Biosynthesis of Membrane Lipids Membrane Lipids Glycerolipids Glycerophospholipid

Biosynthesis of Membrane Lipids

Membrane Lipids: Glycerolipids Glycerophospholipid

Glycerophospholipid Head Groups

Membrane Lipids: Sphingolipids Sphingomyelins with phosphocholine or phosphoethanolamine Neutral Glycolipids • • cerebrosides (1 sugar) globosides (> 2 sugars) Gangliosides • complex carbohydrates with sialic acid (Neu 5 Ac)

Biosynthesis of Membrane Lipids • Glycerolipids • Sphingolipids • Cholesterol

Biosynthesis of Phosphatidic Acid NADH NAD+ glycerol 3 -phosphate dehydrogenase ADP ATP glycerol kinase

Biosynthesis of Glycerophospholipids Strategy 1: Prokaryotes: • all glycerophospholipids Eukaryotes: • phosphatidylinositol • phosphatidyglycerol • cardiolipin • phosphatidylserine (yeast) Strategy 2: • • phosphatidylcholine phosphatidylethanolamine

Glycerophospholipid Biosynthesis in E. coli Strategy 1 (CDP-DAG) • phosphatidylserine (PS) by serine replacing CMP • phosphatidylethanolamine (PE) by decarboxylation of PS • phosphatidylcholine (PC) by methylation (3 x) of PE • phosphatidylglycerol (PG) by glycerol 3 -phosphate replacing CMP, then phosphatase • cardiolipin by one PG replacing glycerol on other PG

Biosynthesis in Eukaryotes of Anionic Glycerophospholipids Strategy 1 (CDP-DAG) • phosphatidylglycerol (PG) by glycerol 3 -phosphate replacing CMP, then phosphatase • cardiolipin by PG replacing CMP on CDP-DAG [CDP-DAG instead of PG] • phosphatidylinositol (PI) by inositol replacing CMP • phosphorylation of PI at positions 4 and 5

Cardiolipin Biosynthesis Summary Phosphatidylglycerol cardiolipin synthase (prokaryotic) Glycerol CDP-diacylglycerol cardiolipin synthase (eukaryotic) CMP

Biosynthesis of Phosphatidylcholine and Phosphatidylethanolamine in Mammals Strategy 2: CDP-alcohol • choline is phosphorylated and cytidylated to form CDP-choline • phosphatidylcholine (PC) formed by diacylglycerol replacing CMP • phosphatidylethanolamine (PE) formed by analogous pathway starting with ethanolamine • salvage pathways for choline and ethanolamine in yeast

Biosynthesis of Phosphatidylserine in Mammals Head group exchange • Mammals cannot directly make phosphatidylserine (PS) • PS formed by exchanging serine for ethanolamine on PE (endoplasmic reticulum) • Mammals can decarboxylate PS to form PE (mitochondria) • PC can be made from PE in mammalian liver • Salvage pathways in yeast

Summary of Pathways to Phosphatidylcholine and Phosphatidyethanolamine Enzymes for PE and PC: • • kinases cytidylate transferases DAG transferases methyltransferases (in liver) Also in Mammals: • PE ↔ PS exchange • PS → PE decarboxylation Not in Mammals: • direct PS biosynthesis from CDP-DAG + serine

Biosynthesis of Glycerophospholipids Summary of Strategies: • CDP-diacylglycerol + alcohol (head group) • CDP-alcohol + diacylglycerol • Head group exchange • Head group modification (methylation, decarboxylation)

Sphingolipid Biosynthesis • Serine decarboxylated and condensed on acyl-Co. A • NADPH reduces resulting ketone • Mixed-function oxygenase forms double bond of sphingosine • UDP-glucose for cerebroside • PC exchange for sphingomyelin

Cholesterol Biosynthesis Cholesterol is made in 4 stages: 1. Condensation of Mevalonate from 3 Acetates 2. Conversion of Mevalonate into Two Activated Isoprenes 3. Polymerization of 6 Activated Isoprenes into Squalene 4. Cyclization of Squalene and Modification of Lanosterol

Cholesterol Biosynthesis Stage 1: Condensation of Mevalonate from Acetate 1. Final step in β-oxidation of fatty acids in reverse (cytosolic) 2. Aldol condensation at C 3 carbonyl to form HMG-Co. A 3. Reduction of HMG-Co. A • • Committed step in biosynthesis of isoprenes Requires 2 NADPH for reduction of carboxylate to alcohol

Cholesterol Biosynthesis Stage 2: Conversion of Mevalonate to Activated Isoprenes • Requires 3 ATP’s in 4 enzymatic steps

Cholesterol Biosynthesis Stage 3: Polymerization of Activated Isoprenes • Farnesyl-PP requires: – 1 Dimethylallyl-PP – 2 Δ 3 -Isopentenyl-PP (head to tail polymerization) • Squalene requires: – 2 farnesyl-PP (head to head polymerization) • 1 NADPH required

Cholesterol Biosynthesis Stage 4: Cyclization of Squalene and Modification of Lanosterol • Monooxygenase forms squalene 2, 3 -epoxide • Cyclase reaction: – H+ opens epoxide ring – Cascade of 4 carbocation additions to C=C’s form the 4 rings – 2 hydride migrations, 2 methyl migrations, and H+ loss gives lanosterol • Modification of lanosterol (19 steps) gives cholesterol

This Slide FYI only – Not on Final Exam Cholesterol Biosynthesis Stage 4: Conversion of Lanosterol to Cholesterol 19 -Step process involves: • Oxidative removal of 3 methyl groups as HCO 2 H or CO 2 • 10 Monooxygenase reactions • Oxidation of 15 NAD(P)H • Reduction of 2 NAD+ Overall Cholesterol Biosynthesis: • 18 ATP hydrolyzed • 27 NAD(P)H oxidized (net) from Risley 2002, J. Chem. Educ. 79: 377

Metabolic Fates of Cholesterol 7α-hydroxylase and desmolase are cytochrome P-450 monooxygenases 7α-hydroxylase cholesterol 7 -dehydrocholesterol reductase desmolase OH 7α-hydroxycholesterol Bile (Salts) Acids Catabolism pregnenolone Steroid Hormones hν cholecalciferol (Vitamin D 3) Vitamin D

Cytochrome P-450 Monooxygenases • usually located in smooth endoplasmic reticulum • involved in hydroxylation of steroids or xenobiotics • General Reaction: AH + BH 2 + O–O → A–OH + B + H 2 O

Biosynthesis of Pregnenolone • Steroid hormone synthesis from cholesterol • side chain removed in mitochondria of steroidogenic tissues • Desmolase is a cytochrome P-450 mixed-function oxidase (monooxygenase) • 2 O 2 introduce diols at C 20, C 22 • 3 rd oxidation cleaves the C–C bond with ketone and aldehyde products

Steroid Hormones Pregnenolone

Vitamin D Metabolism in skin: in liver: in kidney: • • • 7 -dehydrocholesterol absorbs ultraviolet B (~300 nm) previtamin D 3 isomerizes to cholecaliferol (vitamin D 3) vitamin D 3 → 1 -hydroxyvitamin D 3 [1 -(OH)D 3] 1 -(OH)D 3 → 1, 25 -dihydroxyvitamin D 3 [1, 25 -(OH)2 D 3] Final 2 steps involve cytochrome P-450 monooxygenases

Bile (Salts) Acids • 7 hydroxycholesterol hydroxylated and oxidized • carboxylate is activated with Co. A • amino groups of glycine or taurine attack activated carboxylate 7α-hydroxycholesterol trihydroxycoprostanoate cholyl Co. A OH taurine glycocholate taurocholate