Lipid Metabolism & Biosynthesis PDF

 Lipid Metabolism Metabolism of dietary lipids. -Oxidation of fatty acids and ketone bodies. Biosynthesis of fatty acids, biosynthesis of triacylglycerols. Biosynthesis of cholesterol and steroids, plasma lipoproteins. 1  Lipid Biosynthesis These are endergonic and...

 Lipid Metabolism Metabolism of dietary lipids. -Oxidation of fatty acids and ketone bodies. Biosynthesis of fatty acids, biosynthesis of triacylglycerols. Biosynthesis of cholesterol and steroids, plasma lipoproteins. 1  Lipid Biosynthesis These are endergonic and reductive reactions, that use ATP as a source of energy and reduced electron carriers usually NADPH as a 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 (Acyl Carrier Protein). 3. the growing F.A is elongated by the sequential addition of 2-carbon units. 4. the reductant in fatty acid synthesis is NADPH, 5. elongation of F.A is stopped at C16 and further elongation or insertion of double bonds is 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.A2are synthesized mainly in the liver and lactating mammary gland and to a lesser extent in adipose tissue and kidney  Formation of Malonyl CoA from acetyl CoA and HCO 3- This step is mediated by Acetyl CoA carboxylase that consists of three domains. Biotin Prosthetic group 2nd unit of 3rd domain Acetyl CoA carboxylase first part of acetyl CoA carboxylase NET 3 : Acetyl CoA + ATP + HCO3- Malonyl CoA + ADP + Pi + H+  Acetyl CoA carboxylase enzyme 4 Biosynthesis of fatty acids 5 6 7 8 9  The F.A grows by 2 carbon units donated by Malonyl CoA for palmitate synthesis 16 C 7 steps 10  The F.A synthase complex consists of 7 separated polypeptides. These portions act together to catalyze the formation of F.A from acetyl CoA and Malonyl CoA Acetyl CoA is attached to KS by -SH group thioester linkage. 11 Malonyl CoA is attached to ACP through -SH group forming thioester linkage.  The -SH of KS is charged Of phosphopantetheine to acetyl CoA by Acetyl - group CoA - ACP transacetylase (AT) KS : - ketoacyl- ACP synthase The -SH group of ACP is bonded to malonyl CoA by the enzyme Malonyl - CoA - ACP transferase (MT) 12  CO2 is released is the Good nucleophile same HCO3- that condensed into acetyl CoA to form malonyl CoA by acetyl CoA carboxylase. Why? Coupling the HCO3- that reacts with decarboxylation and acetyl CoA to form condensation malonyl CoA is released exergonic in the form CO2. Is the first step The cleavage of acetyl in F.A synthesis group from F.A is highly   - ketoacyl - ACP exergonic & condensation synthase of acetyl CoA endergonic. So the introduction of CO2 makes the condensation more thermodynamically favored. 13 * electron donor for this reduction NADPH Enzyme : - ketoacyl ACP reductase (KR) D- 14 D isomer  -D-hydroxyacyl dehydratase enzyme (HD) 15  Enoyl ACP reductase enzyme (ER) NADPH is the electron donor 16  Malonyl CoA-ACP- transferase (MT) This process is catalyzed by acetyl CoA ACP transacetylase (AT) *Then the butyryl group is transferred from phosphopantetheine -SH of ACP to the Cys-SH of - ketoacyl ACP- synthase (KS) 17  By MT By KS 18  The biosynthesis of F.A requires energy Needs ATP and reducing power of NADPH * For synthesis of 16 C F.A (palmitic acid) 7 cycles of condensation, 7 reduction, 7 dehydration and 7 reduction. 7 acetyl CoA + 7 CO2 + 7 ATP 7 malonyl CoA + 7 ADP + 7 Pi Acetyl CoA + 7 malonyl CoA + 14 NADPH + 14 H+ palmitate + 7 CO2 + 8 CoA + 14 NADP+ + 6 H2O 8 acetyl CoA + 7 ATP + 14 NADPH + 14 H+ palmitate + 8 CoA + 6 H2O + 7 ADP + 7 Pi + 14 NADP+ 19  Fatty acid synthase of different organisms F.A synthase from : Bacteria, plants: 7 activities in 7 separated polypeptides. Yeast: 7 activities in two separated polypeptides. Vertebrates: 7 activities in one large polypeptide One large polypeptide with two domains with 7 activities 20  Source of Acetyl Co for F.A synthesis 21  Acetyl-CoA 22  Source of Acetyl Co for F.A synthesis The major source of acetyl CoA is from mitochondrial oxidation of pyruvate and from the catabolism of amino acids. Acetyl CoA is produced from - oxidation of F.A is NOT a significant source of acetate because synthesis and oxidation of F.A are reciprocally regulated. Acetyl CoA is produced in the mitochondrial matrix. Synthesis of F.A occurs in the cytosol. Acetate is shuttled out of mitochondria as citrate because the membrane is NOT permeable for acetate. 23 Source of NADPH for F.A synthesis Oxidation of malate to pyruvate will produce NADPH Reducing power of NADH is converted into NADPH Oxaloacetate ➔ Malate ➔ Pyruvate The other NADPH are produced pentose phosphate pathway 24 Regulation of F.A synthesis and degradation The reaction catalyzed by acetyl CoA carboxylase is the rate limiting step. Palmitoyl CoA : -ve feed back in the biosynthesis, it inhibits the carboxylase enzyme. When mitochondrial acetyl CoA + ATP increase increase production of citrate which transfers out of mitochondria increase cystolic citrate. 1. citrate is an allosteric activator for this carboxylase. 2. citrate is the precursor of cystolic acetyl CoA high level of citrate increase F.A synthesis 25 Regulation of F.A synthesis and degradation Citrate inhibits the PFK1 reducing the flow of carbon through glycolysis. Insulin stimulates F.A synthesis by activating the carboxylase. Glucagon + Epinephrine decrease F.A synthesis by increasing the phosphorylation 26  F.A synthesis and degradation are reciprocally regulated (not active at the same time) Malonyl CoA: the 1st intermediate in the synthesis of F.A inhibits the carnitine acyl transferase I, so inhibits the - oxidation. Regulation of the level of gene expression? Intake of a large amount of CHO for long periods increases the expression of carboxylase and F.A synthase enzyme. 27  Long-chain saturated F.A are synthesized from palmitate by Desaturation by fatty acyl CoA desaturase the Fatty acid elongation (liver) system Found in smooth endoplasmic reticulum and mitochondria The donor for two carbon units = acetyl CoA Act on the F.A in the phospholipid, phosphatidylcholine or linked glycerol 28  Desaturation of F.A occurs in the phospholipid, phosphatidylcholine or linked glycerol 29  Biosynthesis of Triacylglycerols Ingested or synthesized F.A have two fates : Incorporation in TG for the storage of metabolic energy. Incorporation in phospholipids components of membranes. And the activated pathway is dependent on the need of cells. Rapid growth synthesis of membranes increase demand for phospholipid if the organism has a lot of food and fats storage of F.A in TG A few hundred mg of glycogen is stored in the body, but about 15 kg of lipids (in a normal person) can supply the body with energy for 12 weeks. Excess CHO is stored as fats, While fats can’t be used for glucose synthesis. TG and phospholipids have the same precursors “Glycerol 3-phosphate” and “fatty acyl CoA” TG is biosynthesized primarily in the liver and also in adipocytes. 30  Biosynthesis of TG production of L-glycerol 3-phosphate adipocyte liver In liver and kidney mainly Just in liver and adipocytes, (cytosol) 31  Synthesis by acyl CoA synthetase Phosphatidic acid is formed in trace amounts in the body, but it is the central intermediate in the biosynthesis of phospholipids and in TG (diacylglycerol 3-phosphate) 32 TG is synthesized in the liver, adipocytes, mammary glands and intestinal mucosal cells. Is the parent compound of TG and phospholipid Transesterification by acyl transferase 33 Glycerol 3-phosphate is synthesized in the liver and adipocyte from glucose Glucose entrance is Glucose entrance is not insulin dependant insulin dependant Fates of TG in the liver and adipose tissue TG are stored in the cytosol of the cells in anhydrous form, and ready for mobilization in the liver, TG stored are very little. TG with cholesterol and cholesterol esters are packed in lipoprotein form and exported to other organs via blood. 34 Regulation of TG Biosynthesis CHO or protein consumed in excess of energy needs is stored in the form of TG. Biosynthesis and degradation of TG are regulated reciprocally. Depending on the metabolic recourses and requirements. Different hormones affect TG metabolism Insulin, Glucagon, adrenal cortical hormones. Unable to utilize glucose and to convert it to TG - oxidation is continued of F.A (excess acetyl CoA) increase ketone bodies loss of weight 35 ❖ Biosynthesis of membrane phospholipids Synthesis of membrane lipids requires in general : 1. Synthesis of backbone molecule (glycerol or sphingosine) 2. Attachment of F.A to the backbone by ester or amide linkage. 3. Addition of hydrophilic head group through phosphodiester linkage. 4. Alteration or exchange of head group to yield final phospholipids. Synthesis occur in smooth endoplasmic reticulum then goes to Golgi apparatus. The polar head is linked to the glycerol by phosphodiester linkage. All cells except RBC can synthesize phospholipid, but TG is synthesized in the liver, adipocyte, mammary glands and intestinal mucosal cells. 36  Two strategies for attaching head groups In the biosynthesis one of the OH group is activated by attachment of a nucleotide (CDP) CDP-diacyl CDP-Head group Activation of Activation of head group diacyl glycerol 37 Strategy 1 Synthesis of cardiolipin and phosphatidyl inositol (PI) 38 Strategy 2 39 41  Two strategies biosynthesis of membrane phospholipids 40 42  Biosynthesis of Sphingolipids 1st step Palmitoyl CoA + serine 18 Carbons The reducing agent 43 Ceramide is parent compound of Sphingolipids Polar lipids are targeted to specific cell membrane. After the synthesis in the S.E.R. the polar lipids (phospho, sphingo, glyco lipids) they transferred to different cell membranes by Golgi apparatus in form of vesicles. 44 ✓ Degradation of Phospholipids The enzymes responsible for degradation are present in all tissues and pancreatic juice. Phospholipases hydrolyze the esters of phosphodiester bonds of phosphoglycerides. 45 46  Degradation of phospholipids 47 ✓ Degradation of Sphingomyelin Sphingolipids are degraded by sphingomyelinase which removes phosphorylcholine leaving Ceramide. Ceramide is cleaved by ceramidase into sphingosine and free fatty acid. 48  The End. 49