Ecosanoids: Prostaglandins and related compounds

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1 Ecosanoids: Prostaglandins and related compounds Presented by The requirements for the Pharmaceutical Biochemistry II Philadelphia University Faculty of pharmacy

2 Ecosanoids: Prostaglandins and related compounds Prostaglandins (PGs), thromboxanes (TXs) and leukotrienes (LTs), are known as eicosanoids. They are extremely potent compounds that elicit a wide range of responses, both physiologic (inflammatory response) and pathologic (hypersensitivity). They ensure gastric integrity and renal function, regulate smooth muscle contraction (intestine and uterus are key sites) and blood vessel diameter, and maintain platelet homeostasis. Eicosanoids are produced in very small amounts in almost tissues. Eicosanoids act locally and have an extremely short half-life, being rapidly metabolized to inactive products. Their biologic actions are mediated by plasma membrane G protein coupled receptors which are different in different organ systems.

3 Synthesis of prostaglandins and thromboxanes Arachidonic acid, an ω-6 fatty acid (20:4) is precursor of prostaglandins in humans. Arachidonic acid derived by elongation and desaturation of essential fatty acid linoleic acid also known ω-6 fatty acid. Arachidonic acid stored in the phospholipid of cell membrane at C 2 and released by phospholipase A2 (inhibited by cortisol)

4 Synthesis of prostaglandins and thromboxanes Synthesis of prostaglandins PGH2: 1. The oxidative cyclization of free arachidonic acid to yield PGH 2 by: prostaglandin endoperoxide synthase (PGH synthase) in endoplasmic reticulum membranebound protein. PGH synthase has two isoenzymes 1. Cyclooxygenase, COX-1 is constitutively in tissues, and is required for maintenance of healthy gastric tissue, renal homeostasis, and platelet aggregation. 2. Cyclooxygenase, COX-2 is inducible in a limited number of tissues in response to products of activated immune and inflammatory cells. PGH synthase has two catalytic activities: Fatty acid cyclooxygenase (COX), which required two molecules of O2. Peroxidase, which is dependent on reduced glutathione.

5 Synthesis of prostaglandins and thromboxanes PGH2 is converted to a variety of prostaglandins and thromboxanes, as shown in Figure.

6 Inhibition of prostaglandin synthesis: (see the figure in the next slide) The synthesis of prostaglandins inhibited by cortisol (a steroidal antiinflammatory agent) inhibits phospholipase A2 activity therefore, the precursor of the prostaglandins, arachidonic acid, is not available. Aspirin, indomethacin, and phenylbutazone (all nonsteroidal antiinflammatory agents [NSAIDS]) inhibit both COX-1 and COX-2 therefore, prevent the synthesis of PGH2. Inhibitors specific for COX-2 (example, celecoxib1) were designed to reduce pathologic inflammatory processes while maintaining the physiologic functions of COX-1.

8 Cholesterol, Lipoprotein, and Steroid Metabolism Presented by The requirements for the Pharmaceutical Biochemistry II Philadelphia University Faculty of pharmacy

9 Cholesterol Cholesterol, the characteristic steroid alcohol of animal tissues, Functions of cholesterol 1. It is a structural component of all cell membranes for maintain the fluidity of cell membrane. 2. Cholesterol is a precursor of bile acids, steroid hormones, and vitamin D.

10 Major sources of liver cholesterol 1. Dietary cholesterol which converted to chylomicron in intestine then transfer to the liver. 2. Cholesterol from extrahepatic tissues such as HDL which transfer to the liver.

11 Structure of Cholesterol 1. Cholesterol or sterol is a hydrophobic compound, with a single hydroxyl group located at carbon 3 of the A ring. 2. cholesteryl ester A fatty acid attached at C3, producing more hydrophobic cholesteryl ester.

12 Synthesis of Cholesterol All the carbon atoms in cholesterol are provided by 1. acetyl CoA, and 2. NADPH provides the reducing equivalents. The pathway of cholesterol synthesis is endergonic and driven to: 1. Hydrolysis of the high-energy thioester bond of acetyl CoA. 2. Hydrolysis of the terminal high-energy phosphate of ATP.

13 Synthesis of Cholesterol Cholesterol is synthesized in the cytoplasm. A. Synthesis of 3-hydroxy-3-methylglutaryl (HMG) CoA: Catalyzed by thiolase and HMG CoA synthase (see the figure). The first two reaction are similar in cholesterol synthesis and produces ketone bodies.

Synthesis of Cholesterol Cholesterol is synthesized in the cytoplasm. B. Synthesis of mevalonate Reduction of HMG CoA to mevalonate. It is catalyzed by HMG CoA reductase.

14 Synthesis of Cholesterol Cholesterol is synthesized in the cytoplasm. B. Synthesis of mevalonate Reduction of HMG CoA to mevalonate. It is catalyzed by HMG CoA reductase. Theمهم rate-limiting and regulated step in cholesterol synthesis is catalyzed by the smooth endoplasmic reticulum membrane protein, hydroxymethylglutaryl (HMG) CoA reductase.

15 Regulation of cholesterol synthesis (see figure in the next slide) 1. Expression of the gene for HMG CoA reductase is activated when cholesterol levels are low, via the transcription factor, sterol regulatory element-binding protein-2 (SREBP-2), bound to a sterol response element (SRE), resulting in increased enzyme and, therefore, more cholesterol synthesis. 2. Cholesterol or sterol at high level accelerated HMG CoA reductase degradation. 3. Covalently modification (phosphorylation/dephosphorylation) phosphorylated HMG CoA reductase is inactive by adenosine monophosphate activated protein kinase (AMPK) that is activated by AMP and decrease when ATP is high. Dephosphorylation HMG CoA reductase is active by phosphoprotein phosphatase. 4. Hormonal regulation HMG CoA reductase level increase when increase isulin and thyroxine by induction gene expression. Glucagon and glucocorticoids lead to decrease level of HMG CoA reductase.

16 Regulation of cholesterol synthesis

17 Lipoproteins Lipids are not water soluble so lipids transported in the plasma in association with apoproteins. Apolipoproteins are the protein components of the lipoproteins and divided to four groups (apo A, B, C and E) Apolipoproteins important in: 1. Maintaining the structure integrity of the lipoproteins. 2. Promote and control lipid transport through the circulation and lipid uptake into tissues. 3.Serving as activators or coenzymes for enzymes involved in lipoprotein metabolism. 4. Site for cell-surface receptors. Albumin is the carrier of free fatty acids.

Structure of typical lipoprotein particle Lipoproteins consist of Non-polar lipid core (triglyceride and cholesteryl esters) surrounded by Polar lipid surface (phospholipids, apolipoproteins,

18 Structure of typical lipoprotein particle Lipoproteins consist of Non-polar lipid core (triglyceride and cholesteryl esters) surrounded by Polar lipid surface (phospholipids, apolipoproteins, cholesterol). Lipoproteins are classified on the basis of their densities to: 1. Chylomicrons; transport exogeonus triglyceride to other tissues. 2. Very low density Lipoproteins (VLDL); transport endogenous triglyceride from liver to other tissue. 3. Low density Lipoproteins (LDL). 4. High density Lipoproteins (HDL).

19 Composition of Lipoproteins Lipoproteins are classified on the basis of their densities to: 1. Chylomicrons. 2. Very low density Lipoproteins (VLDL). 3. Low density Lipoproteins (LDL). 4. High density Lipoproteins (HDL).

Plasma lipoproteins can be separated on the basis of their electrophoretic mobility, as shown in Figure 18.

20 Size and density of lipoprotein particles: Chylomicrons has lowest in density and largest in size, and contain the highest percentage of lipid and the lowest percentage of protein. HDL particles are the densest. Plasma lipoproteins can be separated on the basis of their electrophoretic mobility, as shown in Figure 18.15, or on the basis of their density by ultracentrifugation. Large size, low density rich in TAGs Figure Small size, high density poor in TAGs

21 Cellular uptake and degradation of LDL Steps of LDLs degradation: (see figure on the next slide). 1. Structure of LDL contain apo-b100 as receptor recognized LDL receptor. 2. LDLs enter the cells by receptor-mediated endocytosis. 3. LDL receptor recycling after endocytosis. 4. Clathrin is a protein that plays a major role in the formation of coated vesicles. 5. LDLs content are degraded in the lysosomes to amino acids, fatty acids, and cholesterol. Regulation of LDLs degradation or metabolism: 1. High level of endocytosed cholesterol led to a. inhibits HMG CoA reductase and b. inhibits decreases synthesis of LDL receptors through preventing of SREBP-2 binding to the SRE. 2. High level of endocytosed cholesterol activated acyl CoA: cholesterol acyltransferase (ACAT) that can be esterified to cholesteryl esters and stored.

23 Amino acids: Disposal of Nitrogen Presented by The requirements for the Pharmaceutical Biochemistry II Philadelphia University Faculty of pharmacy

24 Amino acids Nitrogen enters the body by amino acids. amino acids are not stored by the body, Therefore, amino acids must be obtained from: 1. The diet. 2. synthesized de novo, or produced from normal protein degradation. Nitrogen leaves the body as urea, ammonia, and other products derived from amino acid metabolism amino acids in excess of the biosynthetic needs of the cell are rapidly degraded and forming ammonia which excreted in urine or it is used in synthesis of urea. Nitrogenمهم leaves the body as urea, ammonia, and other products derived from amino acid metabolism

25 Essential amino acids: The nine amino acids humans cannot synthesize are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. Protein contain only L- standard amino acids. Glucogenic amino acid is an amino acid that can be converted into glucose through gluconeogenesis (all amino acid are glucogenic except leucin and arginine). Ketogenic amino acids, which are converted into ketone bodies (some amino acid are ketogenic).

26 Nitrogen metabolism transformations role of body proteins involves two important concepts: 1. amino acid pool 2. protein turnover. 1. amino acid pool: supplied by three source, degradation of endogenous body proteins ( g/day) Dietary protein Synthesis of nonessential amino acids 2. protein turnover: synthesis of body protein ( g/day) amino acids are precursors of essential nitrogen-containing small molecules (ex: see figure). conversion of amino acids to glucose, glycogen, fatty acids, ketone bodies, or CO2 + H2O

Ubiquitin-proteasome proteolytic pathway: Damaged Proteins in cell or unneeded protein degraded by the ubiquitin-proteasome system by: Protein Ubiquitination that is means addition of ubiquitin to

27 Ubiquitin-proteasome proteolytic pathway: Damaged Proteins in cell or unneeded protein degraded by the ubiquitin-proteasome system by: Protein Ubiquitination that is means addition of ubiquitin to cellular protein (catalyzed by ATP). Proteins tagged with ubiquitin are then recognized proteasome. The proteasome unfolds, deubiquitinates, and cuts the target protein into fragments that are then further degraded to amino acids. Need ATP.

28 Digestion of dietary protein Proteolytic enzymes responsible for degrading proteins. Digestion gastric secretion (stomach): 1. Hydrochloric acid (HCL), (ph 2 3) to kill some bacteria and to denature proteins. 2. Pepsin: (acid-stable endopeptidase) Pepsin releases peptides and free amino acids from dietary proteins. Digestion by pancreatic enzymes (pancreas): Trypsin, Chymotrypsin, Elastase, Carboxypeptidas degraded protein to oligopeptide and amino acids Digestion of oligopeptides by aminopeptidase (di & tri- peptidase) to small peptide and amino acids in small intestine.

29 Removal of nitrogen from amino acids: The catabolism of most amino acids is the transfer of their α-amino group to α- ketoglutarate forming α-keto acid and glutamate. Aminotransferase reaction using α-ketoglutarate as the amino group acceptor.

Removal of nitrogen from amino acids (IN CYTOSOL): The first phase of catabolism involves the transfer of the α-amino groups by Pyridoxal phosphate (PLP) dependent transamination.

30 Removal of nitrogen from amino acids (IN CYTOSOL): The first phase of catabolism involves the transfer of the α-amino groups by Pyridoxal phosphate (PLP) dependent transamination. ALT, Transfer amino group from alanine to α- ketoglutarate forming pyruvate and glutamate. AST, Transfer amino group from glutamate to oxaloacetate forming aspartate and α- ketogutarate. (figure 1) PLP PLP Figure 1: Reactions catalyzed during amino acid catabolism. A. Alanine aminotransferase (ALT). B. Aspartate aminotransferase (AST).

reactions) Deamination of glutamate: (reversible مهم oxidative deamination of glutamate by glutamate dehydrogenase, forming ammonia and converted glutamate to α-ketoglutarate.

31 reactions) Deamination of glutamate: (reversible مهم oxidative deamination of glutamate by glutamate dehydrogenase, forming ammonia and converted glutamate to α-ketoglutarate. (figure 2).The enzyme use NAD +. Reductive amination of glutamate by glutamate dehydrogenase, utilizevammonia and converted α-ketoglutarate to glutamate (figure 2).The enzyme use NADPH

Transport of ammonia to the liver Two mechanisms are available for transport of ammonia: Theمهم First: Ammonia transport from peripheral tissues to the liver as glutamine: By glutamine synthetase

32 Transport of ammonia to the liver Two mechanisms are available for transport of ammonia: Theمهم First: Ammonia transport from peripheral tissues to the liver as glutamine: By glutamine synthetase (need ATP), Ammonia combined with glutamate to produce glutamine (non toxic) that is transfer from tissue after to liver (see the figure). Theمهم Second: Ammonia transport from muscle to the liver as alanine: In the liver, the pathway of gluconeogenesis can use the pyruvate to synthesize glucose, which can enter the blood and be used by muscle this pathway called the glucosealanine cycle.

33 urea cycle The urea cycle converts highly toxic ammonia to urea for excretion. The urea cycle takes place primarily in liver. Need 3 ATP. See the figure in the next slide. الجدول مهم ملخص دورة اليوريا Step Reactions Reactants of the urea cycle: Products 1 NH 3 + HCO 3 + 2ATP 2 carbamoyl phosphate + ornithine carbamoyl phosphate + 2ADP + P i Catalyzed by CPS1 Location mitochondria citrulline + P i OTC mitochondria 3 citrulline + aspartate + ATP argininosuccinate + AMP + PP i ASS cytosol 4 argininosuccinate Arg + fumarate ASL cytosol 5 Arg + H 2 O ornithine + urea ARG1 cytosol

35 Amino acid degradation has two part : 1. The amino group is removed by transaminase reaction. 2. The carbon skeleton converted to an intermediate of the TCA cycle.

36 Fate of urea: Urea synthesis in liver transported to the kidneys and filtered and excreted in the urine. A portion of the urea diffuses from the blood into the intestine, and is cleaved to CO 2 and NH 3 by bacterial urease. This ammonia is partly lost in the feces, and is partly reabsorbed into the blood.

37 مهم The Key junction points There are 3 metabolic junctions 1- Glucose-6-Phosphate 2- Pyruvate 3- Acetyl CoA

39 : Noteمهم Ghrelin enhances appetite. Leptin is satiety (satisfication hormone).

40 Science Should be as simple as possible, but not simpler. Albert Einstein

41 References: Biochemistry. Lippincott's Illustrated Reviews. 6 th Edition by, Richard A Harvey, Denise R. Ferrier. Lippincott Williams and Wilkins, a Wolters kluwer business

Chapter 26 Biochemistry 5th edition. phospholipids. Sphingolipids. Cholesterol. db=books&itool=toolbar

http://www.ncbi.nlm.nih.gov/sites/entrez? db=books&itool=toolbar 1 The surface of a soap bubble is a bilayer formed by detergent molecules 2 Chapter 26 Biochemistry 5th edition phospholipids Sphingolipids