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
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 (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
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)
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 CDPethanolamine transferase (correction)
• Long chain alcohol in ether linkage
oxidized with mixed-function oxidase
(monooxygenase)
CDP-ethanolamine
CDP-ethanolamine
transferase
CDP
Sphingolipid Biosynthesis
• 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
Ceramide
UDP- Glucose
UDP
UDP- Galactose
UDP
Ceramide
Glc
Gal
Glc
Gal
UDP- N-Acetylgalactoseamine
UDP
Ceramide
CMP- Sialic Acid
NAc
Neu
CMP
GM2, a ganglioside
Ceramide
Gal
Glc
Gal
Gal
NAc
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
Cholesterol Biosynthesis
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
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 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
Metabolic Fates of Cholesterol
7α-hydroxylase and
desmolase are
cytochrome P-450
monooxygenases
7α-hydroxylase
cholesterol
7-dehydrocholesterol
7-dehydrocholesterol
reductase
desmolase
OH
7α-hydroxycholesterol
Bile (Salts) Acids
Catabolism
pregnenolone
Steroid Hormones
hν
cholecalciferol
(Vitamin D3)
Vitamin D
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:
in liver:
in kidney:
•
•
•
•
•
7-dehydrocholesterol absorbs ultraviolet B (~300 nm)
previtamin D3 isomerizes to cholecaliferol (vitamin D3)
vitamin D3 → 1-hydroxyvitamin D3 [1-(OH)D3]
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
7α-hydroxycholesterol
trihydroxycoprostanoate
cholyl CoA
OH
taurine
glycine
glycocholate
taurocholate
Isoprenoid Compounds and Derivatives
Isoprenoid Biosynthesis
Statins
HMG-CoA
Δ3-Isopentenyl-PP
Mevalonate
Dimethylallyl-PP
Steroid Hormones
Bile Salts
C10
Geranyl-PP
C15
Farnesyl-PP
C20
Geranylgeranyl-PP
x (3 – 7)
C30-50
Oligoprenyl-PP
x (8 – 21)
C55-120
Polyprenyl-PP
Vitamin D
Stigmasterol
Lanosterol
Squalene
Cholesterol
Ergosterol
Retinoids (Vitamin A)
Carotenoids
Chlorophyll
Tocopherols (Vitamin E)
Phytyl-PP
Phylloquinone (Vitamin K)
Plastoquinone
Ubiquinone (Coenzyme Q)
Dolichol
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?