ketone body formation; fatty acid and cholesterol biosynthesis
TRANSCRIPT
Ketone Body Formation; Fatty acid
and Cholesterol Biosynthesis
Ketone Body Formation; Fatty acid
and Cholesterol Biosynthesis
• Most acetyl CoA from β-oxidation is routed through the TCA cycle
• Excess is used to synthesize ketone bodies; β-hydroxybutyrate, acetoacetate and
acetone
• Ketone bodies serve as fuel molecules
• Liver is the site of synthesis in mammals (occurs in the mitochondrial matrix)
H3C-C-S-CoAO
H3C-C-S-CoAO
Acetyl CoA Acetyl CoA
+
Ketone Body Synthesis:
H3C-C-S-CoAO
H3C-C-S-CoAO
Acetyl CoA Acetyl CoA
+
H3C-C-CH2-C-S-CoA
O O
Acetoacetyl CoA
thiolaseHSCoA
H3C-C-CH2-C-S-CoA
O O
Acetoacetyl CoA
H3C-C-S-CoAO
+
Acetyl CoA
H3C-C-CH2-C-S-CoA
O O
Acetoacetyl CoA
H3C-C-S-CoAO
+
Acetyl CoA
3-Hydroxy-3-Methylglutaryl CoA (HMG CoA)
-OOC-CH2-C-CH2-C-S-CoAOOH
CH3
HMG CoA Synthase
HSCoA
3-Hydroxy-3-Methylglutaryl CoA (HMG CoA)
-OOC-CH2-C-CH2-C-S-CoAOOH
CH3
HMG CoA Lyase
H3C-C-S-CoAO
Acetyl CoA
-OOC-CH2-C-CH3
O
Acetoacetate
-OOC-CH2-C-CH3
O
Acetoacetate
β-Hydroxybutyrate
D’HaseNADH
NAD+
-OOC-CH2-CH-CH3
OH
β-Hydroxybutyrate
CO2
CH3-C-CH3
O
Acetone
Ketone Body Breakdown:
-OOC-CH2-CH-CH3
OH
β-Hydroxybutyrate
β-Hydroxybutyrate
D’Hase
NADH
NAD+
-OOC-CH2-C-CH3
O
Acetoacetate
*Liver enzyme catalyzes a near equilibrium
reaction
*Isozyme found in other cells catalyzes
an irreversible reaction
-OOC-CH2-C-CH3
O
Acetoacetate
+-OOC-CH2-CH2-C-S-CoA
O
Succinyl CoASuccinyl CoA Transferase
H3C-C-CH2-C-S-CoA
O O
Acetoacetyl CoA
+
-OOC-CH2-CH2-COO-
Succinate
H3C-C-CH2-C-S-CoA
O O
Acetoacetyl CoA
Thiolase
H3C-C-S-CoAO
Acetyl CoA
H3C-C-S-CoAO
Acetyl CoA
Fatty acid BiosynthesisFatty acid
Biosynthesis
• Occurs through the condensation of C2 units
• Occurs through the condensation of C2 units
• Occurs largely in adipocytes and liver and to a lesser extent in specialized tissues
• Occurs largely in adipocytes and liver and to a lesser extent in specialized tissues
• Occurs in three stages: 1) transport of acetyl Co A to the cytosol; 2) formation of malonyl CoA; 3) assembly of the fatty acid chain
• Occurs in three stages: 1) transport of acetyl Co A to the cytosol; 2) formation of malonyl CoA; 3) assembly of the fatty acid chain
Citrate Transport System:Citrate Transport System:
MatrixMatrixCytosolCytosol
Citrate:Dicarboxylic acid Carrier
Acetyl CoA + OAA
Citrate
α-Kg or
malate
Citrate Transport System:Citrate Transport System:
MatrixMatrixCytosolCytosol
Citrate:Dicarboxylic acid Carrier
Acetyl CoA + OAA
Citrate
α-Kg or
malate
Citrate
α-Kg
or malate
Citrate Transport System:Citrate Transport System:
MatrixMatrixCytosolCytosol
Citrate:Dicarboxylic acid Carrier
Citratecitrate lyase ATP
ADP
OAA + Acetyl CoAmalate d’hase NAD+
NADH
malate
Malic enzyme
NADP+ NADPH
CO2
pyruvatepyruvate translocase pyruvate
carboxylase
• Formation of Malonyl CoA by Acetyl CoA carboxylase
• Loading Step: Transfer of acetyl CoA and malonyl CoA to ACP to form acetyl-ACP
and malonyl ACP
• Successive rounds of condensation; reduction;
dehydration, reduction
Fatty Acid Synthetase Fatty Acid Synthetase }}
H3C-C-S-CoA
O
acetyl-CoA
+ HCO3
bicarbonate
Acetyl CoA Carboxylase
ATP
ADP, Pi
-OOC-CH2-C-S-CoAO
malonyl-CoA
Fatty Acid Synthetase Complex:
Bacteria, PlantsSeven activities in 7 separate proteins
YeastSeven activities in 2 separate proteins
VertebratesSeven activities in 1 large protein
H3C-C-S-CoA
Oacetyl-CoA
-OOC-CH2-C-S-CoAO
malonyl-CoA
Acetyl CoA:ACP transacylase
Malonyl CoA:ACP transacylase
HS-ACP
HS-CoA
HS-ACP
HS-CoA
H3C-C-S-ACP
O-OOC-CH2-C-S-ACP
O
acetyl-ACP malonyl-ACP
Loading Step
H3C-C-S-ACP
O-OOC-CH2-C-S-ACP
O
acetyl-ACP malonyl-ACP
ketoacyl-ACP synthase
Condensation
H3C-C-S-ACP
O-OOC-CH2-C-S-ACP
O
acetyl-ACP malonyl-ACP
H3C-C-CH2-C-S-ACP
OO
acetoacetyl-ACP
ketoacyl-ACP synthaseCO2
HS-ACP
Condensation
H3C-C-CH2-C-S-ACP
OO
acetoacetyl-ACPketoacyl-ACP
reductase
NADP+
NADPH
b-hydroxybutyryl-ACP
H3C-C-CH2-C-S-ACP
OOH
H
Reduction
b-hydroxybutyryl-ACP
H3C-C-CH2-C-S-ACP
OOH
H
b-hydroxyacyl-ACP dehydrase
H20
H3C-C=C-C-S-ACPO
H
H
butenoyl-ACP
Dehydration
H3C-C=C-C-S-ACPO
H
H
butenoyl-ACP
enoyl-ACP reductase
NADPHNADP+
H3C-CH2-CH2-C-S-ACPO
butyryl-ACP
Reduction
• Fatty Acid Synthesis is sometimes called palmitate synthesis because palmitate is the
predominant product (in mammals)
• Rounds of synthesis continue until a C16 palmitoyl group is formed
• Thiolase then catalyzes the formation of palmitate and ACP-SH
Palmitoyl-ACPPalmitate +
ACP-SHthiolase
Overall stoichiometry for palmitate synthesis:
Acetyl-CoA + 7 Malonyl CoA + 14 NADPH, H+
palmitate + 7CoA + 7HCO3 14 NADP+
Regulation:Regulation:• Key regulatory step: acetyl-CoA carboxylase
• Citrate allosterically activates.
• Palmitate is a feedback inhibitor
• Fatty acyl-CoAs allosterically inhibit
• Phosphorylation (stimulated by glucagon and epinephrine) inactivates
Regulation:
• Fatty acid synthesis occurs in the chloroplast (stroma) in plants
• The plant acetyl-CoA carboxylase is not regulated by phosphorylation; it is activated by increases in stromal
pH and magnesium
• Synthesis of unsaturated fatty acids requires the activity of a number of desaturases
• Animal desauturases form double bonds as much as 9 carbons removed from the carboxyl
end of a fatty acid
• Only plant desaturases form double bonds positioned farther than 9 carbons from the
carboxyl end; thus fatty acids such as linoleate (18:2∆9,12) must be acquired in the diet of animals
and are considered essential fatty acids
Linoleate
arachidonic acid (C20:4∆5,8,11,14)
leukotrienes
lipoxygenase
trigger allergic responses
eicosanoids
cyclooxygenase
prostaglandins (smooth muscle contraction; pain; inflammation)
Linoleate
arachidonic acid (C20:4∆5,8,11,14)
leukotrienes
lipoxygenase
trigger allergic responses
eicosanoidsprostaglandins (smooth muscle contraction; pain; inflammation)
cyclooxygenase
Aspirin
X
Cholesterol BiosynthesisCholesterol Biosynthesis
• Pathway substantially active only in liver cells
• All carbon atoms arise from acetyl-CoA
• Squalene, C30 linear hydrocarbon, is an intermediate
• Squalene is formed from 5 carbon units (isoprene)
H3C CH
2
H2C
C CH
Stage 1: Acetyl Co-A to Isopentenyl Pyrophosphate
Stage 1: Acetyl Co-A to Isopentenyl Pyrophosphate
2 Acetyl Co-A Acetoacetyl Co-Athiolase
HMG CoA
HMG CoA synthase
-OOC-CH2-C-CH2-CH2-OH
OH
CH3 Mevalonate
NADPHNADP+
HS-CoA2
2
HMG CoA reductaseHMG CoA reductase
Stage 1: Acetyl Co-A to Isopentenyl Pyrophosphate
Stage 1: Acetyl Co-A to Isopentenyl Pyrophosphate
2 Acetyl Co-A Acetoacetyl Co-Athiolase
HMG CoA
HMG CoA synthase
-OOC-CH2-C-CH2-CH2-OH
OH
CH3 Mevalonate
HMG CoA reductaseHMG CoA reductase
NADPHNADP+
HS-CoA
Lovostatin (Mevacor);Zocor
X
-OOC-CH2-C-CH2-CH2-OH
OH
CH3
melvalonate kinase
ATPADP, PiMevalonate
Phosphomevalonate kinase-OOC-CH2-C-CH2-CH2-OPO3
=
OH
CH3 PhosphomevalonateATP
ADP, Pi
-OOC-CH2-C-CH2-CH2-OP2O63-
OH
CH3Pyrophosphomevalonate
PyrophosphomevalonateDecarboxylase
ATP
ADP, Pi CO2
Isopentenyl pyrophosphateIsopentenyl pyrophosphate
CH2=C-CH2-CH2-OP2O63-CH2=C-CH2-CH2-OP2O63-
CH3CH3
isomerase
CH3-C=CH-CH2-OP2O63-CH3-C=CH-CH2-OP2O63-
CH3CH3
Dimethylallyl pyrophosphateDimethylallyl pyrophosphate
Stage 2: Isopentenyl Pyrophosphate to Squalene
Stage 2: Isopentenyl Pyrophosphate to Squalene
Isopentenyl pyrophosphate + Dimethylallyl pyrophosphate
prenyl transferase
condense head to tail
PPi
OP2O
63-
Geranyl pyrophosphate
(C10)
OP2O
63- OP
2O
63-
+Geranyl pyrophosphate
(C10)
Isopentenyl pyrophosphate
(C5)
OP2O
63-
Farnesyl pyrophosphate
(C15)
prenyl transferase
PPi
OP2O
63-
2 Farnesyl pyrophosphates
condense head to head
2 PPi
NADPHNADP+
Reaction catalyzed by Squalene
synthase
Product is Squalene (C30) !
Stage 3: Squalene to CholesterolStage 3: Squalene to Cholesterol
• Numerous steps involving addition of oxygen (to form squalene 2,3 epoxide) and closure of rings
• Lanosterol is the first intermediate containing all four fused rings; it accumulates in large amounts
in cells actively synthesizing cholesterol
• Lanosterol is converted to cholesterol in 20 more reactions
• Squalene 2,3 epoxide is precursor to plant sterols
Cholesterol metabolism is a source of a large number of other cellular constituents:
Cholesterol metabolism is a source of a large number of other cellular constituents:
• Isopentenyl pyrophosphate is precursor to fat soluble vitamins (A, E, K), ubiquinone in
animal cells, plastoquinone and the phytol side chain of chlorophyll in plant cells, certain oils
such as musk, lemon, eucalayptus
• Cholesterol is precursor for bile salts, vitamin D, steroid hormones, mineralocorticoids, and
is also an important component of membranes.
Regulation of HMG-CoA Reductase:
• Phosphorylation by c-AMP dependent protein kinase inactivates.
• Gene expression: Cholesterol levels control the amout of mRNA. If [cholesterol] is high, mRNA levels are reduced. If [cholesterol] is low, mRNA is increased.
• Degradation of the enzyme: Half-life depends upon the cholesterol level; high cholesterol means a short half-life.
Lipoproteins: Good vs. Bad Cholesterol
Phospholipids, triacylglycerols and cholesterol circulate in blood in the form of lipoproteins.Phospholipids, triacylglycerols and cholesterol circulate in blood in the form of lipoproteins.
Classified according to the relative amounts of lipid and protein in the complex (the more protein and less lipid the denser the complex).
Classified according to the relative amounts of lipid and protein in the complex (the more protein and less lipid the denser the complex).
Thus, we have high-density lipoproteins (HDL); low-density lipoproteins (LDL); intermediate-density lipoproteins (IDL); very-low density lipoproteins (VLDL) and chylomicrons.
Thus, we have high-density lipoproteins (HDL); low-density lipoproteins (LDL); intermediate-density lipoproteins (IDL); very-low density lipoproteins (VLDL) and chylomicrons.
All lipoproteins consist of a core of triacylglycerols or cholesterol esters surrounded by a single phospholipid layer into which is inserted a mixture of cholesterol and proteins. The proteins serve as recognition sites for lipoprotein receptors throughout the body.
HDL and VLDL are synthesized primarily in the ER of the liver; chylomicrons are synthesized in the intestine.
LDL is synthesized from VLDL and is the major circulatory complex for transport of cholesterol and cholesterol esters from liver to other tissues.
Chylomicrons transport triacylglycerols mostly (and some cholesterol esters) from intestines to other tissues.
VLDL released in to the bloodstream is converted to IDL and LDL by the action of lipases which cleave triacylglycerols.
The half-life of LDL is about 24 hours. It is removed from circulation by endocytosis in coated vesicles and degradation by lysosomal acid lipases.
HDL has a half-life of 5-6 days. Newly formed HDL contains no cholesterol esters. These accumulate over time through the action of enzymes that transfer cholesterol esters. HDL functions to return cholesterol and cholesterol esters to the liver and hence remove them from circulation. Hence, HDL is the “good cholesterol”. LDL, because it is the major circulating form of cholesterol is the “bad cholesterol” correlated with increased risk of cardiovascular disease.
Receptor-Mediated Endocytosis
Discovered by Brown and Goldstein (1985 Nobel Prize).Discovered by Brown and Goldstein (1985 Nobel Prize).
ApoB-100, the major protein in LDL particles, is recognized by the LDL receptor.ApoB-100, the major protein in LDL particles, is recognized by the LDL receptor.
Binding of LDL to the LDL receptor initiates endocytosis which brings LDL and its receptors into the cell inside an endosome.
Binding of LDL to the LDL receptor initiates endocytosis which brings LDL and its receptors into the cell inside an endosome.
Endosomes fuse with lysosomes, which contain enzymes that hydrolyze cholesterol esters, releasing cholesterol and fatty acids into the cytosol.
Endosomes fuse with lysosomes, which contain enzymes that hydrolyze cholesterol esters, releasing cholesterol and fatty acids into the cytosol.
ApoB-100 is degraded to amino acids, but the LDL receptor is recycled back to the cell surface.ApoB-100 is degraded to amino acids, but the LDL receptor is recycled back to the cell surface.
Cholesterol entering the cell in this manner is used to synthesize membranes, or is reesterified for storage in cytosolic lipid droplets.
Cholesterol entering the cell in this manner is used to synthesize membranes, or is reesterified for storage in cytosolic lipid droplets.
Defective LDL receptors result in the genetic disease familial hypercholesterolemia. This is characterized by the inability of cells to take up cholesterol; hence blood cholesterol levels are extremely high.
Defective LDL receptors result in the genetic disease familial hypercholesterolemia. This is characterized by the inability of cells to take up cholesterol; hence blood cholesterol levels are extremely high.
The excess blood cholesterol accumulates and contributes to the formation of atherosclerotic plaques. Heart failure from atherosclerosis is the leading cause of death in industrialized societies.
The excess blood cholesterol accumulates and contributes to the formation of atherosclerotic plaques. Heart failure from atherosclerosis is the leading cause of death in industrialized societies.
Clathrin
Coated pit
Receptor with ligandAdaptor