Lipid Synthesis Anabolism Biosynthesis of Fatty Acids & Lipids

Liver cytosol ANABOLISM OUTLINE: Photosynthesis Carbohydrate Biosynthesis in Animals Biosynthesis of Fatty Acids and Lipids Fatty Acids contrasts location & transport Synthesis Triacylglycerides Membrane lipids acetyl-CoA carboxylase fatty acid synthase ACP priming 4 steps Control of fatty acid metabolism ACC Reciprocal control of b-ox Diversification of fatty acids Eicosanoids Prostaglandins,Thromboxanes, and Leukotrienes Glycerophospholipids Sphingolipids Isoprene lipids: Ketone Bodies Cholesterol isoprene synth. Joining Cholesterol Synth. Fates Cholesterol esters Bile acids Steroid Hormones Metabolism & transport Control ANABOLISM II: Biosynthesis of Fatty Acids & Lipids Lipid Biosynthesis Fatty Acid Synthesis Isoprene Biosynthesis Fat Catabolism Fatty Acid Degradation Ketone body Utilization 1 Cholesterol and Steroid Biosynthesis mevalonate kinase Mevalonate to Activated Isoprenes • Two phosphates are transferred stepwise from ATP to mevalonate. • A third phosphate from ATP is added at the hydroxyl, followed by decarboxylation and elimination catalyzed by pyrophosphomevalonate decarboxylase creates a pyrophosphorylated 5-C product: D3-isopentyl pyrophosphate (IPP) (isoprene). • Isomerization to a second isoprene dimethylallylpyrophosphate (DMAPP) gives two activated isoprene compounds that act as precursors for all of the other lipids in this class IPP D 3-isopentyl pyrophosphate Isopentyl-D-pyrophosphate isomerase Cholesterol and Steroid Biosynthesis DMAPP mevalonate kinase Mevalonate to Activated Isoprenes • Two phosphates are transferred stepwise from ATP to mevalonate. • A third phosphate from ATP is added at the hydroxyl, followed by decarboxylation and elimination catalyzed by pyrophosphomevalonate decarboxylase creates a pyrophosphorylated 5-C product: D3-isopentyl pyrophosphate (IPP) (isoprene). • Isomerization to a second isoprene dimethylallylpyrophosphate (DMAPP) gives two activated isoprene compounds that act as precursors for all of the other lipids in this class IPP D 3-isopentyl pyrophosphate Isopentyl-D-pyrophosphate isomerase DMAPP 2 Cholesterol and Steroid Biosynthesis DMAPP Joining Isoprenes IPP H EAD H EAD TAIL TAIL • The two isoprenes join head to tail, displacing one set of diphosphates, forming 10-carbon geranyl pyrophosphate (GPP). geranyl-pyrophosphate synthase (I (DMAPP) Carotenoids Gibberellins Tocopherols Chlorophylls Geranyl-PP synthase Mechanism Geranyl-pyrophosphate synthase Resonance stabilized carbonium ion Farnesyl-PP synthase Ionization by elim ination of PP i Isopentyl pyrophosphate (IPP) Squalene synthase Geranyl pyrophosphate (GPP) H+ deprotonation Cholesterol and Steroid Biosynthesis Formation of Squalene • Geranyl pyrophosphate (GPP) joins to another isopentenyl pyrophosphate. à forms 15-C farnesyl pyrophosphate (tail-to-tail) • Two farnesyl pyrophosphates (FPP) join tail to tail to form phosphate-free squalene. 3 Cholesterol and Steroid Biosynthesis Synthesis of Cholesterol Conversion of Squalene to Four-Ring Steroid Nucleus • Squalene monooxygenase (squalene epoxidase) adds one oxygen to the end of the squalene chain. (2,3-oxidosqualene) à forms squalene 2,3-epoxide • Here, pathways diverge in animal cells versus plant cells. • In plants, the epoxide cyclizes to other sterols, such as stigmasterol. • In fungi, it forms ergosterol. • The cyclization product in animals is lanosterol, which converts to cholesterol. lanosterol synthase ⑤ ② ④① ③ ⑥ ⑦ 1) C14-demethylation (5 rxns) 2) C14-reduction 3) C4-demethylation (10 rxns) 4) C8à7 isomerization 5) C24-reduction 6) C5-desaturation 7) C7-reduction Cholesterol and Steroid Biosynthesis Synthesis of Cholesterol Conversion of Squalene to Four-Ring Steroid Nucleus • Squalene monooxygenase (squalene epoxidase) adds one oxygen to the end of the squalene chain. (2,3-oxidosqualene) à forms squalene 2,3-epoxide • Here, pathways diverge in animal cells versus plant cells. • In plants, the epoxide cyclizes to other sterols, such as stigmasterol. • In fungi, it forms ergosterol. • The cyclization product in animals is lanosterol, which converts to cholesterol. lanosterol synthase ⑤ ② ④① ③ ⑥ ⑦ 1) C14-demethylation (5 rxns) 2) C14-reduction 3) C4-demethylation (10 rxns) 4) C8à7 isomerization 5) C24-reduction 6) C5-desaturation 7) C7-reduction 4 Cholesterol and Steroid Biosynthesis Fates of Cholesterol After Synthesis Vitamin D pre-VitD 3 7-dehydro-Chol •In vertebrates, most cholesterol is synthesized in the liver, then exported. -They are exported as bile acids, biliary cholesterol, or cholesteryl esters. -Bile is stored in the gall bladder and secreted into the small intestine after fatty meal. -Bile acids such as taurocholic acid emulsify fats. –They surround droplets of fat, increasing surface area for attack by lipases. ( Oxysterols) •Other tissues convert cholesterol into steroid hormones and Vitamin D From degradation of Cys Lecithin-Cholesterol Acyl TransferaseCatalyzed Reaction Occurs in HDL (LCAT) •Adrenal gland-synthesized steroids: – mineralcorticoids • control electrolyte balance, reabsorption of Na+, Cl−, HCO3− from kidney – glucocorticoids • regulate gluconeogenesis, reduce inflammation •Gonad-synthesized steroids: – progesterone, testosterone, estrogens Steroid Biosynthesis Steroid Hormones Derived from Cholesterol • Takes place in mitochondria • The “side chain” on C-17 of the D ring is modified, then cleaved. ox=C3O & iso=C4/5 C-11 C-17 C-21 C-21 C-11 C-17 –acetateC-20/21 –C-19 aromatase C-20 C-22 C-18 ox-C18O • Two adjacent carbons are hydroxylated. • Desmolase cleaves • All three use mixedfunction oxidases, NADPH and cytochrome P450 5 Steroid Biosynthesis Steroid Hormones Derived from Cholesterol • Takes place in mitochondria • The “side chain” on C-17 of the D ring is modified, then cleaved. ox=C3O & iso=C4/5 C-11 C-17 C-21 C-21 C-11 C-17 –acetateC-20/21 –C-19 aromatase C-20 C-22 C-18 ox-C18O • Two adjacent carbons are hydroxylated. • Desmolase cleaves • All three use mixedfunction oxidases, NADPH and cytochrome P450 Cholesterol and Steroid Biosynthesis TABLE 21-1 Major Classes of Human Plasma Lipoproteins: Some Properties Composition (wt %) Lipoprotein Density (g/ml) Protein PL Free cholesterol Cholesteryl esters Triacylglycerols Chylomicrons <1.006 2 (ApoB-48,-E,-CII) 9 1 3 85 VLDL 0.95–1.006 10 (ApoC-I,CII) 18 7 12 50 LDL 1.006–1.063 23 (ApoB-100) 20 8 37 10 HDL 1.063–1.210 55 (ApoA-I,-II) 24 2 15 4 Source: Data from D. Kritchevsky, Nutr. Int. 2:290, 1986. • Lipids are carried through the plasma on spherical particles. –surface is made of protein (called apolipoprotein*) and a phospholipid monolayer –interior contains cholesterol, TAGs, and cholesteryl esters, which are more nonpolar than cholesterol • Named based on position of sedimentation (density) in centrifuge *“apolipoprotein” refers to the protein part of a lipoprotein particle. • Composition varies between class of lipoprotein • Includes four major classes shown in Table 6 Cholesterol and Steroid Biosynthesis Biological Roles and Characteristics of Lipoproteins TABLE 21-2 Apolipoproteins of the Human Plasma Lipoproteins Apolipoprotein Polypeptide molecular weight Lipoprotein association Function (if known) ApoA-I 28,100 HDL Activates LCAT; interacts with ABC transporter ApoA-II 17,400 HDL Inhibits LCAT ApoA-IV 44,500 Chylomicrons, HDL Activates LCAT; cholesterol transport/clearance ApoB-48 242,000 Chylomicrons Cholesterol transport/clearance ApoB-100 Binds to LDL receptor 512,000 VLDL, LDL ApoC-I 7,000 VLDL, HDL ApoC-II 9,000 Chylomicrons, VLDL, HDL Activates lipoprotein lipase ApoC-III 9,000 Chylomicrons, VLDL, HDL Inhibits lipoprotein lipase ApoD 32,500 HDL ApoE 34,200 Chylomicrons, VLDL, HDL Triggers clearance of VLDL and chylomicron remnants ApoH 50,000 Possibly VLDL, binds cardiolipin Roles in coagulation, lipid metabolism, apoptosis, inflammation Source: Information from D. E. Vance and J. E. Vance (eds), Biochemistry of Lipids and Membranes, 5th edn, Elsevier Science Publishing, 2008. Cholesterol and Steroid Biosynthesis Biological Roles of Lipoproteins in Trafficking Cholesterol and TAGs Receptor-Mediated Endocytosis IN: Exogenous (diet) ❶–❹ Pathway HDL Pathway 10 – 13 OUT: Endogenous Pathway Enterohepatic Pathway ❺–❾ 14 7 Cholesterol and Steroid Biosynthesis Cholesterol and Steroid Biosynthesis Regulation of Cholesterol Metabolism •Major regulation is by gene expression: Three levels – Sterol regulatory element-binding proteins (SREBPs) • When sterol levels are high, oxysterols are produced, which retain SREBPs in the ER membrane with other proteins. • When sterol levels fall, the complex is cleaved and moves to the nucleus. • It activates transcription of HMG-CoA reductase and LDL receptor, as well as other genes. – Translational control on mRNA stability • Increased mevalonate and other isoprenes destabilize the HMG-CoA mRNA – Post-translational control • High cholesterol produces oxysterol, which allosterically binds HMG-CoA reductase • Destabilizes by a conformational change that allows ubiquitination • Proteolytic degradation of HMG-CoA reductase •Minor regulatory mechanisms: Phosphorylation/de-phosphorylation – Covalent modification provides short-term regulation – Glucagon, epinephrine • cascades lead to phosphorylation, ¯ activity • AMP-dependent protein kinase Other effects of high cholesterol Activation of ACAT, which increases esterification for storage Transcriptional regulation of the LDL receptor gene activation • also when AMP rises, kinase phosphorylates the enzyme à activity ¯, cholesterol synthesis ¯ cascades lead to dephosphorylation,­ activity – Insulin: 8 Cholesterol and Steroid Biosynthesis Regulation of Cholesterol Synthesis by Sterol regulatory elementbinding protein (SREBP) Cholesterol ↓ HMG-CoA reductase↓ • Insig (insulin-induced gene protein) senses cholesterol levels through oxysterol. HMG-CoA reductase↑ LDL receptor↑ – Also helps sequester SREBP in ER – triggers ubiquination of HMG-CoA reductase – targets the enzyme for degradation by proteasomes Cholesterol and Steroid Biosynthesis We learned that: Summary • synthesis of fatty acids is a multistep process starting from acetyl-CoA and its carboxylated product, malonyl-CoA; it uses a poly-enzyme protein • After synthesis of palmityl-CoA, fatty acids are elongated and desaturated; PUFAs are used for prostaglandin, thromboxane, and leukotriene synthesis • phospholipids are a precursor to TAGs • phospholipids and TAGs are built on a glycerol backbone that can be derived from dihydroxyacetone phosphate or glycerol • head groups are attached using one of two methods, both use CDP carrier • pathways to the synthesis of specific head groups vary by organism and may use salvage pathways • cholesterol is derived from isoprene units, which comes from HMG-CoA • production of isoprene for cholesterol biosynthesis occurs via the mevalonate pathway and starts with multiple acetyl-CoA • cholesterol can be metabolized and modified in a variety of ways • cholesterol and TAGs are trafficked in lipoproteins; classified by density • Regulation of lipid biosynthesis…. 9