Lipids and Fatty Acids

Transcription

1 Lipids and Fatty Acids Objectives: 1. What are Lipids? properties glycerolipids vs. isoprenoids glycerolipid structure glycerolipid nomenclature 2. Fatty acid biosynthesis ellular localization Substrate and its origin Enzymes of de novo fatty acid synthesis De novo fatty acid elongation (2>18) References: Buchanan et al Biochemistry and Molecular Biology of Plants. American Society of Plant Physiologists, Rockville MD. hapter 10.

2 What are lipids? Vitamin D2 Triacylglycerol (TAG)

3 What are lipids? Macromolecules defined on the basis of their physical properties: Insoluble in water Soluble in nonpolar organic solvents

4 Lipids are chemically heterogeneous Triacylglycerol (TAG) Glycerolipid Vitamin D2 Isoprenoid Originate from different biosynthetic pathways

5 What are lipids? Macromolecules defined on the basis of their physical properties: Insoluble in water Soluble in nonpolar organic solvents eteropolymers consisting of different subunits (glycerolipids), or do not form polymers (isoprenoids=terpenoids)

6 Glycerolipids Fats and oils Fatty acyl group Triacylglycerol (TAG) sn1 sn2 sn3 Glycerol backbone

7 Triacylglycerols form oil droplets Oilseed embryo

8 Glycerolipids Membrane lipids sn1 Fatty acyl group Glycerol backbone sn2 2 O sn3 O O O O ead group Acyl group O Monogalactosyldiacylglycerol (MGDG)

9 Glycerolipids Membrane lipids sn1 O Fatty acyl group Glycerol backbone sn2 O O sn3 O P O Acyl group ead group Phosphatidylcholine (P)

10 Membrane lipids are arranged in bilayers Why? (polar) (nonpolar) Membrane lipids are amphipathic

11 Membrane lipids are arranged in bilayers Water ydrophilic heads of lipids ydrophobic fatty acids omputer simulation of lipid bilayer showing waterfree hydrophobic core

12 Forces that stabilize membrane bilayers ydrophilic interactions between polar lipid head groups and water formation of hydrogen bonds. ydrophobic association between nonpolar fatty acids that don t interact with water and are pushed together by water which is forming hydrogen bonds with lipid head groups.

13 Glycerolipid nomenclature 1. Glycerol backbone 2. Fatty acyl groups on sn1 and sn2 positions Saturated no double bonds Unsaturated one or multiple double bonds 3. ead group on sn3 position Sugar moiety > Glycolipids Phosphate + alcohol > Phospholipids No head group > Acyl glycerols

14 Oleic acid 18:1 D9 or ω9 Number of arbon atoms Number of Double bonds Trivial name Double bond position (counting from carboxyl end or methyl end) cis configuration unless indicated Fatty acid nomenclature trans double bond E configuration arboxyl end Methyl end O O O O O O O O cis double bond Z configuration

15 ommon Eukaryotic Fatty Acids Symbol Systematic name Trivial name Saturated fatty acids 16:0 exadecanoic acid Palmitic acid 18:0 Octadecanoic acid Stearic acid 16:1 D7 exadecenoic acid Palmitoleic acid 16:1 D3 t transhexadecenoic acid Unsaturated fatty acids 16:2 D7D10 exadecadienoic acid 16:3 D7,D10,D13 exadecatrienoic acid 18:1 D9 Octadecenoic acid Oleic acid 18:2 D9,D12 Octadecadienoic acid Linoleic acid 18:3 D9,D12,D15 Octadecatrienoic acid Linolenic acid

16 haracteristics of ommon Fatty Acids 16 or 18carbon unbranched acyl chain (even number) Zero to 3 double bonds Double bonds are methylene interrupted (Allylic) omponents of membrane and storage lipids Allylic 2 group = =

17 Site: PLASTID Substrate: acetyloa De novo Fatty Acid Synthesis Pyruvate kinase Fig. 109

18 De novo Fatty Acid Synthesis AcetyloA arboxylase reaction 1 st committed step in fatty acid synthesis Fig. 1011

19 Enzymes: FAS Enzymes 1. Aase = AcetyloA arboxylase 2. MalonyloA:AP transacylase 3. ONDENSING ENZYMES key enzymes of FAS involved in bond formation (O 2 release IRREVERSIBLE!) different substrate specificities: 4. KR = 3ketoacylAP reductase 5. D = 3hydroxyacylAP dehydratase KAS III 2 KAS I 4 14 KAS II ER = 23 trans enoylap reductase AP = Acyl arrier Protein primer for FAS, carries the fatty acyl chain

20 Organization of FAS Enzymes TYPE I (Animals and Fungi) Location: ytosol Organization: A complex of 2 large subunits (250 kda each) catalyzing all the reactions of FAS KAS ER KR D TYPE II (Plants and Bacteria) Location: Plastid stroma Organization: Enzyme activities reside on individual proteins this reflects prokaryotic origin of plastids KAS ER KR D

21 Fatty Acid Synthesis A KASIII TA KAS I KR ER D Ohlrogge and Browse (1995) Plant ell 7:795

22 Fatty Acid Synthesis FAS hloroplast lipids AT FATB 16:0 16:0AP 16:0oA LAS KASII AT FATB 18:0 18:0AP 18:0oA LAS DES AT FATA 18:1 18:1AP 18:1oA LAS FAS = fatty acid synthesis FAT = acylap thioesterase LAS = acyloa synthetase AT = acyl transferase DES = stearoylap desaturase Main products of FAS are 16:0 and 18:1

23 Main products of FAS are 16:0 and 18:1 AT FAT Lipid synthesis in the chloroplast Export to the ER ow do we know that? From ( 14 )acetate labeling experiments of isolated plastids in vitro 16 and 18 fatty acyl chains get radioactively labeled Are there longer chain fatty acids in the cell?