Integrated Metabolism in Tissues

1. Integrated Metabolism in Tissues

2. Overview: • Catabolism of TAGs • Oxidation and Synthesis of Fatty Acids • Transfer of Acyl-CoA • Ketone Bodies • Catabolism of Cholesterol • Essential Fatty Acids

3. Catabolism of TAGs and Fatty Acids • The complete hydrolysis of Triacylglycerols gives us: • a glycerol and three fatty acids.

4. How does this happen? • Hydrolysis occurs through: • Lipoprotein lipase: non-hepatic tissue • Intracellular lipase: in liver and adipose tissue • Activated by epinephrine, norepinephrine, glucagon and ACTH via cAMP • Activated lipase hydrolyzes one fatty acid at a time

5. Glycerol • Glycerol is used by the liver for energy • Glycerokinase converts glycerol to glycerol phosphate • Glycerol phosphate can enter the glycolytic pathway •  Energy oxidation • or  Gluconeogenesis • (Adipose tissue cannot metabolize glycerol)

6. *Fatty acids are a rich source of energy • Process: • Fatty acids enter cell • Activated by Coenzyme A  Acetyl-CoA (using 2 ATP’s) • Catalyzed by Acyl-CoA synthetase • Pyrophosphate produced quickly hydrolyzed = irreversible reaction

7. Mitochondrial Transfer of Acyl-CoA • Fatty acid oxidation occurs in mitochondrial matrix • Energy produced through Oxidative Phosphorylation* • S-C Fatty acids pass directly into mitochondrial matrixAcyl-CoA derivatives • *L-C Fatty acids and the CoA derivatives cannot • -Carnitine, CAT 1, CAT 2

8. Mitochondrial transfer of acyl-cOA

9. Beta-Oxidation of Fatty Acids • *Breakdown of fatty acids into acetyl-CoA • Mitochondrion • *Cyclic Degradative Pathway • *Dehydrogenases • Long fatty acids • Short fatty acids

12. Beta oxidation • Dehydrogenation forms a double bond between alpha and beta carbons • Hydrogenation to unsaturated acyl-coa • B-hydroxy group oxidized to ketone by NAD+ • B-ketoacyl-CoA cleaved resulting in the insertion of CoA and cleavage of B-carbon • Products are acetyl-CoA that enters Krebs cycle • And saturated coA-activated fatty acid with 2 fewer carbons that continues the b-oxidation cycle

13. *Beta-Oxidation not regulated except by TAG lipase • Even number carbons due to 2 carbon loss at a time • 16 carbons= 8 Acetyl-CoA molecules produced • If fatty acid has an uneven # carbons, B12 and Biotin required to oxidize • Unsaturated fatty acid oxidation

14. energy produced • Each cleavage of saturated carbon-carbon bond  4 ATPs produced • For each Acetyl-CoA oxidized 10 ATP produced • The complete B-oxidation of one palmitic acid, including the oxidation of the FADH2 and NADH produced during this cycle yields about 106 molecules of ATP. *

15. Formation of Ketone Bodies* • Another way for Acetyl-CoA to catabolize in liver • Ketogenesis- ketone bodies formed • Ketone bodies are three chemicals that are produced as by-products when fatty acids are broken down for energy. • Only in Mitochondria

16. Ketone body formation normally very low in blood. • Situations of accelerated fatty acid oxidation with low-carb intake => very high levels (Starvation, Low-carb diet, or diabetes)*: • As carb intake diminishes, oxidation of fatty acids accelerates to provide energy through production of TCA substrates (acetyl-CoA) • *Shift to fat catabolism  accumulation of Acetyl-CoA • Ketosis

17. Catabolism of Cholesterol

18. cholesterol • Cholesterol is not an energy producing nutrient • Its four ring structure remains intact through catabolism, eliminated through billary system. • The biliary system creates, transports, stores, and releases bileinto the duodenumto help in digestion. • The biliary system includes the gallbladder, bile ducts and certain cells inside the liver, and bile ducts outside the liver.

19. *Delivery Excretion • Delivered to the Liver • In the form of Chylomicron Remnants • & LDL-C and HDL-C • (low density lipoprotein cholesterol, high density lipoprotein cholesterol) • 2 ways: • 1. Hydrolyzed by esterases to free form • -secreted directly into bile canaliculi • Converted into bile acids before entering the bile

20. Metabolic ChangesCholesterol to Bile Acid • Key Metabolic Changes: • Hydrocarbon Side Chain reduction at C17 • Carboxylic Acid addition on shortened chain • Hydroxyl group addition to ring system of molecule • Effect of these is to enhance water solubility of sterol facilitating its excretion in the bile • Enterohepatic circulation can return absorbed bile salts to the liver • *Hypercholesterolemia treated with removal of bile salts

21. Fatty Acid Synthesis • Non Essential Fatty Acids can be synthesized from simple precursors • Assembly of starter molecule • Acetyl-CoA and Malonyl-CoA • Acetyl-CoA + CO2 = Malonyl-CoA • Occurs in Cytosol • Catalyst- Acetyl-CoA carboxylasehas biotin as prosthetic group= “carboxylation”: Incorporates carboxyl group into a compound using ATP

22. Acetyl-Coa production & movement to cytosol • Production mostly occurs in mitochondria from pyruvate oxidation, oxidation of fatty acids and degradation of some amino acids • Some formed in cytosol through amino acid catabolism. • Fatty acid synthesis localized in cytosol, but acetyl-CoA produced in matrix is unable to exit through mitochondrial membrane. • Acetyl-CoA gets to cytosol by reacting with oxoloacetate to form citrate, which can pass through inner membrane. • Citrate lyase converts the citrate back to oxaloacetate and acetyl-CoA.

23. Mitochondrial matrix transfer • http://www.dnatube.com/video/641/Fatty-Acid-Biosynthesis

25. Fatty acid synthase system • Enzymes involved in fatty acid synthesis arrangement. • In cytosol • *Enzymes: ACP (Acyl Carrier Protein) & CE (Condensing Enzyme) • Both have free SH group that Acetyl-CoA and Malonyl- CoA attach to before synthesis can begin • Acetyl-CoA transferred to ACP, losing its CoA  Acetyl-ACP • Acetyl group then transferred again to SH of CE leaving ACP-SH • Malonyl group attaches to this molecule, losing it’s CoA • Now the fatty acid chain can be extended

26. Starter molecule

28. Steps of Chain Elongation • Carbonyl carbon of acetyl group to C2 of Malonyl-Acp, lose CO2 with malonyl carboxyl group • B-Ketone reduce using NADPH (from PPS) • Alchohol dehydrated  double bond • Double bond reduced to butyryl-ACP from NADPH • Butyryl transferred to CE exposing ACP SH site to a 2ndmalonyl-coa molecule • The second malonyl-coA condenses with ACP • Second condensation rxn takes place, with coupling of butyryl group on the CE to C2 of malonyl-ACP. 6C chain reduced and transferred to CE in a repetition of steps 2-5. • The cycle repeats to form a c16 fatty acid (palmitic)

29. *Essential Fatty Acids • Humans cannot introduce double bonds beyond D-9 site • Linoleic and alpha linoleic- Plant products • Prostaglandins, Thromboxanes and Leukotriene's can be formed from LA (n-6) (favored in the western diet) & ALA (n-3)

30. EFA’s Metabolism and role • EFA’s enter Smooth ER for metabolism • LA  y-linoleic acid  dihomo-Y-linoleic acid arachidonic acid • ALA  Eicosapentaenoic acid (EPA) • N-6 and n-3 fatty acids compete for enzymes and take the same path, which can affect the conversion of one or the other • Eicosanoids transferred to membranes in the form of TAGs or phospholipids. Go through further elongation and desaturations in smooth ER, transferred to the peroxisome and undergo B-oxidation to DHA.

31. AA, ALA, EPA and DHA containing phospholipids or TAG are incorporated into any of the cell’s membranes or the neutral lipid. AA is predominant in membranes. • The higher fluidity from unsaturation = better expression of hormone receptors • Eicosanoids- Important for hormone-receptor binding sites*

32. AA (n-6) vs. epa and dha(n-3) AA EPA AND DHA Pro-inflammatory Pro-arrythmic Activate platelets Vasoconstrictors • Anti-inflammatory • Anti-Arrythmic • Inhibits platelets • Vasodilators • DHA: nervous system, vision, neuroprotection, successful aging, and memory.* • Deep-water fish: Herring, Salmon, Tuna

34. Sytnhesis of triaclyglycerols • Precursors: CoA-activated • fatty acids and G-3-P • De novo,(a Latin expression meaning "from the beginning,”),major route • Salvage pathway increases • when a deficiency of essential • amino acid methionine exists.

35. Synthesis of cholesterol • Nearly all tissues in body capable of synthesizing cholesterol from acetyl-CoA • Liver = 20% of endogenous synthesis • 80% from extrahepatic tissues, intestine most active • 1 g/day endogenously synthesized • Average daily cholesterol intake 300 mg/day, only half is absorbed • Endogenous synthesis 2/3 total cholesterol

36. 26 steps, 3 stages • Cytoplasmic sequence by which • 3-hydroxy-3-methylutaryl-CoA • (HMG-CoA) formed from 3 mol • acetyl-CoA • 2. Conversion of HMG-CoA to squalene, • including rate limtiing step of • cholesterol synthesis, in which • HMG-CoA reduced to mevalonic • Acid by HMG-CoA reductase • 3. Formation of cholesterol from squalene

37. Cholesterol synthesis • http://www.dnatube.com/video/253/Cholesterol--biosynthesis

38. Cholesterol inhibitors • As total body cholesterol increases, the rate of synthesis decreases. ( negative feedback regulation of HMG-CoA reductase reaction.) • Suppression of cholesterol synthesis by dietary cholesterol is unique to liver. • Statins: HMG-CoA inhibitors, block endogenous cholesterol synthesis

39. Summary • The complete hydrolysis of TAGs  Glycerol and 3 fatty Acids • Fatty Acids are a rich source of energy • Long Chain fatty acids cannot cross inner membrane, require carnitine. • The breakdown of fatty acids into acetyl-CoA “B-Oxidation” • The synthesis of fatty acids is essentially the reverse of B-Oxidation • Ketone bodies are produced when fatty acids are broken down for energy • Ketosis is a result which disrupts the body’s acid/base balance, Diabetes • Cholesterol is secreted into bile canliculi or converted to bile acids. • N-6 EFA’s vs. N-3 EFA’s