CHAPTER 15 (LIPID METABOLISM)-1.pdf

LIPID METABOLISM  Introduction  Lipid plays essential role in cell structure and metabolism.  Triacylglycerols are main storage form of metabolic energy in animals.  Cholesterol is an important component of the cell membrane and precursor for steroid hormones and bile salts....

LIPID METABOLISM  Introduction  Lipid plays essential role in cell structure and metabolism.  Triacylglycerols are main storage form of metabolic energy in animals.  Cholesterol is an important component of the cell membrane and precursor for steroid hormones and bile salts.  Triacylglycerols (fats or triglicerides) made up of about 90% dietary lipids and are major form of metabolic storage energy (consist of FA eg. palmitic and oleic acids).  Major forms of energy – glycogen and triacylglycerol (fat is utilized when glucose is not available). What is a Lipid?  A chemically diverse group of compounds which are not soluble in water but soluble in non-polar organic solvents.  Usually related to fatty acids. What is a Fatty Acid?  A long hydrocarbon chain (containing carbon & hydrogen) with a terminal carboxyl (COOH) group.  General Formula CH3(CH2)nCOOH In what forms are fatty acids found in the body?  Triacylglycerol – adipose tissue  Phospholipids – major component of all membranes  Free fatty acids – Non-esterified fatty acids (NEFA) – plasma  Glycolipids  Cholesterol ester What are the major biological roles of fatty acids?  Energy storage and production.  Protection/Insulation  Essential components of biological membranes – isolate cell/organelles from outside environment – allow communication with outside environment.  Precursors of other bioactive molecules – Eicosanoids (prostaglandins, prostacyclins, thromboxanes & leukotrienes) Fatty Acids Can Differ in 4 Ways  Chain Length.  No. of double bonds. 1 = monounsaturated >1 = polyunsaturated  Position on the chain of the double bonds.  Type of double bond (cis or trans).  No double bond present = saturated FA Fatty acid composition of foods Fatty acid synthesis  Synthesized through the addition of 2 carbon units to the growing end of hydrocarbon chain.  The growing FA chain is attached covalently to acyl carrier protein (ACP).  The linkage is a thioester (esterification of carboxylic acid and thiol-organosulfur containing carbon bonded with sulfhydryl) like in acetyl CoA.  Takes place in the cytosol.  Adult mammals – liver cells and adipocytes.  Some takes place at special organs – mammary glands during lactation. Fatty acid synthesis First step : Production of acetyl ACP and malonyl ACP from acetyl CoA Involves condensation of acetyl and malonyl resulting in 4 carbon precursor (acetoacetyl ACP) and CO2. This precursor functions as primer for elongation of FA. Malonyl ACP is the substrate of FA synthesis. Synthesized is 2 steps: 1) Carboxylation of acetyl CoA forming malonyl CoA. 2) Synthesis of malonyl ACP by the transfer of malonyl from coenzyme A to ACP. Fatty acid synthesis 2nd step : Production of Acetoacetyl ACP. Acetoacetyl ACP is the smallest version of 3-ketoacyl moiety version. Synthesis of long chain FA begins with this molecule. Formed by condensation of 2 carbon substrate (acetyl coA or acetyl ACP) and 3 carbon substrate (malonyl ACP) with by product of CO2. Fatty acid synthesis Next step : Elongation step Malonyl ACP donates 2 Carbon units to acyl ACP and produces CO2 as the by product. (condensation). This generates 3-ketoacyl ACP 3-ketoacyl ACP (keto group at C3) goes through two reduction reactions and a dehydration process. Generates a longer acyl ACP. This acyl ACP becomes the substrate of further condensation process. Fatty acid synthesis Final step : Final product of saturated FA synthesis is 16 and 18 carbon FA chain. Longer chain is not possible because this long chain of FA cannot be accommodated on the binding site of enzyme. Long FA chain is released from ACP generating HS-ACP. Catalyzed by thioesterase. The Elongation Step Fatty Acid Synthesis Fatty acids are synthesized by the repetition of the following reaction sequence: condensation, reduction, dehydration, and reduction. The intermediates shown here are produced in the first round of synthesis. 2 carbons are added in each cycle. Biochemistry. 5th edition. Berg JM, Tymoczko JL, Stryer L. New York: W H Freeman; 2002. Synthesis takes place in the cytosol, in contrast with degradation, which takes place primarily in the mitochondrial matrix. Intermediates in fatty acid synthesis are covalently linked to the sulfhydryl groups of an acyl carrier protein (ACP), whereas intermediates in fatty acid breakdown are covalently attached to the sulfhydryl group of coenzyme A. The enzymes of fatty acid synthesis in higher organisms are joined in a single polypeptide chain called fatty acid synthase. In contrast, the degradative enzymes do not seem to be associated. The growing fatty acid chain is elongated by the sequential addition of two-carbon units derived from acetyl CoA. The activated donor of two carbon units in the elongation step is malonyl ACP. The elongation reaction is driven by the release of CO2. The reductant in fatty acid synthesis is NADPH, whereas the oxidants in fatty acid degradation are NAD+ and FAD. Elongation by the fatty acid synthase complex stops on formation of palmitate (C16). Further elongation and the insertion of double bonds are carried out by other enzyme systems.  Fatty Acid Synthase Inhibitors May Be Useful Drugs  Fatty acid synthase is overexpressed in some breast cancers. Researchers intrigued by this observation have tested inhibitors of fatty acid synthase on mice to see how the inhibitors affect tumor growth.  A startling observation was made: mice treated with inhibitors of the condensing enzyme showed remarkable weight loss.  The results of additional studies revealed that this inhibition is due, at least in part, to the accumulation of malonyl CoA.  Thus, fatty acid synthase inhibitors are exciting candidates both as antitumor and as anti-obesity drugs. Synthesis of Cholesterol  Steroid cholesterol is an essential component of many membranes.  Also a precursor of steroid hormones and bile salts for mammals.  All carbon atoms of cholesterol comes from acetyl CoA.  The stages of the cholesterol biosynthesis pathway is Lipids transported in lipoprotein complexes  Lipid digestion products absorbed by the intestinal mucosa converted by these tissues to triacylglycerol and packed into lipoprotein particles called chylomicrons.  They are released to the bloodstream to the tissues.  Triacylglycerols synthesized by the liver are packed into very low density lipoproteins (VLDL) and released into the blood.  The triacylglycerol components of chylomicrons and VLDL are hydrolized to FA and glycerol in the capillaries of adipose tissue and skeletal muscle by lipoprotein lipase.  The resulting free FA are taken up by these tissues while glycerol is transported to the liver or kidneys. β-Oxidation Fatty acid oxidation  FA released from triacylglycerols by a degradation pathway that removes 2 carbon units at each step.  The 2 carbon units will be transferred to coenzyme A forming acetyl coA and the rest will go through another cycle of degradation in the oxidative pathway.  This is known as β-oxidation as the β-carbon unit (C-3) of the FA is being oxidized.  Process has 2 stages;  a) activation of FA  b)degradation of 2 carbon units Fatty acid oxidation  Oxidation of FA produces NADH and ubiquinol (QH2) – utilised in the electron transport chain.  Acetyl-CoA – citric acid cycle  Carbon atoms from FA can be used as substrates for amino acid synthesis.  Organisms have glyoxylate pathway, acetyl CoA can be used for synthesis of glucose (which pathway?).  FA metabolism supplies fuel for metabolism and source of energy.  In eukaryotes, β-oxidation occurs in the mitochondria, whereas in bacteria, it occurs in the cytosol.  β-Oxidation First step: Oxidation  acyl-CoA dehydrogenase catalyzes the formation of double bond between C2 and C3 atoms of the acyl group forming trans 2-enoyl CoA.  Double bonds formed – electrons from fatty acyl CoA transferred to acyl CoA dehydrogenase electron transfering flavoprotein, (ETF) then finally passed to ubiquinon (ox) (QH2) – oxidized by membrane associated electron transport chain.  β-Oxidation 2nd step: Hydration  Water added to trans 2-enoyl CoA forming L-3-Hydroxyacyl CoA.  Catalyzed by 2-enoyl-CoA hydratase.  β-Oxidation 3rd step: Oxidation  L-3-Hydroxyacyl CoA dehydrogenase catalyze the formation of 3-ketoacyl CoA  with the use of NAD+.  NADH –used in electron transport chain.  β-Oxidation Final step: Thiolysis  The nucleophilic sulfhydryl group of HS-CoA attacks the carbonyl carbon of 3-ketoacyl CoA.  Catalyzed by 3-ketoacyl CoA thiolase.  Releases acetyl CoA and long FA shorter by 2 carbons. Transport of Fatty Acyl CoA into mitochondria Long chain FA cannot enter mitochondia matrix through the mitochondria inner membrane. Transport system aka carnitine shuttle system functions to transport FA into mitochondria. In the cytosol, the acyl group of fatty acyl-CoA is transferred to the hydroxyl site of L-Carnitine forming acylcarnitine. Catalyzed by Carnitine acyltransferase I – associated to the mitochondrial outer space. Transport of Fatty Acyl CoA into mitochondria Acylcarnitine enters the mitochondrial matrix in exchange with free carnitine via a translocase enzyme. In the matrix, carnitine acyltransferase II catalyze the formation of fatty acyl CoA releasing carnitine back to the intermembrane space. The carnitine shuttle system is not used in most eukaryotes where FA oxidation occurs in the peroxisomes. In prokaryotes? β-Oxidation of Odd-Chain and Unsaturated FA  Most fatty acids have even-number of carbon units.  However, odd FA chains usually are synthesized by bacteria and some organisms.  These FA chains are oxidised the same way as the even- chain FA except the final product of the thiolytic cleavage is propionyl CoA (CoA with C3 acyl group) rather than acetyl Co A.  Propionyl –CoA can be converted to succinyl CoA in mammals. ATP generation from FA oxidation  Supplies more energy than the same amount of glucose. Stearate (18 C) is converted to Stearoyl CoA using 2 ATP. 9 molecules of acetyl CoA enters the TCA cycle (one molecule gives 10 ATP). Through 8 cycles of β-oxidation. How many ATP’s generated?.....do your own reading and calculation