1
Chapter 19
Takusagawa’s Note©
Chapter 19: Lipid Metabolism
Triacylglycerols = triglycerides = fats
- Triacylglycerols are the major biological energy source, which are water insoluble.
Glycogen is also biological energy source, but it takes a large space to store since glycogen is
heavily hydrated.
- Typical triacylglycerol is 1-palmitoyl-2,3-dioleoyl-glycerol.
O
H2C1 O C
O
HC2 O C
O
H2C3 O
1.
2.
3.
4.
5.
16
9
18
9
18
Dietary fat is breakdown to two fatty acids and monoacylglycerol by pancreatic lipase.
The fatty acids and monoacylglycerol are absorbed into small intestinal enterocyte cells.
In the cell, fatty acids and monoacylglycerol are reattached to form triacylglycerol.
Triacylglycerols are wrapped by proteins to form chylomicrons.
Chylomicrons enter into lymph duct, and then blood stream.
Dietary fat
(triacylglycerol)
Pancreatic
Lipase
2 fatty acids
Enterocyte cells of intestinal wall
2 fatty acids
+
Monoacylglycerol
2 CoA
Lymph
duct
2 fatty acyl CoA
Monoacylglycerol
Triacylglycerol
Intestinal Lumen
Chylomicrons
Protein
Chylomicrons
Transport of chylomicrons and cholesterol in human is shown in Fig. 11-53.
1
Chapter 19
2
Takusagawa’s Note©
Fatty acids in phospholipids are hydrolytically cleaved by various phospholipase as shown in
Fig. 23-2.
Phospholipase A2
2
Chapter 19
3
Takusagawa’s Note©
Phospholipase A2
- As shown in the previous page, phospholipase A2 cleaves C-2O bond of phospholipid.
- The enzyme contains a Ca2+ and has a His-48 - Asp-99 “catalytic diad”.
- His-48 abstracts a proton from a water molecule in the active site.
- The activated HO- nucleophilic attacks on the 2C (carbonyl carbon) of phospholipid.
- The Ca2+ stabilizes the oxyanion transition state.
3
4
Chapter 19
-
Takusagawa’s Note©
Metabolism of triacylglycerol in adipose cells
When certain hormones, such as epinephrine and norepinephrine, bind to their receptors in
adipose tissue, adenylate cyclase is activated.
Hormone-receptor
interaction
Plasma membrane
Adenylate cyclase
PPi
ATP
Adipose cell
cAMP
Protein kinase
(inactive)
ATP
Protein kinase
(active)
2+
Mg
Triacylglycerol
lipase (inactive)
Triacylglycerol
Triacylglycerol
lipase (active)
2+
Pi
ADP
Mg
P
Fatty acid
Diacylglycerol
Fatty acid
Monoacylglycerol
Fatty acid
Glycerol
The free fatty acids are released into bloodstream. The maximum solubility of fatty acid in
blood is relatively low (~10-6 M). Thus, the fatty acids are bind to albumin that is soluble
protein and comprises about half of the blood serum protein.
4
5
Chapter 19
Takusagawa’s Note©
Fatty acid oxidation
- This is called β-oxidation since the β-carbon of fatty acid is oxidized.
H H O
H3C
-
(CH2)n
C Cα C S CoA
β
H H
The β-oxidation leads acetyl-CoA and 2-carbons less fatty acid at a time.
O
H3C
(CH2)n C
S
CoA
Fatty acid (Cn+2)-CoA
-
HSCoA
O
O
H3C
(CH2)n-2
C
S
Fatty acid (Cn)-CoA
CoA + H3C
C
S
CoA
Acetyl-CoA
CoA is attached to fatty acid by using ATP hydrolysis energy, and acyl-CoA is yielded.
Fatty acid + CoA + ATP → acyl-CoA + AMP + PPi (→ 2Pi)
In this reaction, the negatively charged carboxyl oxygen (⎯O-) of fatty acid attacks the αphosphate of ATP, and the pyrophosphate is cleaved from ATP. This reaction is slightly
endergonic. The subsequent reaction (pyrophosphate hydrolysis) pulls the previous
endergonic reaction, since the hydrolysis of the pyrophosphate is relatively large exergonic
reaction (ΔG = -33.5 kJ/mol).
Endergonic
Exergonic
5
6
Chapter 19
-
Why does nature convert fatty acids to acyl-CoA in order to metabolize them?
Answer will be:
O
O
O
because
CH2 C OH
CH2 C Cl
CH2 C OH can takes
more active
less active
-
O
O
+
R C O R'
-
Takusagawa’s Note©
resonance forms.
R C O R'
However, if the O-R’ is replaced with S-R’, the above resonance is not as important in the
thiol ester. The thiol ester in acyl-CoA behaves like a ketons (no resonance forms, thus less
stable).
O
R C SCoA
-
Therefore the hydrogens on α-carbon are more acidic. Thus, they can react with base (-B:).
O
B + H CH2
δ-
-
O
C SCoA
δ+
BH + CH2
C SCoA
Therefore, attachment of CoA to the fatty acid carboxyl group weakens the Cα-Cβ bond of
acyl group.
Transport of acyl-CoA produced in cytosol
- Acyl-CoA is made in cytoplasm, but β-oxidation is carried out in mitochondrion. Thus,
Acyl-CoA must be transported into mitochondrion. However, Acyl-CoA itself does not
have a specific transport protein. Thus, a special carrier system is used.
- Initially acyl-CoA reacts with carnitine, and acyl-carnitine and CoA are produced.
6
-
Takusagawa’s Note©
7
Chapter 19
The acyl-carnitine is transported through the mitochondrial membrane at the carnitine carrier
protein to mitochondrion. The acyl-carnitine reacts with CoA in mitochondrion, and
produce acyl-CoA and carnitine.
β-oxidation is 4 steps with 4 enzymes (Fig. 23-10).
1. Desaturation between α and β carbon by using FAD to FADH2 reduction, and yield a trans
Δ2 compound. Enzyme: acyl-CoA dehydrogenase
FAD
H H O
H3C
(CH2)n
FADH2
C Cα C S CoA
β
H H
H3C
H O
(CH2)n
C Cα C S CoA
β
H
2. Hydration with H2O to 3 carbon to yield 3-L-hydroxyacyl-CoA. Enzyme: enoyl-CoA
hydratase.
H2O
H O
H3C
(CH2)n
C Cα C S CoA
β
H
H H O
H3C
(CH2)n
C C C S CoA
OH H
3. Oxidation by using NAD+ to NADH, and yield β-ketoacyl-CoA. Enzyme: 3-L-hydroxyacylCoA dehydrogenase.
NAD
H H O
H3C
(CH2)n
+
NADH
C C C S CoA
H3C
O H O
(CH2)n
C C C S CoA
OH H
H
4. Cleavage by CoA to produce Acetyl-CoA and acyl-CoA with n-2 carbons. Enzyme: βketoacyl-CoA thiolase.
O H O
CoAH
H3C (CH2)n C C C S CoA
H H O
O
H3C (CH2)n-2 C C C S CoA + H3C C S CoA
H
H H
7
Chapter 19
-
8
Takusagawa’s Note©
Overall β-oxidation reaction is shown below.
ETF = Electron-transfer flavoprotein
8
Takusagawa’s Note©
9
Chapter 19
Energetic of fatty acid oxidation
1. Saturated even number carbon atoms (n = even, such as 14, 16 and 18)
Cn fatty acid + CoASH + 2ATP → (Cn fatty acid)-CoA
FADH2
6
2
4
5
1
C
3
6
SCoA
2
4
5
1
3
O
O
Acetyl-CoA
+ NADH
FADH2
6
4
SCoA
3
C
5
SCoA
C
6
4
3
C
5
O
SCoA
O
1:
(Cn fatty acid)-CoA + FAD + H2O + NAD + CoASH →
(Cn-2 fatty acid)-CoA + acetyl-CoA + FADH2 + NADH
↓
↓
n/2-1:
(C4 fatty acid)-CoA + FAD + H2O + NAD+ + CoASH →
(C2 fatty acid)-CoA + acetyl-CoA + FADH2 + NADH
From one fatty acid,
(n/2 - 1) NADH and FADH2
(n/2) Acetyl-CoA
For example, C16 fatty acid (palmitic acid):
16/2 - 1 =
7 NADH
= 21 ATP
= 14 ATP
7 FADH2
16/2 =
8 Acetyl-CoA = 96 ATP*
* 3 NADH, 1 FADH2, 1 ATP = 12 ATP/acetyl-CoA
Total
= 131 ATP - 2* = 129 ATP (8.1 ATP/C)
* 2 ATPs are utilized to attach CoA to fatty acid
Note: one glucose produces 38 ATPs (6.3 ATP/C).
+
2. Unsaturated fatty acids.
Double bonds occur most often Δ9 & higher, and occur every three carbons, such as Δ9, 12, 15.
- Example, Δ9 fatty acid:
Δ9 ⎯ ⎯⎯⎯⎯⎯
⎯→ Δ7 ⎯ ⎯⎯⎯⎯⎯
⎯→ Δ5 ⎯ ⎯⎯⎯⎯⎯
⎯→ Δ3 (cis double bond)
Acetyl − CoA
Acetyl − CoA
Acetyl − CoA
- Cis double bond cannot be cleaved by β-oxidation (i.e., cannot hydrate (H2O addition)).
- Δ3 must be transferred to Δ2 by enoyl-CoA isomerase.
- The acetyl-CoA dehydrogenase step (step-1) is skipped (i.e., no FADH2 production).
4
6
O
3
C
5
2
1
enol-CoA isomerase
SCoA
No FADH2
9
2
4
6
5
3
1
C
O
SCoA
-
Two double bonds case, such as Δ9, 12.
Δ9, 12 ⎯ ⎯⎯⎯⎯⎯
⎯→ Δ7,10 ⎯ ⎯⎯⎯⎯⎯
⎯→ Δ5, 8 ⎯ ⎯⎯⎯⎯⎯
⎯→ Δ3, 6
Acetyl − CoA
Acetyl − CoA
Acetyl − CoA
- After isomerization and one cycle β-oxidation, Δ2, 4 (trans Δ2, cis Δ4) is yielded.
- Δ2, 4 is a poor substrate, and must be reduced to Δ3 by 2,4-dienoyl-CoA reductase.
- This process requires one NADPH.
The Δ3 must be transferred to Δ2 by 3,2-enoyl-CoA isomerase.
7
6
4
O
3
6
2
4
8
5
3
6
4
3
8
SCoA
6
7
4
3
C
5
5
8
O
NADPH
Acetyl-CoA
+ NADH
6
8
4
3
7
5
6
8
SCoA
5
C
7
O
3. Odd carbon fatty acids (Cn, n = odd number)
10
SCoA
O
Acetyl-CoA
+ NADH
FADH2
7
1
C
SCoA
1
2
5
7
enol-CoA isomerase
C
8
Takusagawa’s Note©
10
Chapter 19
C
O
SCoA
C
O
SCoA
Takusagawa’s Note©
11
Chapter 19
At final step,
HSCoA
O
O
CH3CH2 C CH2 C SCoA
O
CH3CH2 C SCoA
Propionyl-CoA
O
+
CH3 C SCoA
Acetyl-CoA
Propionyl-CoA is converted to succinyl-CoA by propionyl-CoA carboxylase which has a
biotin prosthetic group.
↓
-
Succinyl-CoA can enter the citric acid cycle, but does not produce energy since succinylCoA cannot metabolize to CO2 in the citric acid cycle. Note: Acetyl-CoA is the only
molecule that enters in the citric acid cycle and produce energy. (Do not misunderstand)
{Propionyl-CoA + <ATP>} → {Succinyl-CoA} → {Oxaloacetate + [GTP + FADH2 +
NADH]} → {Pyruvate + <GTP> + [ATP]} → {Acetyl-CoA + [NADH]} → {2CO2 +
[FADH2 + 3NADH + GTP] (in CAC)} = 2FADH2 + 5NADH + 2GTP - ATP = 4ATP +
15ATP + 2ATP – ATP = 20 ATP
- Succinyl-CoA is also converted to glucose in gluconeogenesis.
Succinyl-CoA →→ OAA → PEP →→ glucose
11
Example
C18Δ15
#ATP
Total
Takusagawa’s Note©
12
Chapter 19
#AcetylCoA (12)
9
108
108
#Propionyl- #Oxidation
CoA (20)
(5)
0
8
0
40
108
148
1st , 3rd =
bond (-2)
1
-2
146
2nd , 4th =
bond (-3)
0
0
146
CoA attach
(-2)
1
-2
144
C18Δ12,15
#ATP
Total
9
108
108
0
0
108
8
40
148
1
-2
146
1
-3
143
1
-2
141
C18Δ9,12,15
#ATP
Total
9
108
108
0
0
108
8
40
148
2
-4
144
1
-3
141
1
-2
139
C18Δ6,9,12,15
#ATP
Total
9
108
108
0
0
108
8
40
148
2
-4
144
2
-6
138
1
-2
136
C18Δ6,9,12,15
-CoA
#ATP
Total
9
0
8
2
2
0
108
108
0
108
40
148
-4
144
-6
138
0
138
C19Δ6,9,12,15
#ATP
Total
8
96
96
1
20
116
8
40
156
2
-4
152
2
-6
146
1
-2
144
8
1
8
2
1
0
96
96
20
116
40
156
-4
152
-3
149
0
149
C19Δ9,12,15CoA
#ATP
Total
12
Chapter 19
13
Takusagawa’s Note©
Methylmalonyl-CoA mutase contains a coenzyme B12, and catalyzes unusual carbon
skeleton rearrangement.
Methylmalonyl-CoA mutase contains Co atom in a hemelike corrin.
- One of six ligands is deoxyadenosyl group, i.e., C5⎯Co
SCoA
H C O
H C C H
H CO2
Methylmalonyl-CoA
13
SCoA
O C H
H C C H
H CO2
Succinyl-CoA
Chapter 19
14
Takusagawa’s Note©
Mechanism of methylmalonyl-CoA mutase
1. C5⎯Co bond is cleaved when Ado-CH2 receives H from the substrate (H-C1).
2. The substrate form the C1⎯Co bond.
3. When the C1⎯Co bond is cleaved, rearrangement of X occurs, i.e., C1-X.
4. The H of Ado-CH3 is returned to C2 of the substrate.
5. Ado-CH2 forms a bond with Co, C5⎯Co, to regenerate the B12.
- This reaction begins with homolytic cleavage of C⎯Co bond (C••Co → C• + •Co).
14
Chapter 19
15
Takusagawa’s Note©
Fatty acid oxidation also takes place in peroxisome
- Very long chain fatty acids are shortened by peroxisomal β-oxidation since the β-oxidation
of very long chain fatty acid is relatively slow in mitochondrion.
- The shorter chain fatty acids enter mitochondrial β-oxidation.
- Very long chain fatty acids diffuse into peroxisome. Thus carnitine is not utilized.
- Peroxisomal very long chain acyl-CoA synthetase is one of the unique enzyme.
- Deficiency of the peroxisomal acyl-CoA synthetase causes X-adenoleukodystrophy (XALD).
Ketone Bodies
Acetyl-CoA
Acetoacetate
+
β-hydroxybutyrate
Ketone Bodies
CAC
-
Sometimes acetyl-CoA gets very high, then acetyl-CoA forms “Ketone bodies” in liver’s
mitochondria.
Formation of ketone bodies from acetyl-CoA is shown in Fig. 23-22.
15
16
Chapter 19
β-hydroxybutyrate
16
Takusagawa’s Note©
-
-
Takusagawa’s Note©
17
Chapter 19
Ketone bodies are used as energy source by other tissues, heart and skeletal muscle.
Although brain uses only glucose as its energy source, ketone bodies become the brain’s
major fuel source during starvation.
Metabolism of ketone bodies is shown in Fig. 23-23.
Accumulation of acetoacetate may cause non-enzymatic breakdown of acetoacetate to
acetone and CO2 as shown below. The resulting acetone is excreted by urea (ketoneurea)
and breath (sweet smell).
CO2
O
O
O
-
H3C C CH2 C O
Acetoacetate
H3C C CH3
Acetone
17
Chapter 19
18
Takusagawa’s Note©
Fatty acid synthesis
Fatty acid synthesis takes place at liver (cytosol).
As always seen all biological processes, oxidation and synthesis take different pathways.
- In thermodynamics, if one direction is exergonic pathway, the opposite direction must be
endergonic pathway. Thus, it is impossible to occur both directions in the same system.
- It is easy to control the pathway if they are separated.
Differences between oxidation and synthesis are summarized in Fig. 23-25.
Fatty acid oxidation: takes place in mitochondrion and fatty acid carrier: CoA.
Fatty acid synthesis: takes place in cytosol and fatty acid carrier: acyl-carrier protein (ACP).
Both CoA and ACP have long phosphopantetheine groups in their structures.
The fatty acids are attached on the end of cysteamine residues (HS-CH2-CH2-NH-).
18
19
Chapter 19
Takusagawa’s Note©
Acetyl-CoA carboxylase
- Acetyl-CoA carboxylase catalyzes the first committed step of fatty acid biosynthesis and one
of its rate-controlling steps.
- Malonyl-CoA is synthesized from acetyl-CoA and CO2 catalyzed by acetyl-CoA
carboxylase. In this reaction, biotin is the cofactor.
ATP ADP + Pi
O
O
HCO3-
+
CH3-C-SCoA
-
OOC-CH2-C-SCoA
- Acetyl-CoA carboxylase is inhibited by palmitate (product) and activated by citrate.
- High [citrate] indicates high [acetyl-CoA], i.e., OAA + acetyl-CoA↑ ⎯→ citrate↑.
- Thus, the excess of acetyl-CoA enters into fatty acid synthesis.
- Malonyl-CoA carries an ATP hydrolysis energy which drives the condensation reaction to
produce acetoacetate-ACP.
Fatty acid synthase
Reaction sequence of palmitate synthesis is shown below.
19
Chapter 19
20
Takusagawa’s Note©
- Fatty acid synthase is composed of multi-functional subunits. Those are:
β-ketoacyl-ACP synthase activity
malonyl-CoA-ACP transacetylase activity
acetyl-CoA-ACP transacetylase activity
β-hydroxyacyl-ACP dehydrase activity
enoyl-ACP reductase activity
β-ketoacyl-ACP reductase
acyl-carrier protein
palmitoyl thioesterase activity
-
Fatty acid synthase is a dimer and has all enzyme activities.
Multi-functional enzymes are very efficient since the substrate diffusion time is very short.
20
Chapter 19
21
Start here
21
Takusagawa’s Note©
Chapter 19
22
Takusagawa’s Note©
Sequential reactions of palmitate synthesis (shown in previous page)
1. Acetyl from acetyl-CoA binds on of Cys-SH of β-ketoacyl-ACP (β-keto acyl synthase).
2. Malonyl from malonyl-CoA binds on pant-SH of ACP.
3. Condensation reaction --- transfer acetyl group to malonyl group, and decarboxylation →
acetoacetyl-ACP and free the Cys-SH.
4. Reduction
5. Dehydration
6. Further reduction → butyryl-ACP.
7. Transfer of acyl group to Cys-SH (β-keto acyl synthase) and free pant-SH of ACP.
Repeat 2-7 steps six more times.
8. Transfer palmitoyl group to pant-SH and free Cys-SH.
9. Hydrolysis of C-S bond and release product (palmitate) and free pant-SH.
-
All processes are carried out on the fatty acid synthase, and in cytosol.
Overall reaction of palmitate synthesis is:
Acetyl-CoA + 7malonyl-CoA + 14NADPH + 7H+ →
palmitate + 7CO2 + 14NADP+ + 8CoA + 6H2O
Since 7malonyl-CoA are derived from acetyl-CoA as follows:
7Acetyl-CoA + 7CO2 + 7ATP → 7malonyl-CoA + 7ADP +7Pi + 7H+
Therefore, overall stoichiometry is:
8Acetyl-CoA + 14NADPH + 7ATP → palmitate + 14NADP+ + 8CoA + 6H2O + 7ADP +7Pi
22
Chapter 19
23
Takusagawa’s Note©
Transport of acetyl-CoA from mitochondrion to cytosol
Since acetyl-CoA is synthesized in a mitochondrion, acetyl-CoA must be transported to cytosol
via the tricarboxylate transport system.
1. Mitochondrial pyruvate is converted to oxaloacetate.
2. Acetly-CoA + oxaloacetate → citrate.
3. Citrate is transported to cytosol via tricarboxylate transport system (membrane protein).
4. Cytosol citrate is converted to oxaloacetate + acetyl-CoA
5. Oxaloacetate → malate → pyruvate in cytosol.
23
Chapter 19
24
Takusagawa’s Note©
Elongation and desaturation (Post-modification)
- Elongation processes (elongases) are present in both the mitochondrion and endoplasmic
reticulum.
- In endoplasmic reticulum: the same processes of fatty acid biosynthesis (malonyl-CoA is
precursor).
- In mitochondrion: the reversed processes of β-oxidation, except for the final reaction that
uses NADPH rather than FADH2 as its redox coenzyme.
-
Desaturation is catalyzed by the chain-length specific fatty acyl-CoA desaturases, such as Δ9fatty acyl-CoA desaturase.
Mammalian cells contain: Δ4-, Δ5-, Δ6-, Δ9-fatty acyl-CoA desaturases.
Plant cells contain :
Δ4-, Δ5-, Δ6-, Δ9-, Δ12-, Δ15-fatty acyl-CoA desaturases.
24
-
Double bond insertion occurs three carbons away from as existing on toward the CoA at the
carbon. For example,
C18Δ 9,12
-
Takusagawa’s Note©
25
Chapter 19
-2H
C18Δ 6, 9,12
+2C
C20Δ 8, 11,14
-2H
C20Δ 5, 8, 11,14
+2C
C22Δ 7, 10, 13, 16
The desaturases are non-heme iron-containing enzymes that catalyze the general reaction:
H H
CH3 (CH2)x
C C
O
+
+
(CH2)y C SCoA + NAD + H + O2
H H
O
CH3 (CH2)x
C C
(CH2)y C SCoA + 2H2O + NADH
H H
Actually, desaturase is complexed with other redox proteins, namely, desaturase, cytochrome b5,
and NADH-cytochrome b5 reductase as shown below.
Summary of β-oxidation and fatty acid biosynthesis
β-oxidation at mitochondrion:
1. By using 2 ATP, CoASH is attached to fatty acid to produce acyl-CoA.
2. Desaturation of Cβ-Cα bond by oxidation (cofactor: FAD → FADH2).
3. Hydration of -Cβ=Cα- to attach -OH to Cβ.
4. Oxidation of Cβ-OH to Cβ=O with cofactor (NAD+ → NADH).
5. Cleavage of Cβ-Cα bond by addition of CoASH.
Fatty acid biosynthesis at cytosol:
1. Formation of malonyl-CoA from acetyl-CoA + CO2
2. Reverse reactions of β-oxidation as described above. Note: different cofactors (both
reductions utilize NADPH).
3. Acyl group is grown on the end of acyl-carrier protein (ACP) instead of CoA in β-oxidation.
Synthesis of triacylglycerols
Triacylglycerols are synthesized from fatty acyl-CoA esters and glycerol-3-phosphate or
dihydroxyacetone phosphate (Fig. 23-34).
25
Chapter 19
26
26
Takusagawa’s Note©
Chapter 19
27
Regulation of fatty acid metabolism
Major regulation site
27
Takusagawa’s Note©
Chapter 19
28
Takusagawa’s Note©
Hormonal control
- Glucagon, epinephrine --- stimulate fatty acid oxidation.
Short effect: ↑cAMP, ↑adipose lipase
Long effect: ↓acetyl-CoA carboxylase (when the enzymes are phosphorylated, they are
connected to protomers. The protomers are inactive)
↓synthesis of acetyl-CoA carboxylation (synthesis of malonyl-CoA)
↓fatty acid synthesis
- Insulin --- stimulate fatty acid synthesis.
Short effect: ↓cAMP, ↑cAMP-independent protein kinase
Long effect: ↑acetyl-CoA carboxylase by dephosphorylation
↑synthesis of fatty acid
↓lipoprotein lipase
- Note: Effects of glucagon & epinephrine ↔ Insulin
Cholesterol metabolism
- Cholesterol is in cell membrane, and precursors of hormones, bile acids and vitamin D.
- Cholesterol has C27 skeleton and is produced from acetates that are first converted to
isoprene units, C5,
CH3
H2C C CH CH2
Isoprene
Acetate → isoprenoid intermediate (C5) → squalene (C30) → cyclization product (C30) →
cholesterol (C27).
Pathway of isoprenoid metabolism
28
Takusagawa’s Note©
29
Chapter 19
Key enzyme
β-hydroxy-β-methylglutaryl-CoA (HMG-CoA) is a key cholesterol precursor
- HMG-CoA is synthesized from condensation of acetyl-CoA’s.
Thiolase
2(Acetyl − CoA) ⎯ ⎯⎯⎯
⎯→ β − Ketobutyryl − CoA + Acetyl - CoA
HMG − CoA synthase
⎯ ⎯⎯⎯⎯⎯⎯⎯⎯
⎯→ HMG − CoA
- Isopentenyl pyrophosphate and dimethylallyl pyrophosphate are synthesized from HMGCoA.
CH3
H2C
C
C
H2
H2
C
O
CH3
O
-
H3C
O P O P O
-
-
C
C
H
H2
C
O
O
-
O P O P O
-
-
O
O
Dimethylallyl pyrophosphate
O
O
Isopentenyl pyrophosphate
29
30
Chapter 19
2Acetyl-CoA
Thiolase
β -Ketobutyryl-CoA + CoA
Acetyl-CoA
HGM-CoA synthase
β -Hdroxy-β -methylglutaryl-CoA (HMG-CoA)
HMG-CoA reductase
(cytosol)
Mevalonate
+
CoA
HMG-CoA lyase
(mitochondria)
β -Ketobutyrate
+
Acetyl-CoA
Keytone bodies
30
Takusagawa’s Note©
Chapter 19
31
Takusagawa’s Note©
-
The last step of reaction is an ATP-dependent concerted dehydration-decarboxylation.
-
Formation of dimethylallyl pyrophosphate reaction is catalyzed by isopentenyl
pyrophosphate isomerase.
31
Chapter 19
-
32
Takusagawa’s Note©
Formation of squalene is:
- 2 times head-tail condensation reactions to produce geranyl pyrophosphate (C10 unit) →
farnesyl pyrophosphate (C15 unit).
- head-head condensation of two resulting C15 units (farnesyl pyrophosphate) to produce
squalene.
32
Chapter 19
33
Takusagawa’s Note©
-
Cyclization of squalene is shown below (epoxization is an essential process).
-
C30 lanosterol is the product of cyclization of squalene.
-
Additional 19 step reactions are required to convert lanosterol to cholesterol.
33
34
Chapter 19
Takusagawa’s Note©
Summary of cholesterol biosynthesis
1. HMG-CoA (C6 fragment) is synthesized from three acetyl-CoA.
2. HMG-CoA is converted to mevalonate, and then two isoprene pyrophosphates (isopentenylPPi and dimethylallyl-PPi).
3. Isopentenyl-PPi and dimethylallyl-PPi are connected by head-tail to geranyl-PPi.
4. Another isopentenyl-PPi and geranyl-PPi is connected by head-tail to farnesyl-PPi.
5. Two farnesyl-PPi are put head-head condensation to produce the C30 fragment, squalene.
6. Squalene is cyclized to form lanosterol.
7. Lanosterol is converted to C27 cholesterol by 19 step reactions.
-
From cholesterol:
Cholesterol
Bile acid (cholic acid)
glucocorticoids
(cortisol)
Androgens
(testesterone)
Progestins (progesterone)
Mineralcorticoids (aldosterone)
Estrogens
(estradiol)
Cholesterol synthesized by the liver is:
1. converted to bile acids for use in digestive process.
2. esterified to form cholesterol ester and secreted into bloodstream as part of the very low
density lipoprotein (VLDL)
O
R C O
34
Takusagawa’s Note©
35
Chapter 19
Cholesterol Transport
Synthesis (in liver)
Bile acids (in gallbladder)
Phosphatidylcholine (PC)
LCAT
Cholesterol ester
VLDL
Secreted into bloodstream
LCAT = Lecithin-cholesterol acyl transferase
Formation and secretion of VLDL in liver is illustrated below.
Review of Fig. 11-53 for VLDL, IDL, LDL and
HDL.
-
VLDL/IDL/LDL delivers cholesterol to
tissues.
HDL delivers cholesterol to liver.
LDL is the major cholesterol carrier of the
bloodstream.
Fig. 11-50.
35
Chapter 19
36
Takusagawa’s Note©
LDL enters into cells through endocytosis. LDL is recognized by the LDL receptors which are
synthesized on the endoplasmic reticulum.
ACAT = Acyl-CoA Cholesterol Acyltransferase
Note:
- Cholesterols released from LDL in cells enters into endoplasmic reticulum and are utilized to
form membrane of ER.
- Presence of cholesterol in cytosol decreases the rate of:
HMG-CoA reductase synthesis.
LDL receptors synthesis.
36
Takusagawa’s Note©
37
Chapter 19
Control of cholesterol levels
HMG-CoA reductase is the primary control site for cholesterol biosynthesis.
HO
CH3
C
2NAD+ CoA
O
C
H2C
CH2 C SCoA
O
CH3
HO
C
H2C
-O
2NADH
HMG-CoA
reductase
-O
C
CH2 CH2 OH
O
Mevalonate
HMG-CoA
1. Hormonal regulation of HMG-CoA reductase
- HMG-CoA reductase activity is regulated by phosphorylation/dephosphorylation with
protein kinases (PKs) and phosphatases.
37
Takusagawa’s Note©
38
Chapter 19
-
This enzyme cascade system is quite similar to the glycogen metabolism regulation.
When [cAMP] ↑, activity of HMG-CoA reductase ↓ and thus cholesterol biosynthesis ↓.
The [cAMP] is regulated by hormones, i.e., insulin and glucagon.
Insulin: ↓ [cAMP], thus ↑ cholesterol biosynthesis (in general, ↑ all biosyntheses).
Glucagon: ↑ [cAMP], thus ↓ cholesterol biosynthesis (in general, ↓ all biosyntheses).
-
Compactin and Lovastatin (fungal products) are competitive inhibitor of HMG-CoA
reductase. Lovastatin is now in routine use for treatment of hypercholesterolemia.
HO
-
COO
OH
O
R = H Compactin
R = CH3 Lovastatin
O
CH3
CH3
R
2. Regulation of synthesis of HMG-CoA reductase mRNA
- Presence of relatively high cholesterol in cytosol decreases the synthesis of mRNA of HMGCoA reductase.
3. Degradation of HMG-CoA reductase
- The half-life (t1/2) of HMG-CoA reductase is decreased when [cholesterol] is increased.
- Cholesterol in cytosol stimulates degradation (proteolysis: cut peptide bonds) of the Cterminal domain of HMG-CoA reductase, which is the catalytic domain.
Proteolysis
The other regulation of cholesterol levels are:
- LDL receptor synthesis (is decreased by high [cholesterol] in cytosol).
- Regulation of LCAT (lecithin-cholesterol acyl transferase).
38
Chapter 19
39
Takusagawa’s Note©
LDL level in blood depends on the rate of removal from bloodstream by liver.
(a) If liver has enough LDL receptor, the
[cholesterol] in blood is decreased.
(b) Genetically defective LDL receptor
synthesis cannot produce enough
LDL receptor, thus cholesterol levels
in blood of these individuals are
relatively high and called Familial
hypercholesterolemia (FH).
(c) High cholesterol diet decrease the
LDL receptor synthesis, since the
dietary cholesterol enters liver and
the [cholesterol] is increased. High
[cholesterol] in liver decreases the
LDL receptor synthesis.
39
Takusagawa’s Note©
40
Chapter 19
Complexed lipids
Typical complex lipids are: glycerolipid and sphingolipid
O
O
R2
OH H
CH2 O C R1
C O C H
O
R2
C NH C H
CH2 O X
1,2-diacyl-syn-glycerol
Glyceroglycolipid
Glycerophospholipid
CH C C
CH2
O X
N-acylsphingosine (ceramide)
Sphingoglycolipid
Sphingophospholipid
Activated CDP-choline, CDPethanoamine, etc. are attached to
diacylglycerol as shown below.
Fig. 23-67
40
CH3
H
Glycerophospholipid syntheses
Phosphatidylcholine and
phosphoethanolamine syntheses
- Polar head groups (phosphatidylcholine
and phosphoethanolamine) are activated
by attaching CDP.
- Note: CTP is used for energy source.
UTP is used for glycogen synthesis.
(Carbohydrates).
-
(CH2)12
X=H
X = carbohydrate
X = phosphate
Chapter 19
41
Takusagawa’s Note©
Phosphatidylserine synthesis
- Ethanoamine group of phosphatidylethanolamine is replaced with serine by head group
exchange reaction.
Cardiolipin synthesis
- Cardiolipin is synthesized by two phosphatidylglycerol condensation reaction.
- Cardiolipin is an important phospholipid in heart tissue.
41
Chapter 19
42
Takusagawa’s Note©
Phosphatidylglycerol and phosphatidylinositol syntheses
- Diacylglycerol portion (phosphatidic acid) is activated by attaching CMP (CDPdiacylglycerol)
- Then glycerol-3-phosphate and inositol are connected by using the CMP hydrolysis energy.
- Phosphatidylinositol is an important signal transduction substance (second messenger).
42
Takusagawa’s Note©
43
Chapter 19
Sphingolipids biosynthesis
- Basic fragments of sphingosine and ceramide (N-acetylsphingosine) are synthesized from
palmitoyl-CoA, serine and acyl-CoA.
Palmitate
Acyl
group
Serine
43
Chapter 19
44
Takusagawa’s Note©
-
Sphingomyelin is synthesized from ceramide (N-acetylsphingosine) and
phosphatidylcholine.
-
Cerebrosides (ceramide + sugar) is
synthesized from ceramide and UDP-sugar
(such as UDP-glucose, UDP-galactose).
44