1. Biosynthesis of Membrane Lipids

2. Membrane Lipids: Glycerolipids
Glycerophospholipid

3. Glycerophospholipid Head Groups

4. Membrane Lipids: Sphingolipids
Sphingomyelins
with phosphocholine or phosphoethanolamine
Neutral Glycolipids
cerebrosides (1 sugar)
globosides (> 2 sugars)
Gangliosides
complex carbohydrates with sialic acid (Neu5Ac)

5. Biosynthesis of Membrane Lipids
Glycerolipids
Sphingolipids
Cholesterol

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

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

8. 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

10. 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

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

12. 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

13. 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

14. 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

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

16. 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

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

18. Cholesterol Biosynthesis
Stage 1: Condensation of Mevalonate from Acetate
Final step in β-oxidation of fatty acids in reverse (cytosolic)
Aldol condensation at C3 carbonyl to form HMG-CoA
Reduction of HMG-CoA
Committed step in biosynthesis of isoprenes
Requires 2 NADPH for reduction of carboxylate to alcohol

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

20. 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

21. 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, methyl migrations, and H+ loss gives lanosterol
Modification of lanosterol (19 steps) gives cholesterol

22. Cholesterol Biosynthesis
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

23. Metabolic Fates of Cholesterol
7α-hydroxylase and desmolase are cytochrome P-450 monooxygenases
cholesterol
7-dehydrocholesterol
7-dehydrocholesterol
reductase
7α-hydroxylase
desmolase hν OH
cholecalciferol
(Vitamin D3)
7α-hydroxycholesterol
pregnenolone
Bile (Salts) Acids
Catabolism
Steroid Hormones
Vitamin D

24. 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

25. 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

26. Steroid Hormones
Pregnenolone

27. 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

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