Triacylglycerol synthesis and Lipolysis -1New.pptx

Lecture of Medical Biochemistry on triacylglycerol synthesis and breakdown

1. Dr.Md.Nazrul Islam Professor Department of Biochemistry Rangpur Medical College

2. TAG synthesis Animals can synthesize and store large quantities of triacylglycerols, to be used later as fuel In humans only a few hundred grams of glycogen can be stored in the liver and muscles, barely enough to supply the body's energy needs for 12 hours. In contrast, the total amount of stored triacylglycerol in a 70 kg man of average build is about 15 kg, enough to supply his basal energy needs for as long as 12 weeks . Whenever carbohydrate is ingested in excess of the capacity to store glycogen, it is converted into triacylglycerols and stored in adipose tissue Triacylglycerols share two precursors (fatty acyl-CoAs and glycerol-3- phosphate) and several enzymatic steps in their biosynthesis in animal tissues.

3. TAG synthesis Substrate: Fatty acid and Glycerol (Need to be activated ) Product: TAG Site: Liver, adipose tissue, enterocyte, lactating breas t Rate limiting enzyme:Glycerol-3 –PO4 acyltransferase Compartment: Cytoplasm Nature: Anabolic, stimulated by Insulin Fate: In adipose tissue stored and serves as depot fat I n l i v e r T A G e x p o r t e d a f t e r s y n t hes i s a s VLD L . I f f a i ls t o e x p o r t f a t content of liver gradually increase and finally develops fatty liver

4. S t ep s : Glycerol activated /phosphorylated to glycerol- 3- PO4 by glyceol kinase in liver. Glycerol-3-PO4 also comes from DHAP which is formed in glycolysis (F1,6BP) and converted to glycerol-3-PO4 by glycerol 3 phosphate dehydrogenase in liver & adipose tissue Fatty acid activated to Fatty acyl CoA by thiokinase/acyl CoA synthetases and added sequentially to glycerol-3-PO4 Sequential addition of fatty acid to all three carbons of Glycerol-3-PO4 from three fatty acyl CoA in following steps Glycerol-3-PO4 acylated to 1,acylglycerol3-PO4 ( lysophosphatic acid / Lysophosphatidate ) - Ist carbon 1,acylglycerol3-PO4 acylated to 1,2 diacyl glycerol- 3- PO4 (Phosphatidic acid/ Phohphatidate )- 2nd carbon Remove PO4 from 1,2 diacyl glycerol -3- PO4 to form 1,2 diacylglycrol (Diacylglycerol) Acylation of 1,2 Diacylglycerol makes Triacylglycerol -3rd carbon

5. Li v er only Intestine Li v er and adipose tissue Fatty acid

6. S t ep -1 : Glycerol activated /phosphorylated to glycerol- 3- PO4 by glyceol kinase in liver using ATP. Glycerol-3-PO4 also comes from DHAP(dihydroxyacetone PO4) which is formed in glycolysis from glucose and converted to glycerol-3-PO4 by glycerol 3 phosphate dehydrogenase in liver & adipose tissue. ( DHA-P also converted to acylhydroxyacetone-PO4 and enters the pathway by forming lysophosphatidic acid (1,acylglycerol -3- PO4 ).

7. 2. Activation of fatty acid F a t t y ac i d ac t i v a t i on occ u r in cytoplasm Fatty acid activated into fatty acyl CoA b y thiokinase or fatty acyl CoA synthetase This is a two step reaction needs ATP,Mg++ and CoA Fatty acid reacts with ATP to produce acyladenylate or fatty acid-AMP intermediate and in the next reaction AMP is replaced by coenzyme A to produce fatty acyl CoA

8. S t ep s -3 : Sequential addition of fatty acid to all three carbons of Glycerol-3-PO4 from three fatty acyl CoA in following steps Glycerol-3-PO4 acylated to 1,acylglycerol3-PO4 ( lysophosphatic acid / Lysophosphatidate ) - Ist carbon 1,acylglycerol3-PO4 acylated to 1,2 diacyl glycerol- 3- PO4 (Phosphatidic acid/ Phohphatidate )- 2nd carbon Remove PO4 from 1,2 diacyl glycerol -3- PO4 to form 1,2 diacylglycrol (Diacylglycerol) Acylation of 1,2 Diacylglycerol makes Triacylglycerol -3rd carbon

9. G lu c o se DH AP Glycerol G 3 PD G l y ce r ol kinase Li v er only Liver and Adipo s e tissue Fatty acyl CoA Fatty acyl CoA 1 2 3 Phospholipid 1-acylglycrol3PO4 1,2 diacylglycrol3PO4 1,2 diacylglycrol PO4 Fatty acid activated into Fatty acyl CoA Acyl dihydroxy acetone PO4 1

10. 1.Activation of glycerol to glycerol - 2.Activation of fatty acid to fatty 3.Sequential addition of fatty acid in all three carbons of glycerol-3-PO4 from three fatty acyl CoA C1-OH C2-OH C3-PO4 T A G C1 Fatty acid C2 Fatty acid C3 Fatty acid 3 Fatty acyl CoA 3 C o A+Pi Fatty Acid +ATP+ CoA Thiokinase (Fatty acyl CoA synthase) Glycerol-3-P G l y ce r o l Glycerol Kinase 2 1 ADP 3 C1-FA C2-OH C3-PO4 C1-FA C2-FA C3-PO4 Glycerol 3-P acyltransferase A TP A TP A M P 1 -a c y l g l y c r o l3 P O 4 1,2 diacylglycrol3PO4 1,2 diacylglycrol C1-FA C2-FA C3- 3PO4 Acyl CoA

11. 1.Activation of Glycerol Glycerol activated into Glycerol-3-phosphate can be formed in two ways In addipose tissue Glycerol-3-phosphate pho s p h a t e g e n e r a te d d u r i ng gl y col y s i s by c a n a r i se f r om d i h y d r o x y a c e t o n e the a c t i on o f t he c y t o s o li c N A D- li n k e d g l y c ero l - 3 p h o s pha t e de h y drog e n a se , in w h i c h re du c e d t o NADH . In liver ,kidney intestine and other tissue Glycerol-3-phosphate is also formed from phosphorylation of glycerol by the action of glycerol kinase with the used of ATP G l y c ero l k i na s e a b s e n t i n addi p o s e t iss u e a n d c a n no t u s e d G l y c ero l f r o m t he breakdown of TAG in this tissue.

12. 2. Activation of fatty acid F a t t y ac i d ac t i v a t i on occ u r in cytoplasm Fatty acid activated into fatty acyl CoA b y thiokinase or fatty acyl CoA synthetase This is a two step reaction needs ATP,Mg++ and CoA Fatty acid reacts with ATP to produce acyladenylate or fatty acid-AMP intermediate and in the next reaction AMP is replaced by coenzyme A to produce fatty acyl CoA

13. 3. Two acylation of glycerol-3- PO4 Glycerol-3-phosphate is acylated at 1 carbon position to form 1-acyl-Glycerol 3 Phosphate (Lysophosphatidic acid- L yso PA ) catalyzed by the enzyme 1-acyl- Glycerol 3 Phosphate transferase . Another acyl group added at 2 carbon position to form 1,2 Diacyl -glycerol 3 phosphate 1,2 Diacyl-glycerol 3 phosphate also known as Phosphatidate or Phosphatidic acid (PA)

14. 4.Dephosphorylation of 1,2 DAG & synthesis of TAG Phosphatidic acid spilt off the phosphate group by phosphatidic acid phosphatase and 1,2 Diacyl-glycerol 3 phosphate converted into 1,2 Diacyl – glycerol An acyl group added at 3 carbon position and 1,2 Diacyl -glycerol converted to triacylglycerol catalyzed by Diacyl –glycerol acyl transferase

15. O x i d a t i o n glycolysis of g l u c ose via to p r o d uce glycerol 3-PO4 Oxidation of pyruvate to acetyl CoA by PDH Oxidation of glucose via HMP shunt to produce NADPH Synthesis of fatty acid from acetyl CoA and NADPH Synthesis of TAG through the esterification of glycerol -3-PO4 with Fatty acid Synthesis of TAG from glucose in liver and adipose tissue

16. Synthesis of fat(TAG) from other nonlipid substances Fat (TAG) can be synthesized from non carbohydrate precursor like amino acid, lactate etc. These precursor enters into the gluconeogenesis(reverse glycolytic ) pathway produce dihydroxyacetone. Then dihydroxyacetone form glycerol-3-PO4 and synthesized triacylglycerol with fatty acid.

17. Phosphtidic acid is the common source of TAG & GPL 1,2 Diacyl-glycerol 3 phosphate also known as Phosphatidic acid/Phosphatidate 2 f a t ty a c id a t t a ch e d a t c arbon 1 and 2 and PO4 at carbon 3. If 3 carbon PO4 replaced by another fatty acid then produced TAG . If nitrogen base attached to the PO4 of carbon 3 then produced glycerophospholipid (GPL). That is why phosphatidic acid is a common sources of fat(TAG) and glycerophospholipid C1-FA C2-FA C3-PO4 C1-FA C2-FA C3-PO4-NB C1-FA C2-FA C3-FA 1,2 diacylglycrol3PO4 (Phosphatidate/ Phosphatadic acid) T A G

18. Lipolysis or Breakdown of triacylglycerol Breakdown or Hydrolysis of triacylglycerol into fatty acid and glycerol by lipase enzyme. It is a three steps breakdown process Substrate: TAG Site: In the intestine dietary TAG hydrolysed by pancreatic lipase ,Adipose tissue, Liver (Storage TAG-hydrolysed by Hormone sensitive lipase, HSL and hepatic lipase ) and hydrolysis of TAG of chylomicron (exogenous) and VLDL (endogenous) also takes place in peripheral tissue capillary by lipoprotein lipase , LPL . Product: 3 Fatty acid and Glycerol Compartment: Cytoplasm Nature: Anabolic Rate limiting enzyme: Hormone sensitive lipase (Inhibited by Insulin)

19. Lipase enzymes There are two types of lipase hydrolyze TAG . Hormone sensitive lipase and lipoprotein lipase . Hormone sensitive lipase –present in the tissue and lipoprotein lipase present in plasma attached in capillary endothelial wall Hormone sensitive lipase undergoes dephosphorylation and phosphorylation, deposphorylated is inactive form and phosphorylated form is active form.Activated protein kinase A converts inactive to active from. Protein Kinase A activated by cAMP Epinephrine and glucagon c o n c e n t r a t i o n and i n c r e a s e i n c reas e c AM P li po l y s i s and In s u li n de c reas e cAM P c o n c e n t r a t i o n a nd decrease lipolysis

20. A.Lipolysis of TAG in circulation ( chylomicron and VLDL ) In peripheral tissue circulation , l i po l y sis c a t a l y z e d by lipoprotein lipase Substrates are TAG in chylomicron and VLDL Normally lipoprotein lipase present as inactive form attached in membrane of capillary endothelial cells. It is released by heparin and activated by apoliprotein C-II and Phospholipid which are present in Chylomicron and VLDL Whe n Chylomicron and VLDL goes to peripheral circulation, lipoprotein li p ase b e c om e a c ti v a t ed by apoliprotein C-II and phospholipid and hydrolyse TAG present in chylomicron and VLDL into fatty acid and glycerol

21. Lipolysis in circulation G l y c e r o l G l y c e r o l

22. B.Lipolysis of storage TAG TAG is the storage form of fat mainly present in the adipose tissue and liver, kidney, intestine in lesser extent. In these tissue TAG hydrolyzed by HSL stimulated by insulin In adipose tissue TAG breakdown/hydrolyzed into glycerol and Fatty acid by HSL(Hormone sensitive lipase) TAG hydrolyzed step by step with removal of fatty acid ATGL=Adepocyte glycerol lipase, DGL=Dacylglycrol lipase, MGL=Mo no acylglycerol lipase , *All are HSL(Hormone sensitive lipase)

23. Hydrolysis of Triacylglycerol Hydrolysis of triacylglycerol is a three step breakdown process The adipose triacylglycerol lipase, is specific for triacylglycerols containing long-chain fatty acids, preferentially cleaving ester bonds in the  sn -1 or  sn -2 position (but not  sn -3), and yields 2,3 or 1,3 diacylglycerols and free fatty acids as the main products .In next step 1or 3 position breaks to produce 2-monoacylglycerol and next step breaks into fatty acid and glycerol . Adipose triacylglycerol lipase is a hormone-sensitive lipase stimulated by glucagon and adrenalin and inhibited by insulin, is a rate limiting for the first step in triacylglycerol hydrolysis. ATGL=Adepocyte glycerol lipase, DGL=Dacylglycrol lipase, MGL=Moacylglycerol lipase , *All are HSL(Hormone sensitive lipase)

24. Fate of FA and glycerol Fate of FA: Reutilization in situ (tissue) to make TAG and diffuse to blood and taken up by other tissue for oxidation or re esterification to TG. Fate of glycerol : Cannot used in situ as adipose tissue due to lack of glycerol kinase. Glycerol produced in tissue and plasma goes to liver where it activated to glycerol-3-PO4 by glycerol kinase. Glycerol-3-PO4 esterified with fatty acid to form TAG or produce dihydroxyaceton phosphate (DHAP) which enters into the glycolytic pathway or g luconeogenic pathway to produce glucose

25. T A G Fate of fatty acid Fate of glycerol