Lipids Metabolism Lecture Notes PDF

Beni-suef University Faculty of science Biochemistry division Lecture notes In Lipids metabolism 3rdyear students Dr. Basant Mahmoud 1|Page Contents Introduction about biochemistry...

Beni-suef University Faculty of science Biochemistry division Lecture notes In Lipids metabolism 3rdyear students Dr. Basant Mahmoud 1|Page Contents Introduction about biochemistry Lipid definition Role of liver in lipid digestion Definition of metabolism Fatty acid metabolism ( fatty acid synthesis and oxidation) Calculate energy gained from fatty acid oxidation Ketone bodies metabolism (synthesis and degradation) Calculate energy gained from Ketone bodies oxidation Triglycerides metabolism (synthesis and degradation) Cholesterol metabolism (synthesis and degradation) Role of acetyl CoA in lipid metabolism 2|Page  Introduction Lipids are absorbed from the intestine and undergo digestion and metabolism before they can be utilized by the body. Most of the dietary lipids are fats and complex molecules that the body needs to break down in order to utilize and derive energy from. Lipids found in the body as fatty acids, triglycerides, phospholipids, ketone bodies, steroids and their metabolic products. Digestion begin at mouth then stomach then small intestine and pancreas. Gall bladder in liver secrets bile salts. It has no digestive enzymes but give substances help in digestion of fats. During digestion large units converts into Simple units, then absorption occur from Small intestine. Simple units enters large intestine where bacterial action occur. When bacteria act on carbohydrates make Fermentation. When bacteria act on protein make putrefaction. Due to these processes we have undesirable substance which go out by stools. Finally simple units directed to tissue cells with the blood at which formation g new compounds occur (Anabolism) and break down of other compounds occur (Catabolism). Digestion and absorption processes accompanied with anabolism and catabolism processes. All these process are called { Metabolism }. Role of liver in lipid metabolism Liver cells called hepatocytes perform several important roles concerning fat ("lipid") cells. These include: 1. Break-down of fatty acids - generating adenosine triphosphate (ATP), which is important for the contraction and relaxation of muscles. 2. Synthesis of lipoproteins, which are important for the movement of fatty acids, cholesterol and triglycerides to from cells. 3|Page 3. Storage of certain triglycerides. 4. Synthesis of cholesterol (as well as using cholesterol to produce bile salts). Metabolism ✓ Means all chemical reactions occur inside the living cells and tissues beginning with absorption of digested food till the waste products excreted. I- FATTY FATTY ACID METABOLISM Biosynthesis of fatty acid 1- ACTIVATION STEP Occur in cytosol of liver, brain and adipose tissue. Occur in 2 steps Acetyl CoA Carboxylase acetyl CoA + CO2 Malonyl CoA ACP-SH malonyl transacylase CoA-SH Malonyl ACP a- Acetyl CoA Carboxylase reaction 4|Page  This is an irreversible reaction. AcetylCoA carboxylation is a rate-limiting step of F.A biosynthesis. Acetyl CoA carboxylase is under allosteric regulation. Citrate is a positive effector. palmitoyl CoA is a negative effector. b- Malonyl transacylase reaction Intermediates covalently linked to acyl carrier protein (ACP). malonyl transacylase OOC-CH2 -COS-CoA + ACP-SH OOC-CH2 -CoS-ACP + CoA-SH + Pi malonyl CoA malonyl acyl carrier protein 2- FATTY ACID SYNTHASE COMPLEX The enzymes of fatty acid synthesis are packaged together in a complex called as fatty acid synthase (FAS). Consists of 3 enzymes a) acetyl transacylase : transfer acetyl group b) malonyl transacylase : transfer malonyl group c) β ketoacyl ACP synthase : catalyze condensation reaction 3- FOUR-STEP FATTY ACID SYNTHESIS Condensation Reduction Dehydration reduction 5|Page  4- FATTY ACID ELONGATION The product of is palmitic acid. (16:0). Modifications of this primary FA leads to other longer (and shorter) FA and unsaturated FA. The fatty acid molecule is synthesized 2 carbons at a time FA synthesis begins from the methyl end and proceeds toward the carboxylic acid end. Thus, C16 and C15 are added first and C2 and C1 are added last. C15 and C16 are derived directly from acetylCoA. For further step-wise 2-carbon extensions, acetylCoA is first activated to malonyl CoA, a 3-carbon compound, by the addition of a CO2. Microsomal chain elongation occur in liver Malonyl CoA add to palmitate under action of FA synyhase giving stearate. CoASH used instead of ACP This may involve saturated and unsaturated fatty acids Mitochondrial chain elongation (anaerobically) Here use acetyl CoA instead of malonyl CoA Use NADH & NADPH as reducing agent 6|Page 7|Page THE NET RESULT OF ENERGY PRODUCED 2 acetyl CoA +7 malonyl CoA + 14 NADH + 14 H Palmitate + 7CO2 + 8CoA + 14 NADP + 6 H2O Desaturation of fatty acid Addition of double bond occur using one of the following enzymes Δ9, Δ5, Δ6 desaturase enzymes Δ9 desaturase R CH2 CH2 (CH2)7 COOH R CH = CH (CH2)7 COOH + H2O Fatty acid oxidation Fatty acid oxidation occur in mitochondria of all cells except brain and RBCs. Because they use glucose as a sole energy source. palmitoyl CoA was transferred to mitochondria then can be oxidized Types of oxidation 1. α-oxidation 2. β-oxidation 3. ω-oxidation 8|Page 1- Transport of fatty acid in blood Fatty acid transport to blood as free FA (non-esterified) combined to albumin. 2- Fatty acid activation It occurs as following: Fatty acid(palmitic acid) + CoASH + ATP Acyl CoA synthase ( thiokionase) Acyl CoA (palmitoylCoA) + AMP + PPi 3- Transfer of fatty acid into mitochondrial matrix. Mitochondrial membrane is impermeable to acylCoA so carried on carnitine to enter mitochondria (carnitine synthesized in liver and kidney from lysine and methionine) Carnitine acyl transferrase l Acyl CoA + carnitine acyl carnitine + CoASH (outer memb.) (Impermeable) (Permeable) Through mitochondrial membrane Carnitine acyl transferrase ll Acyl CoA + carnitine acyl carnitine + CoASH (inner memb.) 4- Mitochondrial β-oxidation of palmitoyl CoA (Even carbon fatty acid) Here 2- carbon removed till the last 2 carbon fragments obtained 9|Page 10 | P a g e Energy gained from β-oxidation of palmitoyl CoA Palmitoyl CoA+7CoA+7FAD+7NAD+7H2O 8-AcetylCoA+7FADH2+7NADH+7H 7 FADH2 +14ATP 7NADH +21ATP 8-AcetylCoA +96 ATP(8*12) in Kreb's or TCA. sum 131 ATP Activation step -2ATP Total energy 129 Oxidation of odd chain fatty acid It undergoes β-oxidation by Poroducing acetyl CoA till 3-carbon fragments remains (propionyl CoA). propionyl CoA is the only glucogenic part in β-oxidation that converts to succinyl CoA by methyl malonyl CoA ( gluconeogensis from propionate). succinyl CoA enters TC ) tricarboxylic acid cycle) 11 | P a g e  II- KETONE BODIES METABOLISM The most common ketone bodies in human body are: Aceto acetate, Acetone and β- hydroxy butyrate. 1- Ketogenesis Occurs in liver and mitochondria then transport via blood to extra hepatic tissues 12 | P a g e 2- Ketolysis Ketone bodies utilization occur by extra hepatic tissues( heart muscle and skeletal muscles) because liver lack enzymes necessary to utilize them β-hydroxy butyrate oxidized to aceto acetate Acetoacetate oxidized to acetoacetyl CoA and then to acetyl CoA Acetone oxidized to pyruvic acid which then oxidized throug carbohydrate pathway 13 | P a g e  ✓ Energy gained from ketone bodies oxidation β-hydroxy butrate aceto acetate this step gives NADH = 3ATP Aceto acetate 2 acetyl CoA each acetyl CoA molecule gives 12 ATP i.e this step gives 24 ATP TOTAL ENERGY IS 3ATP+24ATP= 27ATP i.e energy gained from ketone bodies is 27ATP Ketogenesis important in starvation case. Why? Because carbohydrate store depleted and oxidation of fat is the major source of energy. Normally brain use glucose as the only fuel source, so when glucose consumed, use ketone bodies for energy because it haven't free fatty acids to oxidize them Ketosis Ketosis means that the rate of formatiom of ketone bodies in liver is greater than its utilization in extra hepatic tissues Cases of ketosis 1. Starvation. 2. un controlled diabetes mellitus. ▪ Here glycogen increase, So fat utilization decrease, So glycogen / insulin ratio increase leads to lipolysis. FA increase in blood leads to increase its oxidation so increse acetyl CoA conc. But no enough oxaloacetate due to starvation so cannt enter kreb’s cycle so all of them converts to ketone bodies lead to ketosis case 14 | P a g e  III- TRIGLYCERIDES METABOLISM TG. Synthesis Most triacylglycerols are synthesized in liver and adipose tissue using glycerol- 3-phosphate and acyl CoA esters. Sometimes alternative pathway using dihydroxy aceton phosphate is used in synthesis. Triglycerides are esters of glycerol with various fatty acids. Since the 3 hydroxyl groups of glycerol are esterified, the neutral fats are also called. Esterification of glycerol with one molecule of fatty acid gives monoglyceride, and that with 2 molecules gives diglyceride. O O HO C R1 CH2 OH H2C O C R1 O O R2 C O C H HO C R2 + HO C H O O CH2 OH 3 H2O H2C O C R3 HO C R3 Glycerol Triglycerides Fatty acids (Triacylglycerol) 15 | P a g e Types of triglycerides 1. Simple triglycerides: If the three fatty acids connected to glycerol are of the same type the triglyceride is called simple triglyceride, e.g., tripalmitin. 2. Mixed triglycerides: if they are of different types, it is called mixed triglycerides, e.g., stearo-diolein and palmito-oleo-stearin. 3. Natural fats: are mixtures of mixed triglycerides with a small amount of simple triglycerides. O CH2 O C (CH 2)14 CH3 O CH3 (CH2)14 C O C H O CH2 O C (CH2)14 CH3 Tripalmitin (simple triacylglycerol) O CH2 O C (CH2)16 CH3 O CH3 (CH2)7 CH CH (CH2)7 C O C H O CH2 O C (CH2)7 CH CH (CH2)7 CH3 1-Stearo-2,3-diolein (mixed triacylglycerol) O CH2 O C (CH 2)14 CH3 O CH3 (CH2)7 CH CH (CH 2)7 C O C H O CH2 O C (CH2)16 CH3 1-palmito-2-oleo-3-stearin (mixed triacylglycerol) 17 | P a g e Lipolysis of Triglycerides Involve hydrolysis of TG. Into free fatty acid and glycerol Rate limiting step catalyzed by hormone sensitive lipase (HSL) HSL Triglyceride Diacyl gycerol+F.F.A. diacylglcerol lipase diacylglcerol lipase gycerol+F.F.A. monoacyl gycerol+F.F.A. IV- CHOLESTEROL SYNTHESIS CH3 CH3 CH3 CH3 CH3 HO Cholesterol Physical propeties It has a hydroxyl group on C3, a double bond between C5 and C6, 8 asymmetric carbon atoms and a side chain of 8 carbon atoms. It is found in all animal cells, corpus luteum and adrenal cortex, human brain (17% of the solids). 18 | P a g e  In the blood (the total cholesterol amounts about 200 mg/dL of which 2/3 is esterified, chiefly to unsaturated fatty acids while the remainder occurs as the free cholesterol. Cholesterol is the most abundant sterol (sub group of steroid) in the body. Act as precursor of vitamins and steroid hormones. Synthesis occur mainly in liver and may also occur in intestine, skin and adrenal cortex 19 | P a g e  Cholesterol catabolism 20 | P a g e  Fate of cholesterol includes 1- Excretion in bile 2- Conversion to bile acids 3- Production of steroid hormones 4- Pathological deposit as plaques in the arteries Draw a schematic diagram for cholesterol metabolism 21 | P a g e  Answer the following question 1- Draw a schematic diagram for role of acetyl Co enzyme A in metabolic pathways inside human body 2- Discuss oxidation of palmitic acid and energy gained from this process 3- If you have a fatty acid with the following structure CH3-(CH2)17-COOH i- Follow up this fatty acid after its synthesis till its oxidation ii- Calculate energy result from this pathway Notice : 1 propionyl CoA will be transformed into succinyl CoA & this succinyl CoA after entering TCA cycle produces 1 GTP(equivalent to 1 ATP), 1 FADH2 & 1 NADH2. 4- From glycerol how can you obtain 1-stearo 2,3 dipalmetine 5- How many ATP is gotten from a saturated nineteen carbon fatty acid? 6- Ketogenesis important in starvation case. Discuss this statement and define ketosis 7- Show with complete equations the role of each enzyme of the following during metabolic processes inside human body i- Acetyl CoA carboxylase and malonyl transacylase in the synthesis of stearic acid ii- Carnitine acyl transferrase enzymes in oxidation of fatty acid 22 | P a g e Choose the most correct answer for each of the following 1- How many ATP molecules produced from the complete degradation of a C10 fatty acid molecule? a) 87 b) 106 c) 120 d) non of the above 2- The correct sequence for the four reactions of the fatty acid oxidation in terms of “functional group acted upon” is a) alkene, alkane, ketone, 2º alcohol. b) ketone, 2º alcohol, alkene, alkane. c)alkane, ketone, 2º alcohol, alkene. d) alkane, alkene, 2º alcohol, ketone. 3- Before a fatty acid can enter the fatty acid oxidation, it must be activated and then shuttled across the inner mitochondrial membrane. The activating agent and shuttle molecule are, respectively, a) CoA and carnitine. b) CoA and citrate. c) acetyl CoA and carnitine. d) acetyl CoA and citrate 4- How many FADH2 and NADH molecules are produced, respectively, during 3 turns of the fatty acid degradation cycle? a) 1, 1 b) 1, 2 c) 2, 1 d) 3, 3 5- Which of the following is a correct ordering of cholesterol intermediates as they are encountered during the biosynthesis of cholesterol? a) mevalonate, lanosterol, squalene, isopentenyl pyrophosphate b) lanosterol, squalene, isopentenyl pyrophosphate, mevalonate c) squalene, isopentenyl pyrophosphate, mevalonate, lanosterol d) mevalonate, isopentenyl pyrophosphate, squalene, lanosterol 23 | P a g e 6- Three of the following compounds are classified as ketone bodies. The one that is not is a) acetoacetate. b) β-hydroxybutyrate. c) acetobutyrate. d) acetone. 7- The carrier of the “two carbon units” used to biosynthesize a fatty acid is a) acetyl CoA. b) acetyl ACP. c) malonyl CoA. d) malonyl ACP. 8- Which of the following is a possible fate for the acetyl CoA produced from the degradation of a fatty acid? a) conversion to pyruvate b) ketone body formation c) cholesterol biosynthesis d) more than one correct answer 9- Which of the following compounds is an intermediate in the conversion of glycerol to phosphatidic acid ? a) acetoacetate b) acetone c) 3-phosphoglycerate d) no correct response 10- The reducing agent needed in the process of lipogenesis is a) NADPH. b) ATP. c) FADH2. d) ADP 24 | P a g e  References Metabolism of Lipids: 5 Cycles (With Diagram) (biologydiscussion.com) Lipid metabolism - Wikipedia Baynes D (2014). Medical Biochemistry. Saunders, Elsevier Limited. pp. 121–122. ISBN 978-1- 4557-4580-7 Jo Y, Okazaki H, Moon YA, Zhao T (2016). "Regulation of Lipid Metabolism and Beyond". International Journal of Endocrinology. 2016: 5415767. doi:10.1155/2016/5415767. PMC 4880713. PMID 27293434. Abiotic Stress and Legumes.Tolerance and Management.Book 2021 25 | P a g e