of acetyl CoA  Fatty Acid Synthase 7 enzymatic centers + carrier group Dimer – Phosphopantetheine esterified to OH group of serine – Cysteine SH group Regulation ACP Cys Pan SH SH SH SH Pan Cys ACP  Fatty acid synthesis Decarboxylation  Reduction  Dehydration  Reduction  Fatty acid synthesis An acetyl group (from AcCoA) is esterified to ACP to activate the fatty acid synthase complex The acetyl group is transferred to Cys-SH carrier group Malonyl CoA esterified to ACP MalonylCoA is decarboxylated, then condenses with the acetyl group from Cys-SH Reduction – gain of H Dehydration – introduces unsaturation into the molecule Reduction – gain of H 4C molecule generated is transferred to the Cys-SH and the ACP can accept another malonyl group The process is repeated until a fatty acid of desired length is formed (16C palmitate) The fatty acid is cleaved from fatty acid synthase  Overall reaction for palmitic acid 8 AcCoA + 14 NADPH + 14 H + 7 ATP Palmitic acid + 8 CoA + 14 NADP+ + 7 ADP + 7Pi + 7 H2O ELONGATION DESATURATION ER – fatty acid elongase Desaturase Mitochondrion  Fat storage (triacylglycerol) Glycerol-3-Phosphate  TAG synthesis Fatty acids are esterified through their COO- group resulting in a loss of negative charge and formation of neutral fat In triacylglycerols the glycerol molecule is esterified to 3 fatty acid molecules Glycerol phosphate is the initial acceptor of fatty acids during TAG synthesis. Glycerol can be produced from glucose, using first the reactions of the glycolytic pathway to produce DHAP, next DHAP is reduced by glycerol phosphate dehydrogenase to glycerol phosphate A second pathway found in the liver but not in aduipose tissue uses glycerol kinase to convert free glycerol phosphate  Fate of Phosphatidic Acid Cardiolipin TAG Phospholipids  Dipalmitoyl Phosphatidylcholine + Phosphatidylglycerol Pulmonary surfactant ACUTE PULMONARY DISTRESS SYNDROME Thank you for your attention