Dr.S.Chakravarty,MD

1. BIOSYNTHESIS OF FATTY ACIDS Dr.S.Chakravarty,MD

2. CLEARING CONCEPTS !!! MCQ 1 • Which of the following substances will be elevated in a cell following a high fatty meal ? • A. c AMP • B. 5’ AMP • C. ADP • D.GDP • E. glucagon AND WHY ??????

3. Learning Objectives • At the end of this lecture the students will be able to :- • List the sources of NADPH required for fatty acid synthesis • List the key enzymes of fatty acid synthesis • Describe the role of citrate in bringing acetyl Co-A to cytosol for FA synthesis • Explain the regulation of acetyl Co-A carboxylase. • Explain the functions of adipose tissue in the body • Explain the importance of glycerol kinase in the liver.

4. Biomedical Importance • In most mammals, glucose is the primary substrate for lipogenesis, but in ruminants it is acetate. • Critical diseases of the pathway have not been reported in humans. • Inhibition of lipogenesis occurs in type 1 (insulin-dependent) diabetes mellitus • No LIMIT to AMOUNT OF LIPIDS SYNTHESIZED OBESITY • Unsaturated fatty acids in phospholipids of the cell membrane are important in maintaining membrane fluidity. • A high ratio of polyunsaturated fatty acids to saturated fatty acids (P:S ratio) in the diet is considered to be beneficial in preventing coronary heart disease.

5. No limit to storage of Lipids !!

6. DENOVO SYNTHESIS OF FATTY ACIDS • Tissues:- Many tissues like -adipose tissue & Liver (both have lower activity than other animals) - brain, lung , kidney, mammary gland • Subcellular site:- Cytosol(synthesis) E.R – chain elongation

7. Substrates for synthesis: • Acetyl Co-A • NADPH • ATP • Biotin • HCO3 – source of CO2.

8. Steps in fatty acid synthesis: • Transport of acetyl Co-A to cytosol • Synthesis of Palmitate from acetyl CO-A • Elongation of Palmitate – to increase the length of fatty acid • Desaturase system – to produce unsaturated fatty acids.

9. Sources of NADPH: • HMP pathway • MALIC enzyme Malate  Pyruvate • Cytosolic Isocitrate dehydrogenase Isocitrate  α-ketoglutarate USMLE !!

10. The provision of acetyl CoA and NADPH for lipogenesis 1 2 3

11. Transport of acetyl-CoA to cytosol USMLE!!

12. Condensation Reduction The fatty acid synthase complex

13. Production of Malonyl-CoA Is the Initial & Controlling Step in Fatty Acid Synthesis The enzyme is a multienzyme protein containing a variable number of identical subunits, each containing biotin, biotin carboxylase, biotin carboxyl carrier protein, and transcarboxylase, as well as a regulatory allosteric site. The reaction takes place in two steps: carboxylation of biotin involving ATP transfer of the carboxyl group to acetyl-CoA to form malonyl-CoA.

14. Advantages of multienzyme complex • Rapid catalysis by rapid exchange of intermediates • Freedom from interfering reactions , thus achieving compartmentalization without erection of permeability barriers • Synthesis of all enzymes in the complex is coordinated as it is encoded by a single gene.

16. Microsomal elongase system for fatty acid chain elongation MalonylCoA as acetyl Donor

17. CLEAR YOUR CONCEPTS AGAIN !! Regulation 1)The Nutritional State Regulates Lipogenesis(main controlling factor) Synthesis of long chain fatty acids (mol/g/h) Serum FFA (mol/mL)

18. ALLOSTERIC REGULATION Citrate activates acetyl CoA carboxylase. Allosteric activation of enzyme involves aggregation from a dimeric to a polymeric configuration. Rem- citrate increases in well fed state and is a indicator of plentiful supply of Acetyl CoA.

19. Acetyl-CoAcarboxylase as a dimer is inactive, it needs formation of the active polymer

20. Acetyl Co-A Carboxylase – key regulatory enzyme of FA synthesis • Allosteric regulation: positive  Citrate Feed forward regulation Negative (feedback ) Palmitoyl Co-A • feed back inhibition (FEEDBACK INHIBITION Acetyl CoAcarboxylase is inhibited by long chain acylCoA molecules.) Decreased esterification to triglyceride or increased lipolysis – during starvation

21. Regulation by Insulin and Glucagon- Covalent modification

22. Pyruvate dehydrogenase is also regulated by acyl CoA • Acyl CoA causes an inhibition of pyruvate dehydrogenase by inhibiting the ATP-ADP exchange transporter of the inner mitochondrial membrane . • This leads to increased intramitochondrial ATP/ADP ratios and therefore convert active PDH to inactive PDH. Also , oxidation of acyl CoA due to increased levels of FFA may increase NADH/NAD and acetyl CoA/CoA and inhibit PDH

23. Monounsaturated Fatty acids are synthesized by Δ9desaturase

24. Synthesis of PUFA involves desaturase and elongase systems

25. MCQ 2 • Acetyl CoA is produced in mitochondria .Fatty acid synthesis occurs in cytosol .Which of the following enzyme is responsible for regeneration of acetyl CoA in cytosol for this process ? • A. Acetyl CoAcarboxylase • B. PDH • C.Isocitratedehydrogenase • D. Citrate synthase • E. ATP citrate lyase

26. FUNCTIONS OF ADIPOSE TISSUE • RESERVE OF ENERGY • CUSHIONING TO BODY ORGANS • INSULATION ( for conserving HEAT ) • THERMOGENESIS –BROWN ADIPOSE TISSUE

27. Location • In humans, adipose tissue is located in the following places :- • beneath the skin (subcutaneous fat), • around internal organs (visceral fat), • in bone marrow (yellow bone marrow) • and in breast tissue

29. White adipose tissue • Mainly concerned with storage . • Made up of spherical cells , with very few mitochondria. • Triglycerides are major components of white adipose tissue ( 80%).

32. Changes in adipose tissue

33. ADIPOSE TISSUE IN FED & FASTING STATE • LIPID BIOSYNTHESIS AND LIPID STORAGE IN FORM OF TRIACYLGLYCEROL OCCURS. • LIPOLYSIS OCCURS DURING FASTING OR STARVATION

34. INCREASED GLUCOSE METABOLISM REDUCES THE OUTPUT OF FREE FATTY ACIDS In FED STATE :-When glucose utilization is increased , the FFA outflow decreases. Glucose HMP shunt  CO2 Glycerol 3 P DHAP TRIACYL GLYCEROL Acetyl CoA Fatty acid synthesis CO2 • With high glucose conc. , a larger portion is oxidized and converted to CO2. • When glucose concentration decreases , majority is directed towards synthesis of GLYCEROL 3 P and esterification of acylCoA. NADPH,RIBOSE FATES OF GLUCOSE IN ADIPOSE TISSUE

35. Esterification and storage of TAG • TAG is synthesized by esterification of fatty acylCoA with Glycerol 3-P. The provision of Glycerol 3-P Regulates Esterification. • GLYCEROL KINASE ACTIVITY IS VERY LOW IN ADIPOSE TISSUE • SOURCE OF GLYCEROL 3 P  dihydroxy acetone phosphate ( DHAP) from GLYCOLYSIS . Glycolysis  DHAP Glycerol 3P NADH + H + NAD+ GLYCEROL 3P DEHYDROGENASE

36. Triglyceride synthesis Modified from Harper 28th

37. FFA are taken by action of Lipoprotein Lipase • There are two fatty acid pools in the body :- • 1) from ADIPOSE TISSUE • 2) DIETARY pool

38. Harper 28th

39. HORMONES REGULATE FAT MOBILIZATION • Insulin • Enhances lipogenesis • Enhances synthesis of acylglycerols. • Inhibits release of FFA from adipose tissue, so there is a fall in circulating FFA • Increases oxidation of glucose to CO2 both via glycosis and HMP shunt.

40. SEVERAL HORMONES PROMOTE LIPOLYSIS • Lipolysis requires the presence of epinephrine , norepinephrine, glucagon, adrenocorticotropic hormone (ACTH), α-and β-melanocyte stimulating hormones , Thyroid stimulating hormone (TSH), Growth Hormone (GH ) , and vasopressin. • Many of these activate Hormone sensitive Lipase. • This is done by increasing the concentration of c-AMP either by stimulating the enzyme adenylylcyclaseor by inhibiting the enzyme Phosphodiesterase. • Glucocorticoids can stimulate HSL directly.

41. CLEAR YOUR CONCEPTS !!

42. BROWN ADIPOSE TISSUE • It is especially abundant in newborns and in hibernating mammals. • NON SHIVERING THERMOGENESIS • LOCATION :- • In neonates (newborn infants), brown fat, which then makes up about 5% of the body mass and is located on the back, along the upper half of the spine and toward the shoulders, is of great importance to HYPOTHERMIA.

44. BROWN ADIPOSE TISSUE • HIGH NUMBER OF MITOCHONDRIA- HENCE BROWN. • WELL DEVELOPED BLOOD SUPPLY. • LOW CONTENT OF ATP-SYNTHASE. • PRESENCE OF GLYCEROL KINASE UNLIKE NORMAL ADIPOSE TISSUE. • OXIDATION BUT NO PHOSPHORYLATION apart from SUBSTRATE LEVEL PHOSPHORYLATION- THERMOGENIN IS THE UNCOUPLER.

46. THE ADIPOSE TISSUE IS AN ENDOCRINE ORGAN • Adipose derived hormones include: • Adiponectin • Resistin • Plasminogen activator inhibitor-1 (PAI-1) • TNFα • IL-6 • Leptin • Estradiol (E2) • Adipose tissues also secrete a type of cytokines (cell-to-cell signalling proteins) called  adipokines (adipocytokines), which play a role in obesity-associated complications.

48. Leptin- Hunger and Obesity Harrison’s Principles of Internal Medicine

49. Leptin is a product of the Ob gene. • Leptin acts on receptors in the hypothalamus of the brain where it inhibits appetite by • (1) counteracting the effects of neuropeptide Y (a potent feeding stimulant secreted by cells in the gut and in the hypothalamus); • (2) counteracting the effects of anandamide (another potent feeding stimulant) • (3) promoting the synthesis of α-MSH, an appetite suppressant. 

50. THANK YOU