Biosynthesis of Membrane Lipids, Cholesterol, Steroids and Isoprenoids CH353 February 5, 2008
Jan 20, 2016
Biosynthesis of Membrane Lipids, Cholesterol, Steroids and Isoprenoids
CH353 February 5, 2008
Summary
• Review of membrane lipid structures and nomenclature– Lehninger Chapter 10
• Biosynthesis of membrane lipid components– Lehninger Chapters 21.3, 21.4
• Membrane Lipids– glycerolipids– sphingolipids– sterols (cholesterol)
• Other Complex Lipids– steroids– isoprenoids
Membrane Lipids: Glycerolipids
Glycerophospholipid
Plasmalogen (ether lipid)
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 (Neu5Ac)
Membrane Lipids: Sterols
• Sterols are polymerized from isoprene units
• Rigid 4-ring structure• Membrane sterols:
– Cholesterol (animals)– Stigmasterol (plants)– Ergosterol (fungi)– None in bacteria
Biosynthesis of Membrane Lipids
• Glycerolipids• Sphingolipids• Cholesterol
Biosynthesis of Phosphatidic Acid
ATPADP
glycerol kinase
NADH NAD+
glycerol 3-phosphate dehydrogenase
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 (3x) 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
Glycerol
CDP-diacylglycerol
CMP
cardiolipin synthase(prokaryotic)
cardiolipin synthase(eukaryotic)
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)
Biosynthesis of Ether Lipids & Plasmalogens
• 2 NADPH required for reducing carboxylate to alcohol
• 1 NADPH for reducing dihydroxyacetone phosphate
Biosynthesis of Ether Lipids and Plasmalogens
• CDP-Ethanolamine substrate for CDP-ethanolamine transferase (correction)
• Long chain alcohol in ether linkage oxidized with mixed-function oxidase (monooxygenase) CDP-ethanolamine
transferase
CDP-ethanolamine
CDP
• Serine decarboxylated and condensed on acyl-CoA
• NADPH reduces resulting ketone• Mixed-function oxygenase forms
double bond of sphingosine• UDP-glucose for cerebroside• PC exchange for sphingomyelin
Sphingolipid Biosynthesis
Sphingolipid Biosynthesis
Ceramide
Ceramide
UDP- Glucose
UDP
UDP- Galactose
UDP
Glc Gal
Ceramide
UDP- N-Acetylgalactoseamine
UDP
Glc Gal Gal
Ceramide Glc Gal Gal
NAc
Neu
NAcCMP- Sialic Acid
CMP
GM2, a ganglioside
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 C3 carbonyl to form HMG-CoA
3. Reduction of HMG-CoA• 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
Stage 3: Polymerization of Activated Isoprenes
• Farnesyl-PP requires:– 1 Dimethylallyl-PP– 2 Δ3-Isopentenyl-PP
(head to tail polymerization)
• Squalene requires: – 2 farnesyl-PP
(head to head polymerization)
• 1 NADPH required
Cholesterol Biosynthesis
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
Cholesterol Biosynthesis
Stage 4: Conversion of Lanosterol to Cholesterol
19-Step process involves:• Oxidative removal of 3 methyl
groups as HCO2H or CO2
• 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
This Slide FYI only – Not on Final Exam
Metabolic Fates of Cholesterol
OH
7α-hydroxycholesterol
cholesterol
pregnenolone
7α-hydroxylase desmolase
7-dehydrocholesterol
7-dehydrocholesterolreductase
hν
cholecalciferol(Vitamin D3)
Bile (Salts) AcidsCatabolism
Steroid Hormones Vitamin D
7α-hydroxylase and desmolase are cytochrome P-450 monooxygenases
Cytochrome P-450 Monooxygenases
• usually located in smooth endoplasmic reticulum• involved in hydroxylation of steroids or xenobiotics• General Reaction:
AH + BH2 + O–O → A–OH + B + H2O
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 O2 introduce diols at C20, C22
• 3rd oxidation cleaves the C–C bond with ketone and aldehyde products
Steroid Hormones
Pregnenolone
Vitamin D Metabolism
in skin: • 7-dehydrocholesterol absorbs ultraviolet B (~300 nm)
• previtamin D3 isomerizes to cholecaliferol (vitamin D3)
in liver: • vitamin D3 → 1-hydroxyvitamin D3 [1-(OH)D3]
in kidney: • 1-(OH)D3 → 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]
• Final 2 steps involve cytochrome P-450 monooxygenases
Bile (Salts) Acids• 7 hydroxycholesterol hydroxylated and oxidized• carboxylate is activated with CoA• amino groups of glycine or taurine attack activated carboxylate
trihydroxycoprostanoate7α-hydroxycholesterol
glycine taurine
cholyl CoA
glycocholate taurocholate
OH
Isoprenoid Compounds and Derivatives
Isoprenoid Biosynthesis
Dimethylallyl-PPΔ3-Isopentenyl-PP
Geranyl-PP
Farnesyl-PP
Geranylgeranyl-PP
Oligoprenyl-PP
SqualeneStigmasterolLanosterolErgosterol
Cholesterol
Carotenoids
x (3 – 7)
Polyprenyl-PP
x (8 – 21)
Phytyl-PPChlorophyllTocopherols (Vitamin E)Phylloquinone (Vitamin K)
PlastoquinoneUbiquinone (Coenzyme Q)
Dolichol
x 2
Retinoids (Vitamin A)x 2
Mevalonate
Vitamin DBile Salts
Steroid Hormones
HMG-CoA
Statins
C10
C15
C20
C30-50
C55-120
Inhibitors of HMG-CoA Reductase
• Statins: synthetic analogs of mevalonate• Competitive inhibitors of HMG-CoA reductase• For inhibiting cholesterol synthesis
Study Problem
• Statins are widely prescribed drugs for lowering high cholesterol which may lead to atherosclerosis
• They are effective in preventing synthesis of cholesterol by inhibiting HMG-CoA reductase
• Since statins effectively block the entire isoprenoid pathway, there are concerns of potential side effects
• What possible metabolic consequences may statins have by inhibiting isoprenoid biosynthesis?
• What dietary supplements may be prescribed for overcoming possible side effects of statins?