PHTH1011 005 Mobilization of Fatty Acids for Oxidation PDF

Recap on naming of fatty acids ▪ The fatty acid name is derived from the parent hydrocarbon by substitution of oic for the last e. ▪ In common nomenclature, the suffix is usually -ic. 1. From Butane -Butanoic acid CH3(CH2)2COOH 2. from Hexane-Hexanoic acid CH3(CH2)4COOH Recap on naming of fatty acid...

Recap on naming of fatty acids ▪ The fatty acid name is derived from the parent hydrocarbon by substitution of oic for the last e. ▪ In common nomenclature, the suffix is usually -ic. 1. From Butane -Butanoic acid CH3(CH2)2COOH 2. from Hexane-Hexanoic acid CH3(CH2)4COOH Recap on naming of fatty acids ▪ The C18 saturated fatty acid is called Octadecanoic Acid from parent hydrocarbon ▪ The C18 unsaturated fatty acid called Octadecenoic Acid ▪ The C18 unsaturated fatty acid called Octadecadienoic Acid ▪ The C18 unsaturated fatty acid called Octadecatrienoic Acid Octadecane with 1 double bond is with 2 double bond is with 3 double bond is Fatty Acid Oxidation (β Oxidation of Fatty Acids) Definition of β-Oxidation ▪ Beta oxidation is the process by which Fatty acid molecules, are broken down in Mitochondria and or peroxisomes to generate Acetyl-CoA for the Krebs Cycle. Mobilization of Fatty Acids for β Oxidation ▪The primary sources of fatty acids for oxidation are dietary and mobilization from cellular stores. ▪ Fatty acids from the diet can are delivered from the gut to cells via transport in the blood. ▪Fatty acids are stored in the form of triacylglycerols primarily within adipocytes of adipose tissue. ▪ In response to energy demands, the fatty acids of stored triacylglycerols can be mobilized for use by peripheral tissues. ▪ The release of metabolic energy, in the form of fatty acids, is controlled by a complex series of interrelated cascades that result in the activation of hormonesensitive lipase. Hormones and the release of metabolic energy ▪ Glucagon, epinephrine or β-corticotropin stimulute the activation of this cascade, in adipocytes. ▪ These hormones bind cell-surface receptors that are coupled to the activation of adenylate cyclase upon ligand binding. ▪ The resultant increase in cAMP leads to activation of PKA, which in turn phosphorylates and activates hormonesensitive lipase. ▪ Hormone sensitive lipase hydrolyzes fatty acids from carbon atoms 1 or 3 of triacylglycerols Generation of fatty acids from triacylglycerols for β-Oxidation ▪ Hormone-sensitive lipase hydrolyzes FA from triacylglycerols ▪ The resulting diacylglycerols are substrates for either hormonesensitive lipase or for the non-inducible enzyme diacylglycerol lipase. ▪ Finally the monoacylglycerols are substrates for monoacylglycerol lipase. ▪ The net result of the action of these enzymes is three moles of free fatty acid and one mole of glycerol. Epinephrine binds its' receptor and activates adenylate cyclase. The increase in cAMP activates PKA which then phosphorylates hormone-sensitive lipase. This hydrolyzes FA from triacylglycerols and diacylglycerols. Finally FA is released from monoacylglycerols through the action of monoacylglycerol lipase. Inhibition of the mobilization of Fatty acids ▪ The hormonal activation of adenylate cyclase and hormone-sensitive lipase generate FA from triglyceride. ▪ In adipocytes, the mobilization of fat from adipose tissue is inhibited by numerous stimuli. ▪ The most significant inhibition is that exerted upon adenylate cyclase by insulin. When an individual is well fed state, insulin released from the pancreas prevents the inappropriate mobilization of stored fat. ▪ Excess fat and carbohydrate are incorporated into the triacylglycerol pool within adipose tissue. Activation of fatty acid in the cytoplasm as a prelude to β -oxidation ▪ Fatty acids is activated in the cytoplasm before oxidation in the mitochondria. ▪ Activation is catalyzed by fatty acyl-CoA ligase (also called acyl-CoA synthetase or thiokinase). ▪ The net result of this activation process is the consumption of 2 molar equivalents of ATP. Fatty acid + ATP + CoA -------> Acyl-CoA + PPi + AMP Fatty acids must be transported to Mitochondria before β -Oxidation occurs ▪ Oxidation of fatty acids occurs in the mitochondria. ▪ The transport of fatty acyl-CoA into the mitochondria is via an acyl-carnitine intermediate, which itself is generated by the action of carnitine acyltransferase I, an enzyme that resides in the outer mitochondrial membrane. ▪ The acyl-carnitine molecule is then transported into the mitochondria where carnitine acyltransferase II catalyzes the regeneration of the fatty acyl-CoA molecule. Transport of fatty acid into the mitochondria ▪ Fatty acids is transported from the cytoplasm to the inner mitochondrial space for oxidation. ▪ Following activation to a fatty-CoA, the CoA is exchanged for carnitine by carnitinepalmitoyltransferase I. ▪ The fatty-carnitine is then transported to the inside of the mitochondrion where a reversal exchange takes place through the action of carnitinepalmitoyltransferase II. ▪ Once inside the mitochondrion the fatty-CoA is a substrate for the β-oxidation machinery. Reaction Sequences of β Oxidation. (Reactions take place in the mitochondrial matrix) ▪ Acyl-CoA dehydrogenation occurs between the α and β carbons (C2 and C3) in a FAD-linked reaction catalyzed by acyl CoA dehydrogenase. ▪ The product contains a Trans-double bond. Involvement of the β-carbon in this and subsequent steps gives the pathway its name. The product is trans Δ 2- enoyl-CoA 2nd Step in β-Oxidation Hydration of the double bond is catalyzed by enoyl CoA hydratase. The product is an L-3hydroxyacyl CoA. 3rd step in β-Oxidation( 2nd Dehydrogenation ) A, 2nd dehydrogenation of the alcohol occurs in a NAD-linked reaction catalyzed by βhydroxyacyl CoA dehydrogenase. The product is a ketone. FORMATION OF CoA Thiolytic cleavage of the thioester is catalyzed by β-ketoacyl CoA thiolase. Products of β-Oxidation ▪ Acyl CoA and a long chain fatty acyl CoA that is 2 carbons shorter than the original fatty acyl CoA. ▪ The shortened fatty acyl group is now ready for another round of β-oxidation. ▪ After the fatty acyl CoA has been reduced to acetyl or propionyl CoA, β-oxidation is complete. Regulation ▪ This reaction is inhibited by high concentrations of acetyl CoA. ▪ β-Oxidation is regulated as a whole primarily by fatty acid availability ▪ Once fatty acids are in the mitochondria they are oxidized so long as there is adequate NAD+ and CoA. Energy yield from fatty acid oxidation ❑ Each round of β-Oxidation produces: 1. 1 mole of NADH 2. 1 mole of FADH2 3. 1 mole of acetyl-CoA → enters the TCA and is oxidized to CO2 with the concomitant generation of : 4. 3 moles of NADH 5. 1 mole of FADH2 6. 1 mole of ATP. The NADH and FADH2 from the TCA enter the respiratory pathway for the production of ATP. Comparison of yields (fatty acids : carbohydrates) ▪ The oxidation of fatty acids yields significantly more energy per carbon atom than does the oxidation of carbohydrates. ▪ The net result of the oxidation of one mole of oleic acid (an 18-carbon fatty acid) will be 146 moles of ATP (2 mole equivalents are used during the activation of the fatty acid). ▪ 114 moles of ATP will be generated from an equivalent number of carbohydrate carbon atoms. B-Oxidation of fatty acids with odd number of carbons ▪ The majority of natural lipids contain an even number of carbon atoms. ▪ Fatty acids with an odd number of carbon: upon complete β-oxidation yield acetyl-CoA units + 1mole of propionyl-CoA. ▪ The propionyl-CoA is converted, in an ATP-dependent pathway, to succinyl-CoA. ▪ The succinyl-CoA then enters the TCA cycle for further oxidation. β -Oxidation of lipids with odd number of carbons requires additional enzymes: Fatty acids with an odd number of carbons in their chains require a means of handling the three-carbon propionyl CoA that is the final fragment produced by β-oxidation of such a chain: 1. Carboxylation (1st step in B-Oxidation of lipid with odd number of carbons) The first step is carboxylation by the biotindependent propionyl CoA carboxylase in an ATPrequiring reaction. 2. Intermediate step in B-Oxidation (FA with odd number of carbons) The D- isomer, which is the product, is then converted to the L- isomer by methylmalonyl CoA racemase. 3. In the final step, the L- isomer is converted to succinyl CoA by methylmalonyl CoA mutase. (Succinyl CoA can then be metabolized via TCA) Cis double bond has to be converted to trans before β -Oxidation occurs ▪ The action of enoyl CoA isomerase is required to handle double bonds at odd-numbered carbons because β-oxidation generates or requires pre-existing double bonds at even-numbered carbons. ▪ If there is a double bond at an odd-numbered carbon (e.g., 18:1 9), the action of enoyl CoA isomerase is required to move the naturally occurring cis- bond and convert it to the trans- bond used in β-oxidation. β-Oxidation of unsaturated fatty acids ▪ The oxidation of unsaturated fatty acids is essentially the same process as for saturated fats. ▪ When a double bond is encountered. The bond is isomerized by a specific enoyl-CoA isomerase and oxidation continues. Clinical Significance of Fatty Acids (diseases) ▪ Deficiencies in Carnitine: Deficiencies in carnitine lead to an inability to transport fatty acids into the mitochondria for oxidation. ▪ This can occur in newborns and particularly in pre-term infants. Carnitine deficiencies are also found in patients undergoing hemodialysis or exhibiting organic aciduria. ▪ Carnitine deficiencies may manifest systemic symptomology or may be limited to only muscles. ▪ Symptoms can range from mild occasional muscle cramping to severe weakness or even death. Treatment is by oral carnitine administration. Clinical Significance of Fatty Acid (diseases) Carnitine Palmitoyltransferase I (CPT I) Deficiency: Deficiencies in this enzyme affect primarily the liver and lead to reduced fatty acid oxidation and ketogenesis. Carnitine Palmitoylransferase II (CPT II) deficiency results in recurrent muscle pain and fatigue and myoglobinuria following strenuous exercise. Carnitine acyltransferases may also be inhibited by sulfonylurea drugs such as tolbutamide and glyburide. Clinical Significance of Fatty Acid ▪ Deficiencies in Acyl-CoA Dehydrogenases: A group of inherited diseases that impair β-oxidation. This results from deficiencies in acyl-CoA dehydrogenases. ▪ Medium Chain Acyl CoA Dehydrogenase Deficiecy is the most common form of acyl-CoA dehydrogenase deficiency. ▪ In the first years of life this deficiency will become apparent following a prolonged fasting period. ▪ Symptoms include vomiting, lethargy and frequently coma. Excessive urinary excretion of medium-chain dicarboxylic acids as well as their glycine and carnitine esters is diagnostic of this condition. ▪ In the case of this enzyme deficiency taking care to avoid prolonged fasting is sufficient to prevent clinical problems. Clinical Significance of Fatty Acid (diseases) Refsum's disease is a rare inherited disorder in which patients lack the mitochondrial α-oxidizing enzyme. Patients accumulate large quantities of phytanic acid in their tissues and serum. Severe symptoms: such as cerebellar ataxia, retinitis pigmentosa, nerve deafness and peripheral neuropathy. The restriction of dairy products and ruminant meat from the diet can ameliorate the symptoms of this disease.