Biosynthesis of Fatty Acid *Prepared By* Mr. Ansari Altamash Shakeel Ahmad Assistant Professor Y. B. Chavan College of Pharmacy, Aurangabad Contact No: 9823967266

Biosynthesis of Fatty Acid Introduction Reactions Pathways Fatty acid Synthase Regulation Reference By: Altamash Ansari

Introduction The dietary carbohydrates and amino acids, when consumed in excess, can be converted to fatty acids and stored as triacylglycerol's. De novo (new) synthesis of fatty acids occurs predominantly in liver, kidney, adipose tissue and lactating mammary glands. The enzyme machinery for fatty acid production is located in the cytosomal fraction of the cell. Acetyl CoA is the source of carbon atoms while NADPH provides the reducing equivalents and ATP supplies energy for fatty acid formation. By: Altamash Ansari

Reactions The fatty acid synthesis may be completed in 3 stages Production of acetyl CoA and NADPH Conversion of acetyl CoA to malonyl CoA Reactions of fatty acid synthase complex. By: Altamash Ansari

1. Production of acetyl CoA and NADPH Acetyl CoA is the starting material for de novo synthesis of fatty acids. Acetyl CoA is produced in the mitochondria by the oxidation of pyruvate, fatty acids, degradation of carbon skeleton of certain amino acids & from ketone bodies. Mitochondria are not permeable to acetyl CoA. An alternate or a bypass arrangement is made for the transfer of acetyl CoA to cytosol. Acetyl CoA condenses with oxaloacetate in mitochondria to form citrate. Citrate is freely transported to cytosol by tricarboxylic acid transporter. By: Altamash Ansari

Oxaloacetate in the cytosol is converted to malate. In cytosol it is cleaved by ATP citrate lyase to liberate acetyl CoA & oxaloacetate. Oxaloacetate in the cytosol is converted to malate. Malic enzyme converts malate to pyruvate. NADPH & CO2 are generated in this reaction. Both of them are utilized for fatty acid synthesis By: Altamash Ansari

Transfer of acetyl CoA from mitochondria to cytosol HMP Shunt Glucose Mitochondrial matrix Cytosol Pyruvate Pyruvate NADPH + H+ CO2 Malic enzyme PDH Complex Pyruvate carboxylase NADP+ Malate Amino acids FA NAD+ Fatty acid MDH NADH + H+ Oxaloacetate Oxaloacetate Acetyl CoA ADP+Pi Citrate lyase Acetyl CoA Citrate synthase ATP CoASH Citrate Citrate By: Altamash Ansari

2. Conversion of acetyl CoA to malonyl CoA Acetyl CoA is carboxylated to malonyl CoA by the enzyme acetyl CoA carboxylase. This is an ATP-dependent reaction & requires biotin for CO2 fixation. The mechanism of action of acetyl CoA carboxylase is similar to that of pyruvate carboxylase. Acetyl CoA carboxylase is a regulatory enzyme By: Altamash Ansari

Conversion of acetyl CoA to malonyl CoA CH3 – C – SCoA Acety CoA CO2, ATP Acetyl CoA carboxylase Biotin ADP + Pi O -OOC – CH2 – C – SCoA Malonyl CoA By: Altamash Ansari

3. Reactions of fatty acid synthase complex. The remaining reactions of fatty acid synthesis are catalysed by a multifunctional enzyme known as fatty acid synthase (FAS) complex. In eukaryotic cells, fatty acid synthase exists as a dimer with two identical units. Each monomer possesses the activities of seven different enzymes & an acyl carrier protein (ACP) bound to 4'-phosphopantetheine. Fatty acid synthase functions as a single unit catalysing all the seven reactions. In Multifunction enzyme, Intermediates of the reaction can easily interact with the active sites of the enzymes. By: Altamash Ansari

The efficiency of the process is enhanced. The sequence of reactions of the extra mitochondrial synthesis of fatty acids (palmitate) is as follows By: Altamash Ansari

Biosynthesis of long chain fatty acid-palmitate By: Altamash Ansari

Biosynthesis of long chain fatty acid-palmitate By: Altamash Ansari

Fatty acid synthase complex It is a multienzyme complex Fatty acid synthase is a dimer composed of two identical subunits (monomers), Each with a molecular weight of 240,000. Each subunit contains the activities of 7 enzymes of FAS & an ACP with 4'-phosphopantetheine SH group. The two subunits lie in anti-parallel (head to tail) orientation By: Altamash Ansari

Fatty acid synthase - multienzyme complex Dehydratase Enoyl reductase Ketoacyl ACP - Thioesterase Ketoacyl synthase Acetyl transacylase Malonyl 4’-Phosphopantetheine I Cys SH SH Functional division Subunit division ------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------- SH SH I Cys 4’-Phosphopantetheine Ketoacyl synthase Acetyl transacylase Malonyl Dehydratase Enoyl reductase Ketoacyl ACP Thioesterase - By: Altamash Ansari

Only the dimer is functionally active. The -SH group of phosphopantetheine of one subunit is in close proximity to the -SH of cysteine residue (of the enzyme ketoacyl synthase) of the other subunit. Each monomer of FAS contains all the enzyme activities of fatty acid synthesis. Only the dimer is functionally active. By: Altamash Ansari

FAS structure has both functional division & subunit division The functional unit consists of half of each subunit interacting with the complementary half of the other. FAS structure has both functional division & subunit division The two functional subunits of FAS independently operate & synthesize two fatty acids simultaneously By: Altamash Ansari

Significance of FAS complex The FAS complex offers great efficiency that is free from interference of other cellular reactions for the synthesis of fatty acids. There is a good coordination in the synthesis of all enzymes of the FAS complex. By: Altamash Ansari

Regulation of fatty acid synthesis Fatty acid production is controlled by enzymes, metabolites, end products, hormones and dietary manipulations. Acetyl CoA carboxylase: This enzyme controls a committed step in fatty acid synthesis. By: Altamash Ansari

Citrate promotes polymer formation & increases fatty acid synthesis. Acetyl CoA carboxylase exists as an inactive protomer (monomer) or an active polymer. Citrate promotes polymer formation & increases fatty acid synthesis. Palmitoyl CoA & malonyl CoA cause depolymerization of the enzyme, inhibits the fatty acid synthesis. By: Altamash Ansari

Reference Textbook of Biochemistry-U Satyanarayana Textbook of Biochemistry-DM Vasudevan By: Altamash Ansari

Thank you By: Altamash Ansari