Transcript Triacylglycerol Metabolism Gone Bad: A major cause of disease

Fatty Acid Synthesis
• Fatty Acid Synthase
– Acetyl-CoA serves as a primer
– Addition of two-carbon units from malonyl-CoA
– Each two-carbon unit added must be reduced by
2 NADPH + 2 H+
– Reaction for the synthesis of Palmitic acid (C:16):
Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH + 14H+
Palmitic acid + 7 CO2 + 14 NADP+ + 8 CoA + 6 H2O
Cytosolic Acetyl-CoA & NADPH Generation (presented as in most
text books, this scheme ignores the specificities of mitochondrial
transporters; a more accurate description is in the handout)
Glycolysis
Mitochondrion
Acetyl-CoA
Pyruvate
Pyruvate
Oxaloacetate
TCA cycle
Citrate
Malate
Citrate
Cytosol
Citrate lyase
Malic enzyme
Pyruvate
Malate
dehydrogenase
Malate
NADPH
NADP+
+ H+ + CO2
ATP + CoA
ADP + Pi
Oxaloacetate + Acetyl-CoA
NAD+
NADH+H+
Fatty acid
synthesis
Acetyl-CoA
carboxylase
ATP + CO2
ADP + Pi
Malonyl-CoA
Fatty Acid Synthesis
• Malonyl-CoA is produced by Acetyl-CoA carboxylase
O
||
CH3-C-S-CoA
O
O
||
||
- O-C-CH -C-S-CoA
2
Acetyl-CoA (cytoplasmic) + HCO3-
ATP
Malonyl-CoA
ADP + Pi
Acetyl-CoA Carboxylase
Requires Biotin
Fatty Acid Synthesis
• Acetyl-CoA Carboxylase
– Rate limiting reaction for fatty acid synthesis
– ACC1 is a liver isozyme
– Small amounts of ACC2 are present in muscle
where malonyl-CoA has a regulatory function
(Fatty acid oxidation)
Fatty Acid Synthesis
• Acetyl-CoA Carboxylase 1
– Highly regulated
• Allosteric activation by citrate; inhibition by
palmitoyl-CoA.
• Inhibited by phosphorylation in the fasting state.
– (low blood glucose inhibits; phosphorylation state is
determined by both glucagon activation of a kinase and
insulin activation of a phosphatase).
• Transcriptional up regulation by ChREBP (high
carbohydrate diet increases amount of ACC1 and most
other enzymes of fatty acid synthetic pathway)
Fatty Acid Synthesis
Transcriptiona
l control
Acetyl-CoA Carboxylase 1
Xylulose-5phosphate
+
Insulin
H2O
PP
Phosphorylated
Acetyl CoA
carboxylase
(Inactive)
Acetyl-CoA
Transcription
Citrate
Palmitoyl-CoA
Pi
+
+
Protein phosphatase
PKA AMPK
ADP + Pi
+
Glucagon
Covalent
modification
+
ATP
AMP
─
CO2
ATP
Acetyl CoA
carboxylase
(Inactive)
Acetyl CoA
carboxylase
(Active)
ADP + Pi
Malonyl-CoA
Allosteric
regulation
Triacylglycerol Synthesis
• Long-term transcriptional regulation by ChREBP
(Carbohydrate Regulatory Element Binding Protein).
– In addition to short term regulation of Acetyl-CoA carboxylase
– Many enzymes of fatty acid & triacylglycerol synthetic pathway are
coordinately regulated by ChREBP.
– ChREBP is inhibited by Protein Kinase A dependent phosphorylation.
– ChREBP is activated by Protein Phosphatase 2A dependent
dephophorylation (PP2A is stimulated by Xyulose-5-P).
Low Glucose:
Glucagon
cAMP
Protein kinase A
Inactive ChREPB-P
Fatty acid synthesis
High Glucose:
Xyulose-5-P
Protein Phosphatase A2
Active ChREPB-OH
Fatty acid synthesis
Fatty Acid Synthesis
• The main product of fatty acid synthase is palmitic
acid (16:0).
• Fatty acids can be elongated by other enzymes that
add two carbon units from malonyl-CoA.
Elongation is particularly important in brain.
• Still other enzymes can add double bonds (usually
at 9 ). Omega-3 and omega-6 fatty acids can not
be synthesized by humans.
Triacylglycerol Synthesis
• Fatty acids must be activated to Acyl-CoA
Fatty acid + CoA + ATP
Acyl-CoA + AMP + PPi
Acyl-CoA synthetase
PPi + H2O
2 Pi
Pyrophosphatase
Triacylglycerol Synthesis
• Glycerol-3-phosphate is required for
triacylglycerol synthesis.
H2C-OH
|
HOCH O
|
|
H2C-O-P-O ||
O-
H2C-OH
|
O=C O
|
|
H2C-O-P-O ||
O-
Glycerol-3-phosphate
dehydrogenase
Dihydroxyacetone Phosphate
Glycerol-3-phosphate
NADH + H+
NAD+
Glycerol-3-phosphate dehydrogenase
Triacylglycerol Synthesis
• Addition of 3 Acyl groups from Acyl-CoA
to Glycerol-3-phosphate
O
||
O H2C - O - C - R1
||
|
R2 - C - O - CH
|
H2C - O - C - R3
||
O
H2C-OH
|
HO-CH O |
|
H2 C-O-P-O ||
O-
Glycerol-3-phosphate
2 Acyl-CoA
Phosphatidate
CoA
Acyl-CoA
Triacylglycerol
CoA + Pi
VLDL formation
Apolipoprotien B-100 has a repeating -helix/-sheet structure:
Lipids are packaged as apolipoprotein B-100 is being synthesized:
From Shelness & Sellers (2001) Curr Opin Lipidology 12:151-157
VLDL formation
• VLDL stands for Very Low Density Lipoprotein
• As it is synthesized, VLDL contains:
•
•
•
•
One molecule of apoliprotein B-100
Triacylglycerol
Phospholipid
Cholesterol ester
• Microsomal Triacylglycerol Transfer Protein(MTP)
assists in the formation of the VLDL
• Other components are added to the VLDL in the blood.
VLDL formation
• Apolipoprotein B-100 synthesis is required for the
transport of lipid out of the liver
– If protein synthesis is reduced (e.g. by malnutrition) fat
droplets accumulate in the liver.
– If the rate of lipid synthesis is greatly elevated with
respect to protein synthesis (e.g. in type I diabetes or
glucose 6-phosphatase deficiency) fat droplets
accumulate in the liver.