Fatty Acid Biosynthesis

• Lipid, considered a principal form of stored energy, phospholipid is the significant component of the Plasma Membrane. 
• The liver, Kidney, adipose tissue, and lactating mammary glands are the organs where the de-novo synthesis of fatty acids occurs. 
• The site for Fatty acid biosynthesis is cytosol. 

1.0Introduction of Fatty Acid Biosynthesis of Fats

The synthesis of fatty acids starting from a simple precursor (acetyl CoA) is known as de- novo lipogenesis. In animals (and yeast), it occurs primarily in the cytosol of various tissues such as the liver, adipose (fat) tissue, central nervous system and lactating mammary glands. In plants, the enzymes are present in chloroplasts. Lipogenesis is an endergonic reductive process. The source of the reductant is NADPH. The fatty acid synthesis intermediates are covalently linked to an acyl carrier protein.  

2.0Fatty Acid Synthase Enzyme

Fatty Acid Synthase complex is a multifunctional enzyme which is made up of dimers with two identical subunits, including ACP (Acyl Carrier Protein). ACP is responsible for transferring the acyl group from acetyl CoA and 2 carbon fragments from malonyl CoA to elongate the carbon chain required for fatty acid biosynthesis, such as 16 carbon compounds called Palmitate. Fatty acid synthase complex is made up of - 

• Acetyl transacylase 
• Malonyl transacylase 
• Ketoacyl synthase 
• Ketoacyl reductase 
• Dehydratase 
• Enoyl reductase
• Thioesterase

3.0Steps Involved in Fatty Acid Synthesis

Production of Acetyl CoA and NADPH

Acetyl CoA is produced in mitochondria by oxidation of pyruvate (PDH complex) and fatty acid oxidation. Oxidation of Palmitic acid (fatty acid) gives 8 molecules of acetyl CoA. However, the problem is that acetyl CoA cannot permeate mitochondria as it has to go to the cytosol for FAB. So, in mitochondria, acetyl CoA condenses with OAA to form citrate. Citrate can pass through the mitochondrial membrane and enter the cytosol, where an enzyme called citrate lyase cleaves citrate into OAA and Acetyl CoA. OAA is converted into malate in cytosol and malate into pyruvate by malic enzyme with NADPH formation, a reducing equivalent required for fatty acid biosynthesis.

Citrate + ATP + HSCo + H2O → AcetylCoA + ADP + Pi + OAA

Formation of malonyl CoA

It is formed by the carboxylation of acetyl CoA by an enzyme called acetyl CoA carboxylase. This enzyme is ATP-dependent and requires biotin as a cofactor. Reactions are catalyzed by the Fatty Acid Synthase complex to form long-chain fatty acid called Palmitic acid/Palmitate

The acyl group from acetyl CoA and the malonyl group from malonyl CoA are transferred to fatty acid synthase complex by Acetyl CoA-ACP transacylase and Malonyl CoA-ACP transacylase. 

Step 1 . Condensation Reaction

Acyl (from acetyl CoA) is the first acyl group, and 2 carbons derived from malonyl extend the acyl chain by 2 carbons. This condensation of both molecules is associated with decarboxylation, and the product form is 𝛃- keto butyryl-ACP. The enzyme is 𝛃- ketoacyl-ACP synthase.

Step 2 . Reduction Reaction 

𝛃- keto butyryl-ACP is then undergoing reduction, and the ketoacyl group is turned into the hydroxyl group. The electron donor is NADPH. The enzyme is 𝛃- ketoacyl-ACP reductase. The final product is 𝛃- hydroxybutyryl-ACP 

Step 3 . Dehydration reaction 

𝛃- hydroxybutyryl ACP undergoes dehydration and forms enoyl ACP. The enzyme is Beta hydroxyacyl-ACP dehydratase. Water is eliminated during the dehydration reaction, and the product formed is butenoyl-ACP. 

Step 4. Reduction reaction 

Enoyl ACP reductase catalyzes this reaction using NADPH as a reducing equivalent and forms Acyl ACP. 

The 4-carbon unit attached to ACP is butyryl. ACP is the carrier molecule that transfers the carbon chain to the cysteine part of the fatty acid synthase enzyme complex, and in this way, the reactions mentioned above are repeated 6 more times. Note: A total of 7 reactions were repeated to form 16 carbon palmitate molecules. Each time, the chain is elongated by 2 carbon units. Then, finally, palmitoyl thioesterase separates Palmitate from ACP. In this way, a fully saturated 16-carbon compound is called Palmitate and is formed.

4.0Synthesis of Long Chain Fatty Acids 

• Elongation by the fatty acid synthase complex stops after formation of Palmitate (16 C).  
• Other enzyme systems carry out further elongation and the formation of double bonds.  
• The major product of fatty acid biosynthesis is the 16-carbon Palmitate. Additional enzymes are required to synthesize longer-chain fatty acids.  
• Chain elongation reactions occur both in mitochondria and in microsomes. 
• Microsomes are small membrane-enclosed vesicles derived from the endoplasmic reticulum of cells.  
• Mitochondria and microsomes carry out chain elongation by adding two-carbon units to fatty acids.  
• The microsomal system is highly physiologically important because it provides the long-chain fatty acids (18-24C) required for the myelination of nerve cells in animals.  
• Chain elongation occurs through a condensation, reduction, and dehydration cycle, followed by another reduction that parallels cytosolic fatty acid biosynthesis.  
• The more active elongation system adds two carbons to palmitoyl-CoA to make it steroyl CoA.  
• The elongation mechanism is identical to that used to synthesize Palmitate, except for the enzyme systems and the acyl carrier protein. 

5.0Biosynthesis of Unsaturated Fatty Acids 

• There are two pathways for synthesizing unsaturated fatty acids: aerobic and anaerobic. 
• The aerobic pathway is almost universal, but the anaerobic pathway is rare, and the two pathways have not been reported from the same organism. 
• Most monounsaturated fatty acids (palmitoleic and oleic acid) produced have a ∆-9 double bond. 
• The aerobic pathway is mediated by a desaturase complex with three proteins: a flavoprotein (NADH-cytochrome b5 reductase), cytochrome b5, and the desaturase present in the SER of animal cells. The desaturases belong to mixed-function oxidases as both fatty acid and NADPH (co-substrate) are simultaneously oxidized. Various unsaturated fatty acids can be formed from oleate by combining elongation and desaturation reactions.
• In most cases, the double bonds introduced are in the cis configuration. Eukaryotes ' polyunsaturated fatty acids (PUFA) generally have unconjugated (interrupted by a methylene group) double bonds. 
• Although plants and animals synthesize several long-chain unsaturated fatty acids, the difference arises due to the specific desaturases each one expresses. 
• Both produce ∆-9 desaturases; animal desaturases introduce subsequent double bonds between C-9 and the carboxyl group, whereas plants introduce additional double bonds between C-9 and the methyl group (omega end). Some mammalian desaturases include ∆-6, ∆-5 and ∆-4, while plants have ∆-12 and ∆-15 desaturases. Most bacteria lack polyunsaturated fatty acids.

6.0Regulation of Fatty Acid Synthesis

• Hormones, enzymes, metabolites and end products control it. 
• Acetyl CoA carboxylase: This enzyme is active in polymeric form and inactive in monomeric form. Citrate promotes its polymeric form, whereas palmitoyl CoA and malonyl CoA promote its inactivation. 
• Hormonal control includes cAMP-dependent phosphorylation for inactivation and vice versa for activation. Insulin promotes fatty acid synthesis, and glucagon inhibits it. 
• Availability of NADPH—It is provided by citrate (Acetyl CoA) or PPP/HMS (Hexose monophosphate shunt pathway), which significantly influences Fatty acid synthesis.