1. 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 CO NADP+ + 8 CoA + 6 H2O

2. Oxaloacetate + Acetyl-CoA
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
Citrate
Pyruvate
Oxaloacetate
TCA cycle
Malate
Citrate
Cytosol
ATP + CoA
Citrate lyase
Malate dehydrogenase
ADP + Pi
Malic enzyme
Pyruvate
Malate
Oxaloacetate + Acetyl-CoA
NADPH
+ H+ + CO2
NADP+
NAD+
NADH+H+
ATP + CO2
Acetyl-CoA
carboxylase
ADP + Pi
Fatty acid
synthesis
Malonyl-CoA

3. Fatty Acid Synthesis - O-C-CH2-C-S-CoA
Malonyl-CoA is produced by Acetyl-CoA carboxylase
Acetyl-CoA (cytoplasmic) + HCO Malonyl-CoA O ||
CH3-C-S-CoA
O O
|| ||
- O-C-CH2-C-S-CoA
ATP
ADP + Pi
Acetyl-CoA Carboxylase
Requires Biotin

4. 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)

5. 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)

6. Fatty Acid Synthesis Acetyl-CoA Carboxylase 1 Transcriptional control
Xylulose-5-phosphate
Acetyl-CoA +
Insulin
Transcription
Citrate
Palmitoyl-CoA
H2O Pi + ─
Protein phosphatase +
CO2
ATP
ADP + Pi P P
Phosphorylated
Acetyl CoA carboxylase
PKA
AMPK
Acetyl CoA carboxylase
Acetyl CoA carboxylase + +
(Inactive)
(Active)
ADP + Pi ATP
(Inactive)
Glucagon AMP
Malonyl-CoA
Allosteric regulation
Covalent modification

7. 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

8. 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.

9. Triacylglycerol Synthesis
Fatty acids must be activated to Acyl-CoA
Fatty acid + CoA + ATP Acyl-CoA + AMP + PPi
PPi + H2O Pi
Acyl-CoA synthetase
Pyrophosphatase

10. 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

11. Triacylglycerol Synthesis
Addition of 3 Acyl groups from Acyl-CoA to Glycerol-3-phosphate
Glycerol-3-phosphate Phosphatidate Triacylglycerol
2 Acyl-CoA CoA Acyl-CoA CoA + Pi

12. 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:

13. 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.

14. 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.