Lipid Metabolism - Fatty Acid Synthesis (Lesson 2) Past Paper PDF

Lipid Metabolism. )55 ‫)الدفعة‬ Lesson 2 Fatty Acid Synthesis. Dr. Mohamed Khomsi Fb: ‫ محمد الخمسي‬.‫د‬ Oct / 2024  Fatty Acids Name: - Fatty Acids...

Lipid Metabolism. )55 ‫)الدفعة‬ Lesson 2 Fatty Acid Synthesis. Dr. Mohamed Khomsi Fb: ‫ محمد الخمسي‬.‫د‬ Oct / 2024  Fatty Acids Name: - Fatty Acids Symbol: - F.A. Numbering: (1) Delta numbering system & (2) Omega numbering system Delta (∆) numbering system: Omega (ω) numbering system - Fatty acid are numbered from - Fatty acid are numbered from First carbon Terminal (last) carbon Carboxyl carbon Methyl carbon Delta carbon Omega carbon n carbons The carbons adjacent to the carboxyl carbon i.e. Carbon 2,3,4 are known as α, β, & γ carbons respectively - Enoyl = Double Bond. 1  Fatty Acids Classification of Fatty Acids - - Fatty Acids are Classified According to: (1) Length of Carbon Chain (2) Type of Bond in Chain (3) Nutritional Classification (4) Eicosanoids Classification Type of FA Short FA 1- Length of Carbon Chain Medium FA i.e. Long FA (Number of Carbons) Very Long FA 2- Type of Bond in Chain Saturated FA i.e. Unsaturated FA (Single or Double Bond) 3- Nutritional Classification Essential FA (Needed from Diet or Not) Nonessential FA Prostanoids 4- Eicosanoids Leukotrienes Fatty Acids Length Bond Nutrition Eicosanoids - Short - Saturated - Essential - Prostanoids - Medium - Unsaturated - Nonessential - Leukotrienes - Long - Very Long 2  Fatty Acids Length Classification: (1) Short FA, (2) Medium FA, (3) Long FA, (4) Very long FA Short Medium Long Very long < 5 carbons 6 – 12 carbons 13 – 21 carbons > 22 carbons ** Most biological fatty acids are 12 - 22 chain length ** Bond Classification: (1) Saturated FA , (2) Unsaturated FA Saturated Fatty Acids -They are Fatty acids that Only have Single bonds between carbons. Examples of Saturated Fatty Acids: Number of Carbons Common Name Short-handed Formula Carbons : Double bonds 2 Acetic acid 2:0 3 Propionic acid 3:0 4 Butyric acid 4:0 16 Palmitic acid 16:0 18 Stearic acid 18:0 3  Fatty Acids Unsaturated Fatty Acids - They are fatty acids that have One or More double bonds. Types of Unsaturated FA: (1) Monounsaturated FA, (2) Polyunsaturated FA. Monounsaturated FA Polyunsaturated FA (MUFA) (PUFA) Have 1 double bond Have 2 or more double bonds MUFA Examples of Mono Unsaturated Fatty Acids (MUFA): Common name Short-handed formula Omega Palmitoleic acid 16:1 ∆ 9 ω7 Oleic acid 18:1 ∆ 9 ω9 PUFA Examples of Poly Unsaturated Fatty acids (PUFA): Common name Short-handed formula Omega Linoleic acid 18:2 ∆ 9,12 ω6 α-Linolenic acid 18:3 ∆ 9,12,15 ω3 γ-Linolenic acid 18:3 ∆ 6,9,12 ω6 Arachidonic acid 20:4 ∆ 5,8,11,14 ω6 - Linoleic & α-Linolenic are Essential Fatty acids. - Arachidonic acid is found in Phospholipids of Cell Membranes. - Arachidonic acid is a Precursor of Eicosanoids. 4  Fatty Acid Synthesis There are 3 Systems for Fatty Acid Production (1) Fatty Acid Synthesis System. (2) Fatty Acid Elongation System. (3) Fatty Acid Desaturation System. 5  Fatty Acid Synthesis System Name: - Fatty Acid Synthesis. Other Name: - Lipogenesis. Pathway: - Major, Denovo, Extra-Mitochondrial, Cytoplasmic Pathway. State: - It occurs After meal, in Well-fed State, Stimulated by Insulin. Definition: - It is the Complete Synthesis of Fatty acid from Acetyl-CoA during Well-Fed State. Site (Organ): - In Adult Humans, - It Occurs Primarily in Liver & Lactating Mammary Glands. - To a Lesser Extent in Adipose Tissue. Site (Cell): - It is an Extra-Mitochondrial Fraction. - It Occurs in Cytosol. 6  Fatty Acid Synthesis System Substrate: - The Requirements are: Acetyl CoA, Malonyl CoA, NADPH, ATP. Substrate Function Source Mitochondrial Primer Glucose Acetyl CoA Initiator Molecule Amino Acid (2C Unit) Source of Carbons Malonyl CoA Elongation Molecule Formed by (3C Unit) 2 Carbon Donor Carboxylation Acetyl CoA Pentose Phosphate Pathway NADPH+H Reducing Molecule Oxidation of Malate by Malic Enzyme - In Most Mammals, Glucose is the Primary Substrate for Lipogenesis. - Glucose is the Major Source of Acetyl CoA. - Acetyl CoA is Produced by the Oxidation of Pyruvate & Catabolism of Certain A.A. End Product: - Palmitic Acid (Palmitate) (16:0). - This Pathway Produces a Pool of Palmitic acid. - Palmitic Acid then Can be Modified. Steps: (1) Transportation of Acetyl CoA, (2) Formation of Malony CoA, (3) Synthesis of Palmitic Acid. 7  Fatty Acid Synthesis System (1) Transportation of Acetyl CoA: - This Step Transfers Acetate Units of Acetyl CoA from Mitochondria to Cytosol. - By Transportation System Called Citrate-Malate-Pyruvate Shuttle. - The CoA Portion of Acetyl CoA Cannot Cross the Inner Mitochondrial Membrane, - Only the Acetyl Portion are Shuttled Out of the Mitochondria as Citrate. - Citrate is Produced by the Condensation of Acetyl CoA & Oxaloacetate by Citrate Synthase. - Citrate Crosses Inner Mitochondrial Membrane into Cytosol. - In the Cytosol Citrate is Cleaved to Oxaloacetate & Acetyl CoA by Citrate Lyase. - Oxaloacetate is Reduced to Malate, by Cytosolic Malate Dehydrogenase. - Malate is Converted to Pyruvate Producing NADPH & CO2 by Malik Enzyme. Mitochondria Cytosol Pyruvate Pyruvate Malic Enzyme Pyruvate Carboxylase Malate Malate Dehydrogenase Acetyl CoA + Oxaloacetate Oxaloacetate + Acetyl CoA Citrate Synthase Citrate Lyase Citrate Citrate Malate Malate - Citrate Lyase Requires ATP & Induced by Insulin. - Oxaloacetate = OAA. 8  Fatty Acid Synthesis System (2) Formation of Malonyl CoA: - Carboxylation of Acetyl CoA. - By Acetyl CoA Carboxylase (ACC). - This Requires NADPH, ATP, Biotin, Bicarbonate (Co2), Mn2+. - This is a Committed Step & Point of Regulation of this Pathway. - Acetyl CoA Carboxylase is a Multi-Enzyme protein, Containing: (1) Biotin, (2) Biotin Carboxylase, (3) Biotin Carboxyl Carrier Protein (BCP), (4) Carboxyl Transferase, (5) Regulatory Allosteric Site. - Biotin Carboxylase is Enzyme 1 (E1). - Carboxyl Transferase is Enzyme 2 (E2). Acetyl CoA Carboxylase Acetyl CoA Malonyl CoA (2C) (3C) The Reaction Proceeds (Takes Place) in Two Steps: (Step 1) - Is Catalyzed by E1. - Involves Carboxylation of Biotin. - Biotin Accepts Carboxyl Group (Coo) from Bicarbonate (HCO3). - This Step Uses ATP. (Step 2) - Is Catalyzed by E2. - Involves Carboxylation of Acetyl CoA. - Transfer of Carboxyl Group (Coo) to Acetyl-CoA to form Malonyl-CoA. - Bicarbonate as a Source of CO2. - One Subunit of the Complex Contains All the Components, - And Variable Number of Subunits Form Polymers in the Active Enzyme. 9  Cytoplasmic Synthesis System Key Enzymes: - Acetyl CoA Carboxylase (ACC). Regulation: Stimulation Inhibition Well-Fed State Restricted Caloric Intake Diet High Carbohydrate Diet High Fat Diet Sucrose High Plasma Free F.As Hormonal Insulin Glucagon, Epinephrine, DM (Covalent Modification) (Phosphatase, Dephosphorylation) (Kinase, Phosphorylation) Energy High Energy (ATP) Low Energy (AMP) (Allosteric Regulation) Intermediate Malonyl CoA & Palmitoyl CoA Acetyl CoA & Citrate (Allosteric Regulation) Long Chain Fatty Acyl CoAs Stimulation: - When Sucrose is Fed Instead of Glucose, - Fructose Bypasses PFK Control Point in Glycolysis & Floods the Lipogenic Pathway. - Insulin, Reverses the Effects of Liver Kinase Cascade & Stimulates ACC by Dephosphorylation. - This Promotes the Conversion of the Enzyme to it's Active Polymeric Form. - Insulin Also Promotes Entry of Glucose into Cells which Favors Production of NADPH, Via Entry of Glucose-6-phosphate into the Pentose Phosphate Pathway. - Insulin Activates the Pyruvate dehydrogenase which Promotes Production of Acetyl CoA. - The Presence of Large Amounts of ATP Inhibits isocitrate dehydrogenase of CAC. - This Leads to Accumulation & Increase of Citrate & Isocitrate Mitochondrial Concentration. - This Increases Translocation of Citrate to the Cytosol. - ATP & Citrate are High Energy Signals that Enhance Lipogenesis. - Citrate Promotes Conversion of ACC from Inactive Dimer to Active Polymeric Form. Inhibition: - Glucagon & Epinephrine through the cAMP Dependent Protein Kinase, - Inactivates ACC by Phosphorylation & Depolymeriztion to the Monomeric Form. - Acyl CoA Inhibits the Tricarboxylate Transporter, which Transports Citrate Out of Mitochondria. - This Decreases Citrate Concentration in Cytosol, Favoring Inactivation of the Enzyme. - Increased Concentrations of Plasma-Free Fatty Acids have Inverse Relationship & Hepatic Lipogenesis. 10  Cytoplasmic Synthesis System (3) Formation of Palmitic Acid: (1) An Acetyl group is Transferred from Acetyl CoA to the –SH group of the ACP, By: Acetyl CoA-ACP Acetyl Transacylase Domain. (1) Next, This 2 Carbon Fragment is Transferred to a Temporary Holding Site, The Thiol group of a Cysteine Residue on the Enzyme (2) The Now-Vacant ACP Accepts a 3 Carbon Malonyl group from Malonyl CoA, By: Malonyl CoA-ACP Transacylase Domain. (3) The Acetyl on the Cysteine Residue Condenses with the Malonyl on ACP with the Release of Co2. By: Ketoacyl-ACP Synthase (Condensing Enzyme) Domain. - The CO2 Released is the CO2 originally Added by Acetyl CoA Carboxylase. -This Step Incorporates a 2 Carbon Unit into Growing FA Chain at the Carboxyl End. - This Condensation Step Result in a Four Carbon Unit Attached to the ACP Domain. - The Loss of Free Energy from the Decarboxylation Drives the Reaction. (4) The Next 3 Reactions Convert the 3-Ketoacyl group to the Corresponding Saturated Acyl by a Pair of NADPH-Requiring Reductions & a Dehydration Step. (5) The Keto group is Reduced to an Alcohol β-hydroxybutyryl. By: Ketoacyl-ACP Reductase Domain - (NADPH is the Reductant). (6) A Molecule of Water is Removed, Creating a Trans Double Bond between C2 & C3 (α & β). By: Hydroxyacyl-ACP Dehydratase Domain (7) The Double bond is Reduced to an Acyl (Butyryl). By: Enoyl-ACP Reductase Domain - (NADPH is the Reductant). - This Completes the First Turn in the Spiral of Fatty Acid Synthesis. - These Seven Steps are Repeated Beginning with Transfer of Butyryl from ACP to Cysteine of 3KAS. - Another Molecule of Malonyl-CoA Combines with the SH of ACP, - The Second Turn of the Spiral, Generates Hexanoyl-ACP. - The Elongation Cycles are Repeated 5 More Times, - The Synthetic Process is Terminated with Palmitoyl-ACP. - Finally, Palmitoyl Thioestrase Cleaves Thioester bond & Releases a Free Palmitate. 11  Fatty Acid Synthesis System Acetyl CoA Transacylase Fatty Acid Synthase Acetyl CoA Enzyme Malonyl CoA Malonyl CoA Transacylase Acetyl Malonyl Enzyme 3-Ketoacyl Synthase 3-KetoAcyl Enzyme nzyme Thioestrase 3-Ketoacyl ACP Reductase 3-HydroxyAcyl Enzyme nzyme 3-Hydroxyacyl ACP Hydratase ∆ 2-EnoylAcyl Enzyme nzyme ∆ 2-Enoylacyl ACP Reductase Palmitoyl Enzyme Butaryl Enzyme - Shorter FA are Important End Product in the Lactating Mammary Gland. - All the Carbons in Palmitic acid Have Passed through Malonyl CoA, - Except the 2 Donated by Original Acetyl CoA which are Found at Methyl (ω) End. - The Newly Synthesized Fatty acids are Prevented from Oxidation by Malonyl CoA. 12  Fatty Acid Synthesis System Fatty Acid Synthase Complex In Eukaryotes, - It is a Multi-Catalytic Enzyme Complex. - It is a Dimer formed of 2 Subunits. - The 2 Subunits are Identical, but Arranged Head to Tail, - Each Monomer contain 7 Different Enzymatic Domains & 1 Protein Domain. - Each Monomer contains 2 SH groups to Carry Acyl chain. - One SH is from Cysteine residue on 3-Ketoacyl synthase. - One SH is from 4-Phosphopantothein on ACP. - ACP is a Protein Domain that Covalently Binds to a 4-Phosphopantetheine. - A Derivative of Vit B5 Pantothenic acid Carries Acyl Units on its Terminal Thiol – SH. Biological Importance: - Synthesis of Fatty Acids, - Which is used to Make TAG, - Which Stores Energy of Excess Glucose. Medical Importance: - Critical Diseases of the Pathway Have Not been Reported in Humans. - Variations in Lipogenesis Activity may Affect the Nature & Extent of Obesity. - Type 1 (Insulin-Dependent) Diabetes Mellitus causes Inhibition of Lipogenesis. - Inhibition of Lipogenesis causes Inability to Gain Wight.. 13  Fatty Acid Elongation System Other Name: - Lipogenesis. Pathway: - Minor Pathway, Mitochondrial or Microsomal Elongation System. State: - It occurs After Meal, in Well-Fed State, Stimulated by Insulin. Definition: - It is Elongation of Saturated & Unsaturated F.A to from Long Chain FA for Sphingomyelin Synthesis. Site (Cell): - It Occurs in Mitochondria & Smooth Endoplasmic Reticulum (SER). - But the SER Microsomal System is the Dominant System. Substrate & Steps: Shorter Fatty Acids Saturated & Unsaturated Fatty Acyl CoAs (From C10 Upward) Carbon Donor Malonyl CoA as Acetyl Donor which adds Two Carbons Reducing Molecule NADPH+H Enzyme Catalyzed by Microsomal Fatty acid Elongase - Malonyl CoA Condenses with the Carbonyl group of the Fatty Acyl Residue - Fatty Acyl CoA is Elongated Using Reactions Similar but Not Identical to FA Synthase Complex. End Product: - Very Long Fatty acid C22 or C24. Biological Importance: - Elongation of Steroyl-CoA in Brain Increases Rapidly during Myelination. - Brain Provide C22 & C24 Fatty Acids for Sphingolipids (Myelination). Regulation: - Both Fasting & Low insulin levels Inhibit the Desaturation & Elongation system. 14  Microsomal Desaturation System - MUFA are Non-essential that Can be Synthesized by the body. - MUFA are Synthesized by Several Tissues including Liver. - MUFA are Synthesized from Saturated Fatty Acids. - MUFA are Synthesized by Microsomal ∆9 Desaturase System. - Which Requires: Cytochrome b5, Oxygen (O2) & NADPH & FAD Linked Reductase. - The First Double bond introduced into Saturated FA is Nearly Always in ∆9 Position. - Stearoyl-CoA (18:0) is the Major Substrate for Desaturation. Biosynthesis of Mono Unsaturated Fatty Acids (MUFA) ∆ 9 Desaturase Palmitoyl-CoA Pamitoleyl-CoA (16:0) (16:1 ∆9) ∆ 9 Desaturase Stearoyl-CoA Oleyl-CoA (18:0) (18:1 ∆9) Regulation: - Both Fasting & Low insulin levels Inhibit the Desaturation & Elongation system. - Microsomal = Endoplasmic Reticulum. - Since Animals have ∆9 Desaturase, - They are Able to Completely Synthesize the ω9 Oleic acid - By a Combination of Chain Elongation & Desaturation of Saturated Fatty Acids 15  Microsomal Desaturation System - Animal & Human Tissues Have Limited Capacity for Desaturating Fatty Acids. - And Require Certain Dietary Polyunsaturated Fatty Acids Derived from Plants. - Mammals Can Only Desaturate between the ∆ 9 Position & the Carboxyl End of Acyl Chain. - Humans have Carbon 4, 5, 6, 9 Desaturases, - But Humans Lack the Ability to Introduce Double bonds from Carbon 10 to the ω End. - Whereas Plants Can Desaturate at Positions ∆ 9, ∆ 12, ∆ 15. - For Mammals, Linoleic (ω6) & α-Linolenic (ω3) are Essential & Must be Supplied in the Diet. - These Two Fatty Acids are also Required to Make Other Members of ω6 & ω3 of PUFA. - Linoleic acid is used to Make γ-linolenic acid & Arachidonic acid. - α-Linolenic acid is used to Make DHA. - The Nutritional Requirement for Arachidonate is Dispensed with if There is Adequate Linoleate in the Diet. Biosynthesis of Polyunsaturated Fatty Acids (PUFA) Linoleoyl-CoA (18:2 ∆9, 12) ∆ 6 Desaturase γ-Linolenoyl-CoA (18:3 ∆6,9,12) Elongase Dihomo- γ-Linolenoyl-CoA (20:3 ∆8,11,14) ∆ 5 Desaturase Archidinoyl-CoA (20:4 ∆5,8,11,14) Regulation: - Both Fasting (Starving) & Low insulin levels Inhibit the Desaturation & Elongation system. Medical Importance: - Unsaturated FAs in Phospholipids of Cell Membrane are Important in Maintaining Membrane Fluidity. - Diets with a High P:S (Polyunsaturated : Saturated fatty acid) ratio is Beneficial - Because it Prevents Coronary Heart Disease. - Trans Unsaturated Fatty Acids are Implicated in Various Disorders. - Small Amounts of Trans UFAs are Found in Ruminant Fat E.g. Fat Butter has 2-7%. - These Fats Arise from the Action of Microorganisms in the Rumen. - But the Main Source in the Human diet is form partially Hydrogenated Vegetable Oils E.g. Margarine. - Trans Fatty Acids Compete with Essential FAs & May Exacerbate Essential FA Deficiency. - They are Structurally Similar to Saturated FAs & Promote Hypercholesterolemia & Atherosclerosis. 16 Fatty Acid Synthesis 17