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* Lipid Biosynthesis

* Lipid Biosynthesis - These are endergonic and reductive reactions, use ATP as source of energy and reduced electron carrier usually NADPH as reductant. * Fatty acid synthesis: - F.A synthesis is not the reversal of the degradative pathway, using different sets of enzymes.

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* Lipid Biosynthesis

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  1. * Lipid Biosynthesis • - These are endergonic and reductive reactions, use ATP as source of energy and reduced electron carrier usually NADPH as reductant. • * Fatty acid synthesis: • - F.A synthesis is not the reversal of the degradative pathway, using different sets of enzymes. • 1-synthesis in cytosol, degradation in mitochondria (mitochondrial matrix) • 2-intermediates of F.A synthesis are covalently linked to -SH group of ACP, at higher organism single polypeptide called fatty acid synthase. (while in F.A degradation are bonded to CoA) • 3-the growing F.A is elongated by sequential addition of 2-carbon units. • 4-the reductant in fatty acid synthesis is NADPH, while the oxidant in F.A degradation are NAD+, FAD • 5-elongation of F.A is stopped at C16 and further elongation or insertion of double bonds are carried by other enzyme systems. • * Large proportion of F.A used in the body is supplied by diet excess CHO and protein are converted into F.A. • - F.A are synthesized mainly in liver and lacting mammary gland and to lesser extent in adipose tissue and kidney.

  2. -oxidation of Saturated Fatty Acids * Fatty acids are activated and transported into mitochondria * F.As enter to cytosol from the blood and should move to mitocondrial matrix where the enzymes are exist in three steps “CARNITINE SHUTTLE” F.A fatty acyl CoA This process occurs in the outer mitocondrial membrane Found at outer mitocondrial membrane The hydrolysis of Pull the rxn to the formation of fatty acyl CoA

  3. * Fatty acyl CoA esters : don’t cross the inner mitocondrial membrane intact. But it binds to the -OH group of Carnitine to form fatty acyl-Carnitine complex this complex enters the matrix by facilitated diffusion. In the matrix Carnitine and fatty acyl CoA are regenerated. Acyl-Carnitine / Carnitine transporter Inner face of the membrane Outer face of the membrane Inner membrane • Entrance of F.A to mitocondrial matrix(Carnitine shuttle) • 1- Esterification to CoA • 2- Transestrification to Carnitine followed by transport. • 3- Transestrification back to CoA.

  4. * The -oxidation of saturated F.A has four basic steps • 1- Dehydrogenation (oxidation) • - 3 isomers of AD • a) LCAD(long chain acyl-CoA dehydrogenase) • act on fatty acids 12 - 18 carbons • b) MCAD : 4 - 14 C • c) SAD : 4 - 8 C • - AD bound to the inner membrane of mitochondria • - AD has FAD as prosthetic group • 2- Hydration (addition of water) • - water is added to the double bond. • 3- Dehydrogenation (oxidation) • - the enzyme is specific only for L-isomer • NADH NAD+ and e- are transported to O2 to produce ATP via e-transport chain • NADH NADH dehydrogenase (complex I) • 4- Thiolysis • - free CoA-SH split off the carboxy terminal two carbons 1-oxidation 2e- electron transfer chain 2-Hydration 3-oxidation 4-Thiolysis Thio ester of fatty acid + 2 carbon

  5. 1-oxidation 2e- electron transfer chain 2-Hydration 3-oxidation 4-Thiolysis Thio ester of fatty acid + 2 carbon

  6. -oxidation of Fatty acids

  7. In each cycle of -oxidation one acetyl-CoA, 2 pairs of e- and 4H+ are removed from the fatty acid C16-CoA + CoA + FAD + NAD+ + H2O C14-CoA + acetyl-CoA + FADH2 + NADH + H+ Palmitoyl CoA 7 cycles of -oxidation 8 acetyl CoA + 7 FADH2 + 7 NADH + 7H+ - The four steps are repeated 7 times

  8. Are oxidized to CO2 in the mitocondrial matrix 7 cycles 8 acetyl CoA 16 C palmitic acid * Mitocondrial oxidation of F.A - oxidation: four reactions results in shortening the F.A by 2 carbon units, and these four steps are repeated until the complete degradation of F.A * The energy released by fatty acid oxidation is conserved as ATP

  9. * Energy yields from complete oxidation of palmitic acid • (129 ATP resulted from complete oxidation) • Palmitoil-CoA + 7 CoA + 7 FAD + 7 NAD+ + 7 H2O • 8 acetyl-CoA + 7 FADH2 + 7 NADH + 7 H+ • acyl-CoA dehydrogenase 7 FADH2 • - hydroxyacyl-CoA dehydrogenase 7 NADH • 8 acetyl CoA : citric acid cycle produce • isocitrate dehydrogenase 8 NADH • ketoglutirate dehydrogenase 8 NADH • succinyl CoA-CoA synthatase 8 GTP • succinate dehydrogenase 8 FADH2 • malate dehydrogenase 8 NADH • Total = 31 NADH + 15 FADH2 + 8 ATP • 93 + 30 + 8 = 131 ATP • - For each fatty acid to be activated by thiokinase (fatty acyl CoA synthetase) two high energy bonds are consumed and these should be considered • So 131 - 2 = 129 ATP 24 NADH + 8 FADH2 + 8 GTP 8 ATP

  10. Energy yields from complete oxidation of palmitic acid

  11. Complete oxidation of odd number of Fatty acids (require additional three reactions) * F.A with an odd number of C are common in plants. * Odd F.A oxidized normally by - oxidation, in the last step, Propionyl CoA and acetyl CoA are produced. biotin Propionyl CoA carboxylase Methylmalonyl CoA epimerase Coenzyme B12 Methylmalomyl CoA mutase

  12. - oxidation of polyunsaturated F.A (18 : 2 9,12 linoleate) need additional two enzymes Is removed by continuing the - oxidation. Cis 4 First oxidation step of the second cycle FAD Acyl CoA dehydrogenase FADH2

  13. Is not a substrate forenoyl-CoA hydratase

  14. * oxidation of unsaturated Fatty acids need additional reactions (Monounsaturated) * F.A with double bond is in Cis- configuration can’t acted by enoyl-CoA hydratase that add H2O to the Trans double bond of 2 enoyl-CoA generated during - oxidation. Oxidation of oleate 18 C 18 : 19 Is not a substrate for enoyl-CoA hydratase This is a substrate for hydratase enzyme and - oxidation is continued

  15. -oxidation of unsaturated F.A

  16. * Regulation of fatty acid oxidation • - In the liver : • Fatty CoA formed in the cytosol has two routs • A. enter the mitochondria - oxidation for energy. • B. Synthesis of triglycerols. This depends on the rate of transfer of fatty CoA into the mitochondria. • Malonyl CoA : the first intermediate in the cytosolic synthesis of F.A from acetyl CoA • When there is no need for energy, high level of glucose increase acetyl CoA increase malonyl CoA that inhibit the Carnitine acyl transferase I. • Oxidation of fatty acid is inhibited whenever there is excess CHO and glucose and the extra glucose is converted into fatty acid. While excess fatty acid can’t be converted to glucose . • Also when NADH / NAD+ ratio is high inhibition of - hydroxyacyl CoA dehydrogenase. • When high concentration of acetyl CoA inhibition of thiolase.

  17. Ketone Bodies * Acetyl CoA in the liver cells can enter the citric acid cycle, or can be converted into ketone bodies. synthesized in the liver to be exported to other tissues by the blood, then they can be oxidized by citric acid cycle. Produced in smaller amounts and exhaled • The brain uses glucose as fuel • if it is not available it can use acetoacetate and -hydroxybutyrate. * Ketone bodies can be used as fuel for heart, skeletal muscles and kidney.

  18. Ketone bodies formed in the liver are exported to other organs

  19. *Acetone is formed in very small amount in healthy people. * Untreated diabetes or (starvation) large quantities of acetoacetate is produced increase acetone amount odor to the breath. Or even spontaneous decarboxylation Specific for D not L

  20. Extra hepatic tissues use ketone bodies as fuels Acetoacetate and - hydroxybutyrate is converted into 2 acetyl CoA that enter the citric acid cycle to produce energy.

  21. Extra hepatic tissues use ketone bodies as fuels

  22. Over production of ketone bodies during the diabetes and starvation * During starvation, gluconeogenesis which depleted citric acid cycles intermediates diverting the acetyl CoA to ketone body production. * In diabetes, low level of insulin Tissues can’t uptake glucose to use as fuel Malonyl CoA is not formed the entrance of fatty acids into mitochondria are not inhibited accumulation of acetyl CoA that can’t go through citric acid cycle accelerate the ketone body production, causing low pH and keto acidosis or ketosis.

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