Lecture 9 - Metabolism of Glycolipids, Isoprenoids, and Steroids PDF

Metabolism of glycolipids, isoprenoids and steroids Dr. Ardina Grüber Nanyang Technological University School of Biological Sciences Division of Structural Biology and Biochemistry...

Metabolism of glycolipids, isoprenoids and steroids Dr. Ardina Grüber Nanyang Technological University School of Biological Sciences Division of Structural Biology and Biochemistry Singapore 637551 email: [email protected] According to their chemical structure lipids are organized in some classes of lipids: -ve 1 storage/generate of energy formate of biological membranes /otherlinkagea alcohols lesterified Marks Basic Medical Biochemistry: A clinical Approach, 4th edition  platform created by alcohol glyceral Glycerol fatty acid - I 2 3 X iS - H X is - H ~ - X is ~carbohydrate - X is carbohydrate & X is phosphate ester 2 is phosphate ester The phospholipids, which include both glycerophospholipids and sphingomyelins, are crucial components of membrane structure. They are also precursors of hormones such as eicosanoids and signal molecules. Voet, Voet: BIOCHEMISTRY 3rd edition  in eukaryotes apart from using , glycerol-3-phosphate we Pathways in can , DHAP use diacylglycerol & glycerophospholipid biosynthesis The major phospholipids found - precursor in membranes are shown in for other complex lipids green. Pathways found in both bacterial and eukaryotic cells are highlighted in light purple. Other reactions are confined to eukaryotic cells. DHAP = dihydroxyacetone phosphate; DAG = diacylglycerol; AdoMet = S-adenosylmethionine  - can come fro degradate of triacylglycerols phosphorylati The synthesis of glycerolipids in Fact-acP eukaryotes begins with the formation -cont ~ of phosphatidic acid, which may be C used commonly (used commonly formed from glycerol, diacylglycerol in eukaryotes bacteria in or dihydroxyaceton phosphate. transfer acyl grp from fatty I acyl-CoA molecule to C , Glycerokinase catalyzes the phosphorylation of glycerol to form glycerol-3-phosphate, which is then acylated at both the 1- and 2-positions to yield phosphatic acid. Enkaryotes transfer acyl grp from fatty acyl-col molecule to 22 Diacylglycerol kinase ATP ADP reversible ext I 2 P H20 - Phosphatidic acid can also be formed by phosphatase Garrett, Grisham: Biochemistry 4th edition degradate of triacylglycerol  another pathway > - [H] pathway reduct of before one to -OH transfer acyl grp from fatty acyl-CoA molecule to C , cacylation transfer acyl grp from fatty acyl-CoA molecule to C , I I 202 EN] reduct of before to -OH Eukaryotic systems can also utilize C dihydroxyaceton phosphate as a starting point transfer acyl grp from fatty 1 for synthesis of phosphatidic acid. acyl-CoA molecule to 22 Alternatively, dihydroxyacetone phosphate can be reduced to glycerol-3-phosphate by glycerol-3-phosphate dehydrogenase. 2 Garrett, Grisham: Biochemistry 4th edition  In eukaryotes, phophatidic acid is converted directly either to diacylglycerol or to cytidine diphosphodiacylglycerol (CDP- diacylglycerol). Diacylglycerol is a precursor for synthesis of triacylglycerol, phosphatidylethanolamine, and phosphatidylcholine. Triacylglycerol is synthesized mainly in adipose tissue, liver, and intestines and serves as the principal energy storage molecule. Its biosynthesis in liver and adipose tissues occurs via diacylglycerol acyltransferase, an enzyme bound to the cytoplasmic face of the ER. Garrett, Grisham: Biochemistry 4th edition  acting on triacylglycerols ~ &L& % , catalys clearage of FA yielding mononcylglycerol The biosynthesis of triacylglycerols is different in teflacy GrfrAs intestines. Recall that triacylglycerols from the diet are I broken down to 2-monoacylglycerols by specific lipases. Acyltransferases then acylate 2-monoacylglycerol to produce new triacylglycerols. trsf another acyl grp for to ncyl-cost a 3 Garrett, Grisham: Biochemistry 4th edition ~ interest I lipop to give chylomicroneti:  phosphorylate - nota clieth transfer CMP frm CTP to activate e molecule cmp , is a high energy molecule ~ active enough for condensate phosphoethanolamine is attached &0 of OH of C 3 identical pathway just diff enzymes M 7 PE can be methylated to give PC if molive is not present in sufficient amt Garrett, Grisham: Biochemistry 4th edition for I synthesis of PC  Mammals synthesize phosphatidylserine (PS) in a calcium ion-dependent reaction involving aminoalcohol exchange. The enzyme catalyzing this reaction is associated with the ER and will accept phosphatidylethanolamine (PE) and other phospholipid substrates. matrix A mitochondrial PS decarboxylase can decarboxyla +of subsequently convert PS to PE. PS where is carboxyl ap reversible released & PS converted to PE Garrett, Grisham: Biochemistry 4th edition  CDP-diacylglycerol as precursor of glycerolipids in eukaryotes In eukaryotes, phophatidic acid is converted directly either to diacylglycerol or to cytidine diphosphodiacylglycerol (CDP-diacylglycerol). From these two precursors, all other glycero- phospholipids in eukaryotes are derived. Garrett, Grisham: Biochemistry 4th edition  Phosphatidic acid is activated as CDP-diacylglycerol The reaction of CTP with phosphatidic acid is catalyzed by CDP-diacylglycerol synthase (reaction 1). This reaction is drawn to the right by the enzymatic hydrolysis of pyrophosphate, Lactivated form catalyzed by the ubiquitous of phosphatidic pyrophosphatase (reaction 2). acid energy released by hydrolysis promote format is ? of CPP-diacylglyceral high energy molecule CDP-diacylglycerol is activated for nucleophilic attack by various polar head groups (CMP is a good leaving group). © 2016 Pearson Education, Ltd. Carbohydrate - G3Dattachedtest inositos attached to phosphate gup Eukaryotes also use CDP-diacylglycerol, derived from phosphatidic acid, as a precursor for several other important cleare end phospholipids, including phosphate gup phosphatidylinositol (PI), phosphatidylglycerol (PG), and cardiolipin. ↑ Garrett, Grisham: Biochemistry 4th edition  β-ketosphinganine from synthase (PLP) ~ release cost Palmitoyl-CoA coenzyme fatty acyl-cort attached to serive creates ~ sphingolipids Condensation of palmitoyl-CoA and serine followed by reduction with NADPH yields sphinganine, which is then β-ketosphinganine reductase reduce before to alcohol acylated to N-acylsphinganine. (H) introduce one double bond chain & C, into fatty acyl Mixed function I Oxidase (animals) 14 - asimple ne 2 & more complex fatty acyl attached to N atom of G Lehninger: Biochemistry 4th edition  is carbohydrates interact e q.. galactose glucose , Ceramide is the building uppactivatese galactose / phosphatidyl - block for all other ethanolamina sphingolipids. Sphingomyelin, for example, is produced by [cerebrosides) transfer of phosphocholine galactose molecule attached from phohatidylcholine. to 0 atom & Cs ceramide Glycosylation of ceramide by sugar nucleotides yields cerebrosides, such as galactosylceramide, which phosphocholine make up 15% of the lipids attached to 0 afom & Is uramide of myelin sheath structures. Globosides- more than one sugar. Garrett, Grisham: Biochemistry 4th edition  Cerebrosides that contain one or more sialic acid (N-acetylneuraminic acid) moieties are called gangliosides. (globosides) Several dozen gangliosides have been characterized, and the general form of the Cactivated form) biosynthetic pathway is illustrated for the sialic acid attached to case of ganglioside GM2. and carbohydrate (galactose) - or & M : monosialic acid D : disialic acid Garrett, Grisham: Biochemistry 4th edition  locally acting signaling molecules Eicosanoids, so named because they are derived from C20 fatty acids, are breakdown products of phospholipids. Biologically active eicosanoids are short- lived, locally acting hormones. In humans, the most prevalent precursor of eicosanoids is a arachidonic acid, a C-20 polyunsaturated fatty acid that has four nonconjugated double bonds. cis - 5, 8, 11, 14-eicosatetraenoic acid or Arachidonic acid Voet, Voet: BIOCHEMISTRY 3rd edition  Prostaglandins, thromboxanes and leukotrienes are important eicosanoids synthesized from arachidonic acids Arachidonic acid is the precursor of prostaglandins (PG), thromboxanes (Tx), and leukotriene. Thromboxanes were discovered in thrombocytes. Leukotriene C was originally discovered in the class of white blood cells called polymorphonuclear leukocytes and was named after the source (leukocytes) and the triene structure (three conjugated double bonds). The key step in the synthesis of prostaglandins (involved in inflammation) is PGH synthase, a dual function enzyme. Aspirin inhibits the COX-1 and COX-2 forms of PGH synthase, yielding its anti- ©inflammatory activity. 2016 Pearson Education, Ltd.  flimulate arachidonate release & eicosaurids synthesis can tissue involve injury & inflammate ⑬ ⑰ 2 Bathways arachidonate release from phospholipids ⑪ catalyse I release of arachidonoy attached grp 2 to [2 3 ⑬ - clearage of phosphotidylinositol & (> release diacylglycerol PLA2: phosphatidylcholine , &2) OCC C2FA , C, Catalysed by phosphatidylethanolamine - diacylglycerol lipase cleare another FA cleares PLC: phosphaditylinositol - monracylglycerol lipase FA archidonate off last Voet, Voet: BIOCHEMISTRY 3rd edition  ext w Si introduce 20 All prostaglandins are cyclopentanoic acids creat atom radical & ( - derived from arachidonic acid. COX The biosynthesis of prostaglandins is initiated ↑ expressed in by an enzyme associated with the ER, called most cells , prostaglandin endoperoxide H synthase normal pat I prostaglanding (PGHS), also known as cyclooxygenase (COX). was 2 isoforms PENSI & PARIS2/ O introduced enzyme : Cox 1 & Cox2 in ring The enzyme that converts arachidonic acid to prostaglandin PGH2, possesses two distinct activities: cyclooxygenase (COX) and a ext w Cis gluthatione-dependent hydroxyperoxidase (POX). 15 POX induced by cytokine & etc. inflammatory cells & , is responsible for elevated path of prostaglanding during inflammate Voet, Voet: BIOCHEMISTRY 3rd edition Prostaglandin H2 which is derived from arachidonic acid, is a precursor for many other biologically significant molecules.  a acygup ed to 0 afom stops synthesis of PG X-Ray structure of PGH synthase (PGHS) from sheep. Inactivation of PGH synthase by aspirin. Heme, fluriprofen, and Tyr 385 are labeled. homodimer embedded forma of , radical within nembrane Aspirin (acetylsalicylate) exerts most of its effects by inhibiting the biosynthesis of prostaglandins. Its site of action is PGHS. COX activity is destroyed when aspirin O-acetylates Ser530 on the enzyme. Aspirin exerts its powerful anti-inflammatory effect by inhibiting the first step in their synthesis. Garrett, Grisham: Biochemistry 4th edition  Science 2007: vol. 318 pp. 1258-1265 Cholesterol The basic ring structure of sterols; the perhydrocyclopentanophenanthrene nucleus. Steroids constitute a class of lipids that are classified as isoprenoids because their structures are related to the five-carbon molecule isoprene. Steroids contain four fused rings: three six-carbon rings designated as A, B, C and five-carbon D ring. Cholesterol plays an essential role in mammalian biochemistry. It is not only a component of certain membranes but is also a precursor of steroid hormones and bile salts. Mathews, van Holde, Ahern: Biochemistry 3rd edition Cholesterol biosynthesis can be divided into three distinct processes 1. Conversion of C2 fragments (acetate) to a C6 isoprenoid precursor (mevalonate) activated 2. Conversion of six C6 mevalonates, via activated C5 intermediates, to the C30 squalene 3. Cyclization of squalene and its transformation to the C27 cholesterol 3 don't need to removise all e steps & structures Voet, Voet: BIOCHEMISTRY 3rd edition  The cholesterol biosynthesis pathway begins in the cytosol with the synthesis of mevalonate from Acetyl-CoA. The rate-limiting step in cholesterol biosynthesis is catalyzed by the HMG-CoA reductase, responsible for the formation of 3R-mevalonate from 3-Hydroxy-3-methylglutaryl-CoA. Here, HMG-CoA undergoes two NADPH- dependent reductions to produce 3R-mevalonate. [rl) before to alcohol Garrett, Grisham: Biochemistry 4th edition  phosphorylate The biosynthesis of squalene involves conversion of mevalonate to two key 5- c a r b o n i n t e r m e d i a t e s , i s o p e n t e ny l phosphorylate py ro p h o s p h a te a nd d i m e t hy la l ly l pyrophosphate (next slide), which joins to yield farnesyl pyrophosphate and then squalene. cleared Garrett, Grisham: Biochemistry 4th edition activated 25 intermediate  Isomerization of the double bond in isopentenyl pyrophosphate yields the double bond dimethylallyl pyrophosphate. shifted Condensation of these two 5-carbon intermediates produces geranyl Occurs in the cytosol pyrophosphate; addition of another 5- carbon isopentenyl group gives farnesyl Geranyl pyrophosphate pyrophosphate. 102 Both steps in the production of farnesyl pyrophosphate occur with release of pyrophosphate, hydrolysis of which drives these reactions forward. Squalene synthase (farnesyl transferase) is bound to the membrane of the ER. 152 bounded enzyme Occurs in in ER membrane the ER [H] Garrett, Grisham: Biochemistry 4th edition  Squalene monooxygenase, an enzyme bound to the ER, converts squalene to squalene-2,3-epoxide. This reaction employs FAD and NADPH as coenzymes and requires O2. A second ER membrane enzyme, 2,3-oxidosqualene lanosterol cyclase, catalyzes the second reaction, which involves a succession of 1,2 shifts of hydride ions and methyl groups. Garrett, Grisham: Biochemistry 4th edition  Although lanosterol may appear similar to cholesterol in structure, another 20 steps are required to convert lanosterol to cholesterol. The enzymes responsible for this are all associated with the ER. Leasily transported from Garrett, Grisham: Biochemistry 4th edition  Cholesterol has several fates A small fraction of the cholesterol made in liver is incorporated into the membranes of hepatocytes, but most of it is exported in one of three forms: biliary cholesterol, bile acids and transported cholesteryl esters, easily ~ Cholesterol will be used for the synthesis of steroid hormone and as a precursor of vitamin D.  activate on ~ ~ one atom of oxygen used as -or & introduce - or & (7 other released is as An α-hydroxyl group is formed at position 7 of Hy cholesterol. This reaction, which is inhibited by bile salts, is the rate-limiting step in bile salt synthesis. bond - of double of hydroxyl Marks Basic Medical Biochemistry: A clinical Approach, 4th edition  Nascent HDL is synthesized in liver and intestinal cells. It exchanges proteins with chylomicrons and VLDL. a transport back to live ~ HDL picks up cholesterol (C) from cell membranes. This cholesterol is converted to cholesterol ester (CE) by the enzyme lecithin-cholesterol acyltransferase (LCAT) dependent reaction. HDL transfers CE to VLDL in exchange for triacylglycerol (TG). The cholesterol ester transfer protein (CETP) mediates this exchange.  Overview of LDL receptors in cholesterol uptake and metabolism & LDL molecules synthesized in ER ② & transported to Goldi where molecules mature transported to ③ molecules are plasma memb ① sufficient LOL receptor molecules interact in PM & create coated pits P that ③ LOL receptors recognise LDL molecules & take up have = molecules i in firm of resicles endocytotic ⑧ 2 endocytotic resides fuse tat to create an endosome endosome cause pH to ⑦ At pump in LOL to dissociate drop causing from receptors endosome endosome ⑧ lysosome fuses i , molecules enzyme will degrade LDL ⑨ recycled reside return to PM , recycled ⑩ cholesterol , O , a. n. released & fate © 2016 Pearson Education, Ltd. of cholesteral is diff  don't need to removise - shorters long hydrocarbor chain structures Desmolase hydroxy see The biosynthesis of steroid hormones begins with the desmolase reaction, which converts cholesterol to pregnenolone. Desmolase is found in the mitochondria of tissues that synthesize steroids. Pregnenolone is transported from the mitochondria to the ER, where a hydroxyl oxidation and migration of the double bond yield - progesterone. to before from Ring B to A Pregnenolone synthesis in the adrenal cortex is activated by adrenocorticotropic hormone (ACTH), a peptide of 39 amino acid residues secreted by the anterior pituitary gland. Progesterone is also the precursor for synthesis of the other sex hormone steroids and the corticosteroids. Voet, Voet: BIOCHEMISTRY 3rd edition  Regulation of cholesterol synthesis Rate-limiting step is conversion of HMG CoA to mevalonate. Cholesterol synthesis is regulated by: ~ phosphorylated : inactive , dephosphorylated : active HMG-CoA reductase is regulated hormonal covalently by PP2A, PKA, and by AMPK rate-limiting Step & regulat Glucagon (promotes phosphorylation - inactivation of HMG-CoA reductase;) Insulin (promotes dephosphorylation- activation of HMG-CoA reductase). Intracellular cholesterol concentrations final pot inhibits HMG- Coff reductase (feedback inhibity) & also inhibits RME X represents metabolites of cholesterol that stimulate proteolysis of HMG-CoA reductase (oxysterol). & also prevents RME (dual action] & recycling of ↳ transcriptional regulate of genetic level cholester  To Make a Cholesterol (To the tune of "When Johnny Comes Marching Home") Copyright © 2010 Kevin Ahern Some things that you can build with acetyl-CoAs Are joined together partly thanks to thiolase They come together 1-2-3 Six carbons known as H-M-G And you’re on your way To make a cholesterol To synthesize a mevalonate in the cell Requires reducing HMG-CoA, as well The enzyme is a RE-ductase Controlled in allosteric ways When the cell's impelled To make a cholesterol. The mevalonate made in metabolic schemes Gets decarboxylated down to isoprenes They’re linked together willy-nil To build a PP-geranyl In the cells’ routines To make a cholesterol A single step links farnesyls but that’s not all The squalene rearranges to lanosterol From that there’s nineteen steps to go Before the sterol’s apropos Which you must recall To make a cholesterol The regulation of the scheme’s complex in ways Inhibited by feedback of the RE-duc-tase And statins mimic so they say The look of HMG-CoA So we sing their praise And not make cholesterol