Fatty Acid Biosynthesis

• Biosynthesis of fatty acids is a 3-stage process
• Fatty acid biosynthesis is a stepwise assembly of acetyl-CoA units, in the form of malonyl-CoA units, ending with Palmitate (C-16)

Physiological roles of fatty acids

• Fatty acids serve as building blocks of phospholipids and glycolipids
• Many proteins are modified by covalent attachment of fatty acids, targeting them to membrane locations
• Fatty acids act as fuel molecules
• Fatty acid derivatives function as hormones and intracellular messengers

Synthesis vs Degradation

• Synthesis takes place in the cytosol, while degradation occurs in the mitochondria
• Synthesis requires NADPH, degradation requires NADH and FADH2
• Synthesis uses acyl carrier protein, degradation uses CoA
• Synthesis involves D-isomer, degradation involves L-isomer
• Synthesis involves CO₂ activation and citrate ion, degradation involves no CO2 or citrate
• Synthesis involves a multi-enzyme complex, degradation uses independent proteins
• Synthesis adds 2 carbon units as 3-carbon malonyl CoA, degradation splits off 2 carbon units as acetyl CoA

Fatty Acid Synthesis

• Glucose is converted to acetyl-CoA, which can then become fatty acids, alkanes, or alkenes
• Acetyl-CoA carboxylase (ACCase) converts acetyl-CoA to malonyl-CoA
• Malonyl-CoA: ACP transferase converts malonyl-CoA to malonyl-ACP
• Synthesis utilizes : Acetyl CoA (in cytoplasm), Malonyl CoA, NADPH and a Fatty acids synthase

Starting Material

• Glucose is converted to pyruvate through glycolysis in the cytoplasm
• Pyruvate is converted to Acetyl CoA in the mitochondria

Fatty acid characteristics

• Contains Carboxylic Acid and a Chain of Hydrocarbons
• Amphiphilic
• Synthesis occurs in the cytosol and requires Acetyl CoA and NADPH
• Stored as Triacylglycerols (Triglycerides) in adipose tissue
• The storage is Energy/Mass efficient due to it being anhydrous

Steps of Fatty Acid Synthesis

• Activation
• Elongation
• Termination

Sites of Fatty Acid Synthesis

• Liver
• Kidney
• Adipose tissue
• Lactating mammary glands

Phases of Fatty Acid Synthesis

• Transport of acetyl-CoA from mitochondria to cytosol
• Carboxylation of acetyl-CoA to malonyl-CoA
• Reactions of fatty acid synthase complex

Systems for Fatty Acid Synthesis

• De novo synthesis of FAs in the cytoplasm
• Chain elongation in mitochondria
• Chain elongation in microsomes

Transfer of Acetyl CoA

• In the mitochondria, Acetyl-CoA is converted to citrate
• Citrate is transported to the cytosol
• In the cytosol, citrate is converted back to acetyl-CoA and oxaloacetate
• Oxaloacetate is converted to pyruvate

Committed Steo

• Catalyzed by Acetyl-CoA Carboxylase (ACC)
• Stimulated by Insulin and Citrate
• Inhibited by Epinephrine, Palmitoyl-CoA and Glucagon

Transportation of Acetyl CoA

• In the mitochondrial matrix, glucose is converted to pyruvate, then to Acetyl-CoA
• Acetyl-CoA combines with oxaloacetate to form citrate, which is transported to the cytosol via citrate transporter
• In the cytosol, citrate lyase converts citrate back to acetyl-CoA and oxaloacetate, supporting fatty acid synthesis

Enzymes and Cofactors Involved

• Acetyl coA carboxylase
• Fatty acid Synthase
• Biotin
• NADPH
• Mn++
• Mg++

Carboxylation of Acetyl CoA to Malonyl CoA

• Acetyl-CoA carboxylase, with biotin, carboxylates acetyl-CoA to form malonyl-CoA

Fatty acid synthase complex

• It is a polypeptide containing seven enzyme activities and an acyl carrier protein (ACP) segment
• ACP contains vitamin pantothenic acid in the form of 4'-phosphopantetheine (Pant)
• ACP carries the acyl groups during fatty acid synthesis

Reactions of fatty acid synthase complex

• Fatty acid synthase (FAS) is a multifunctional enzyme, existing as a dimer in eukaryotic cells
• Each monomer possesses the activities of seven different enzymes and an acyl carrier protein (ACP) bound to 4'-phosphopantetheine
• Fatty acid synthase functions as a single unit, catalyzing all seven reactions

First Round of Synthesis

• Fatty acid synthase activates malonyl and acetyl groups on the enzyme
• Acetyl-CoA combines with a cysteine -SH group, catalyzed by acetyl transacylase
• Malonyl-CoA combines with the adjacent —SH, catalyzed by malonyl transacylase

Activation of molnyl group

• The acetyl group from acetyl-CoA is transferred to the Cys-SH group of the-ketoacyl ACP synthase
• This reaction is catalyzed by acetyl- CoA transacetylase

Activation Reactions

• Fatty acid synthesis begins with the formation of acetyl ACP and malonyl ACP
• Acetyl transacylase and malonyl transacylase catalyze these reactions
• Acetyl CoA + ACP results in acetyl ACP + CoA
• Malonyl CoA + ACP results in malonyl ACP + CoA

Condensation Reaction

• Acetyl ACP and malonyl ACP react to form acetoacetyl ACP
• Enzyme involved is acyl-malonyl ACP condensing enzyme, releasing CO₂

Reduction Reaction

• Acetoacetyl ACP is reduced to D-3-hydroxybutyryl ACP
• NADPH is the reducing agent
• Enzyme: β-ketoacyl ACP reductase

Dehydration Reaction

• D-3-hydroxybutyryl ACP is dehydrated to form crotonyl ACP (trans-A2-enoyl ACP)
• Enzyme: 3-hydroxyacyl ACP dehydratase

Reduction Reaction

• The final step is the reduction of crotonyl ACP to butyryl ACP
• NADPH is reductant
• Enzyme - enoyl ACP reductase
• This marks the end of the first elongation cycle (first round)

Step 4: Reduction

• Reduction of the double bond forms butyryl-ACP
• Catalyzed by enoyl-reductase and is NADPH dependent

Second Round

• Butyryl ACP condenses with malonyl ACP to form a C6-β-ketoacyl ACP
• Reduction, dehydration, and a second reduction convert this into a C6-acyl ACP, ready for a third round of elongation

Termination

• Synthesis continues until a C16 palmitoyl group is formed
• Palmitoyl-ACP is hydrolyzed by a thioesterase

Net Production

• Net reaction: 8 acetyl-CoA + 14 NADPH + 7 ATP yields palmitate + 14 NADP+ + 8 CoA + 7 ADP + 7 Pi
• Overall Net Reaction: acetyl-CoA + 7 malonyl-CoA + 14 NADPH yields palmitate + 7 CO₂ + 14 NADP+ + 8 CoA

De novo synthesis of fatty acids

• Acetyl CoA ACP transacylase transfers from Acetyl CoA and Fatty Acid Synthase Complex

Chain Elongation

• A new malonyl-CoA molecule combines with the -SH of 4'-phosphopantetheine, displacing the saturated acyl residue onto the free cysteine SH group
• The sequence is repeated until a saturated 16-carbon acyl radical (Palmityl) is assembled
• catalyzed by Thioesterase (deacylase)

Result of fatty acid synthesis activity

• Seven condensation and reduction cycles produce the 16-carbon saturated palmitoyl group, still bound to ACP
• Chain elongation usually stops here, releasing free palmitate from ACP
• Smaller amounts of longer fatty acids like stearate (18:0) may also form

Fatty Acid Elongation

• Palmitate (16:0) in animal cells is the precursor of other long-chained FAs
• Further elongation, through FA elongation systems in smooth ER and mitochondria, increases chain length
• Palmitate elongates to Stearate (18:0), which then can elongate or desaturate to Oleate, and then elongate further.

Regulation of Fatty acid biosynthesis

• The reaction catalyzed by acetyl-CoA carboxylase is the rate-limiting step
• Long-chain fatty acid synthesis is controlled in the short term by allosteric and covalent modification, and in the long term by gene expression changes

Conclusion

• Fatty acid biosynthesis occurs in the cytosol
• Acetyl CoA serves as the immediate substrate
• Free palmitate is the end product
• Reaction depends on ATP and NADPH
• Reaction rate is regulated by acetyl CoA carboxylase
• 8 Acetyl CoA + 7 ATP + 14 NADPH → Palmitate + 14 NADP+ + 8 CoA + 6H2O + 7 ADP + 7 Pi

Key points

• Synthesis of long-chain fatty acids involves acetyl-CoA carboxylase and fatty acid synthase
• The pathway converts acetyl-CoA to palmitate, requiring NADPH, ATP, Mn2+, biotin, pantothenic acid, and HCO3- as cofactors
• Acetyl-CoA carboxylase converts acetylCoA to malonyl-CoA, while fatty acid synthase assembles palmitate from acetyl-CoA and malonyl-CoA molecules
• Lipogenesis is regulated at acetyl-CoA carboxylase by allosteric modifiers, phosphorylation/dephosphorylation, and enzyme synthesis
• Citrate activates the enzyme, while long-chain acyl-CoA inhibits it
• Insulin activates acetyl-CoA carboxylase, while glucagon and epinephrine have opposite actions