AI Beta-Oxidation of FA + Ketogenesis PDF

05 Beta-Oxidation of FA Ketogenesis.pdf 1) β-Oxidation and ATP Production Overview of β-Oxidation β-Oxidation is the metabolic process through which fatty acids are broken down to generate acetyl-CoA, which can then enter the citric acid cycle (TCA cycle) to produce ATP. This process is crucial fo...

05 Beta-Oxidation of FA Ketogenesis.pdf 1) β-Oxidation and ATP Production Overview of β-Oxidation β-Oxidation is the metabolic process through which fatty acids are broken down to generate acetyl-CoA, which can then enter the citric acid cycle (TCA cycle) to produce ATP. This process is crucial for energy production, especially during periods of fasting or low carbohydrate intake. Fatty acids must first be activated by linking them to coenzyme A (CoA) in a reaction that requires ATP. This activated form, fatty acyl-CoA, is then transported into the mitochondria for oxidation, primarily through the carnitine shuttle for long-chain fatty acids Page 1. Enzymatic Steps of β-Oxidation The β-oxidation of fatty acyl-CoA involves a series of four enzymatic steps that repeat until the fatty acid is completely oxidized: 1. Formation of Enoyl-CoA Fatty acyl-CoA is converted to trans-Δ²-enoyl-CoA by the enzyme acyl-CoA dehydrogenase. This step uses FAD, producing FADH2, which is linked to the electron transport chain (ETC). 2. Hydration to 3-L-Hydroxyacyl-CoA Trans-Δ²-enoyl-CoA is then hydrated to form 3-L-hydroxyacyl-CoA by the enzyme enoyl-CoA hydratase, utilizing water. 3. Oxidation to β-Ketoacyl-CoA 3-L-hydroxyacyl-CoA is oxidized to β-ketoacyl-CoA by 3-L-hydroxyacyl-CoA dehydrogenase, using NAD+ and producing NADH + H+. 4. Cleavage to Acetyl-CoA Finally, β-ketoacyl-CoA is cleaved by β-ketoacyl-CoA thiolase, using CoA-SH, to yield acetyl-CoA and a fatty acyl-CoA that is two carbons shorter Page 6. This cycle continues until the entire fatty acid chain is converted into acetyl-CoA units. Energy Yield from β-Oxidation The complete β-oxidation of fatty acids results in a significant yield of ATP. For example: Palmitic Acid (C16:0): ○ Produces 8 acetyl-CoA, 7 FADH2, and 7 NADH. ○ ATP yield calculation: 8 acetyl-CoA x 10 ATP = 80 ATP 7 FADH2 x 1.5 ATP = 10.5 ATP 7 NADH x 2.5 ATP = 17.5 ATP Total = 80 + 10.5 + 17.5 - 2 (for activation) = 106 ATP. Stearic Acid (C18:0): ○ Produces 9 acetyl-CoA, 8 FADH2, and 8 NADH. ○ ATP yield calculation: 9 acetyl-CoA x 10 ATP = 90 ATP 8 FADH2 x 1.5 ATP = 12 ATP 8 NADH x 2.5 ATP = 20 ATP Total = 90 + 12 + 20 - 2 (for activation) = 120 ATP. The high energy yield from β-oxidation is due to the highly reduced state of carbon in fatty acids, making them a rich source of energy Page 7. 2) Fatty Acid Absorption and Transport Absorption of Fatty Acids Short-chain fatty acids (SCFAs) and medium-chain fatty acids (MCFAs) are absorbed directly into the bloodstream via the portal vein and transported to the liver.  SCFAs: 2-4 carbon atoms MCFAs: 4-12 carbon atoms In contrast, long-chain fatty acids (LCFAs), which have 12-20 carbon atoms, are not absorbed directly into the bloodstream. Instead, they circulate as chylomicrons through the lymphatic system and peripheral circulation. Transport Mechanisms Across Plasma Membranes The transport mechanisms for fatty acids across plasma membranes differ based on the chain length: Short-Chain and Medium-Chain Fatty Acids SCFAs and MCFAs can simply diffuse across the plasma membrane without the need for transport proteins. Long-Chain Fatty Acids LCFAs require specific membrane transporters to facilitate their movement across the membrane. The key transporters include: ○ FA translocase (CD36) ○ FA transport protein F (FATP5) This diagram illustrates the differences in transport mechanisms for SCFAs, MCFAs, and LCFAs across the plasma membrane. Overview of Fatty Acid Absorption and Transport The overall process of fatty acid absorption and transport can be summarized as follows: 1. Dietary fats are emulsified in the digestive system, forming micelles that facilitate absorption by intestinal epithelial cells. 2. SCFAs and MCFAs are absorbed directly into the portal circulation, while LCFAs are packaged into chylomicrons. 3. Chylomicrons enter the lymphatic system and eventually reach the bloodstream, where they deliver fatty acids to various tissues. This diagram provides a detailed overview of the absorption and transport pathways for dietary fats and cholesterol in the human body. 3) Formation of Acyl-CoA Acyl-CoA Synthetase Overview Acyl-CoA synthetase is a crucial enzyme involved in the activation of fatty acids for subsequent metabolic processes. It catalyzes the formation of Acyl-CoA from free fatty acids, which is an essential step for fatty acid oxidation. The reaction can be summarized as follows: 1. Fatty Acid + ATP → Acyl-Adenylate (Mixed Anhydride Intermediate) + AMP + PPi 2. Acyl-Adenylate + CoA → Acyl-CoA + AMP This process is energetically expensive, requiring the hydrolysis of ATP to drive the reaction forward. Mechanism of Acyl-CoA Formation The formation of Acyl-CoA involves several key steps: Step 1: Activation of Fatty Acid The fatty acid (R-COOH) reacts with ATP, leading to the formation of an acyladenylate intermediate. This intermediate is a mixed anhydride where the fatty acid is linked to adenosine monophosphate (AMP). Step 2: Hydrolysis of Inorganic Pyrophosphate (PPi)  The reaction is driven forward by the hydrolysis of inorganic pyrophosphate (PPi) into two inorganic phosphates (2Pi) by the enzyme inorganic pyrophosphatase. This step is crucial as it helps to push the reaction towards the formation of Acyl-CoA. Step 3: Formation of Acyl-CoA Finally, coenzyme A (HS-CoA) is added to the acyladenylate, resulting in the formation of Acyl-CoA and the release of AMP. This diagram illustrates the entire reaction pathway, highlighting the energetically expensive nature of the process, which requires both ATP hydrolysis and the subsequent hydrolysis of PPi to effectively drive the reaction forward. The formation of Acyl-CoA is essential for the activation of fatty acids, allowing them to undergo β-oxidation for energy production. Importance of Acyl-CoA Acyl-CoA plays a vital role in fatty acid metabolism. It is necessary for: Fatty Acid Oxidation: Acyl-CoA is the activated form of fatty acids that can enter the β-oxidation pathway, leading to ATP production. Synthesis of Lipids: Acyl-CoA is also a precursor for the synthesis of various lipids, including phospholipids and triglycerides. In summary, the formation of Acyl-CoA from fatty acids via the action of Acyl-CoA synthetase is a critical step in both the catabolism and anabolism of fatty acids, underscoring its importance in cellular metabolism. 4) Carnitine Shuttle Mechanism Overview of the Carnitine Shuttle Mechanism The carnitine shuttle is a crucial mechanism for transporting long-chain fatty acids (LCFAs) into the mitochondria for β-oxidation. This process involves several key enzymes and steps: 1. Carnitine + LCFA-CoA is converted to Acylcarnitine via the enzyme Carnitine palmitoyl transferase - 1 (CPT-1), which is located on the outer mitochondrial membrane.  2. The Acylcarnitine then passes through the inner mitochondrial membrane via Carnitine-acyl carnitine translocase (CACT). CACT functions as an antiporter, allowing acylcarnitine to enter the mitochondria while transporting carnitine out into the cytosol. 3. Inside the mitochondria, Acylcarnitine is converted back to LCFA-CoA by the enzyme Carnitine palmitoyl transferase - 2 (CPT-2), which is located on the inner mitochondrial membrane. This reaction releases carnitine, which is then transported back to the cytosol via CACT. 4. The LCFA-CoA is now ready to undergo β-oxidation for energy production. This shuttle mechanism is essential because LCFAs cannot cross the mitochondrial membrane in their CoA-activated form. Key Enzymes in the Carnitine Shuttle The carnitine shuttle involves three main enzymes: 1. Carnitine Palmitoyl Transferase - 1 (CPT-1) Location: Outer mitochondrial membrane Function: Converts LCFA-CoA to Acylcarnitine. 2. Carnitine-Acyl Carnitine Translocase (CACT) Location: Inner mitochondrial membrane Function: Transports Acylcarnitine into the mitochondria while moving carnitine out. 3. Carnitine Palmitoyl Transferase - 2 (CPT-2)  Location: Inner mitochondrial membrane Function: Converts Acylcarnitine back to LCFA-CoA, allowing it to enter the β-oxidation pathway. Importance of Carnitine in Fatty Acid Metabolism Carnitine plays a vital role in the transport of fatty acids into the mitochondria, which is essential for: Energy Production: By facilitating the entry of LCFAs into the mitochondria, carnitine enables their oxidation and subsequent ATP production. Regulation of Metabolism: The carnitine shuttle helps regulate the availability of fatty acids for energy, especially during periods of fasting or increased energy demand. The biosynthesis of carnitine involves several enzymatic reactions, starting from Nε-trimethyllysine (TML) and culminating in the formation of L-carnitine, which is distributed across various tissues, including the brain, liver, and kidneys. Understanding the carnitine shuttle mechanism is crucial for comprehending how fatty acids are utilized for energy in the body. 5) Energy Yield Calculations Energy Yield from Palmitic Acid To calculate the energy yield from the β-oxidation of palmitic acid (16:0), we need to consider the following: Palmitic acid produces: ○ 8 acetyl-CoA ○ 7 FADH2 ○ 7 NADH The ATP yield from these products can be calculated as follows: 1 acetyl-CoA = 10 ATP (via TCA cycle) 1 FADH2 = 1.5 ATP 1 NADH = 2.5 ATP However, we must also account for the 2 ATP used for the activation of free fatty acids (via acyl-CoA synthetase). Calculation: The total ATP yield can be calculated using the formula: (8×10)+(7×1.5)+(7×2.5)−2=106(8×10)+(7×1.5)+(7×2.5)−2=106 Thus, the complete β-oxidation of one palmitic acid yields 106 ATP. This high energy yield is due to the highly reduced state of carbon in fatty acids, making them a rich source of energy. Energy Yield from Stearic Acid Next, let's calculate the energy yield from the β-oxidation of stearic acid (18:0). Stearic acid produces: ○ 9 acetyl-CoA ○ 8 FADH2 ○ 8 NADH Using the same ATP yield values: Calculation: The total ATP yield can be calculated as: (9×10)+(8×1.5)+(8×2.5)−2=120(9×10)+(8×1.5)+(8×2.5)−2=120 Thus, the complete β-oxidation of one stearic acid yields 120 ATP. It is important to remember that the initial 2 ATP are used for the activation of the fatty acid, which is why the final yield is 120 ATP instead of 122 ATP. 6) Unsaturated Fatty Acid Catabolism Overview of Unsaturated Fatty Acid Catabolism Unsaturated fatty acids contain one or more double bonds in their carbon chains, which can complicate their catabolism during the process of beta-oxidation. For example, linoleic acid, which has two double bonds, can undergo two normal rounds of beta-oxidation before the presence of double bonds begins to pose challenges. Challenges in Catabolism The catabolism of unsaturated fatty acids presents several challenges: 1. Presence of Double Bonds: The double bonds in unsaturated fatty acids can interfere with the normal beta-oxidation pathway. 2. Formation of 4,5-Double Bonds: During the breakdown of unsaturated fatty acids, the formation of a 4,5-double bond can occur, which is not a substrate for the typical enzymes involved in beta-oxidation. 3. Isomerization Requirement: To continue the beta-oxidation process, isomerization of the double bonds is often necessary, which requires additional enzymatic steps. Pathway of Unsaturated Fatty Acid Catabolism The catabolism of unsaturated fatty acids, such as linoleic acid, can be illustrated in a flowchart format: In this pathway: Linoleic acid undergoes two rounds of beta-oxidation, producing 2 NADH, 2 FADH2, and 2 acetyl-CoA. The pathway highlights the three main problems encountered: ○ Problem 1: Caused by the presence of the double bond. ○ Problem 2: Involves the formation of a 4,5-double bond. ○ Problem 3: Requires isomerization to continue the beta-oxidation process. Enzymes such as acyl-CoA dehydrogenase, 3,2-enoyl-CoA isomerase, and 2,4-dienoyl-CoA reductase play crucial roles in facilitating these reactions, allowing the catabolism of unsaturated fatty acids to proceed despite the challenges posed by their double bonds. 7) Peroxisomal β-Oxidation Overview of Peroxisomal β-Oxidation Peroxisomes are specialized organelles involved in the metabolism of very long-chain fatty acids (VLCFAs), which are defined as fatty acids with more than 20 carbon atoms. In peroxisomes, the β-oxidation of these VLCFAs occurs without the need for the carnitine shuttle, which is typically required for the transport of long-chain fatty acids into mitochondria. Instead, a direct transport system brings VLCFAs into the lumen of the peroxisome for oxidation. Mechanism of β-Oxidation in Peroxisomes The β-oxidation pathway in peroxisomes follows a similar sequence of reactions as in mitochondria, but with some key differences: The first reaction is catalyzed by a peroxisome-specific acyl-CoA dehydrogenase. This enzyme is responsible for the initial oxidation of the acyl-CoA molecule. The process involves the conversion of acyl-CoA to trans-enoyl-CoA, followed by hydration, further oxidation, and thiolysis, ultimately leading to the production of shorter-chain acyl-CoA molecules. Role of Catalase An important aspect of peroxisomal function is the presence of catalase, an enzyme that decomposes hydrogen peroxide (H2O2), a byproduct of fatty acid oxidation. The reaction catalyzed by catalase converts hydrogen peroxide into water (H2O) and oxygen (1/2 O2), thus preventing the accumulation of this potentially harmful compound within the peroxisome. Additional Functions of Peroxisomes In addition to β-oxidation, peroxisomes are involved in the synthesis of bile acids and other lipid-related processes. They are surrounded by the endoplasmic reticulum (ER) and are located close to the Golgi apparatus, highlighting their role in lipid metabolism and cellular function. Visual Representation of Peroxisomal β-Oxidation To better understand the process, refer to the following biochemical diagram illustrating the β-oxidation of very long-chain fatty acids within peroxisomes: This diagram shows the conversion of acyl-CoA molecules through the action of acyl-CoA dehydrogenase enzymes, emphasizing the unique aspects of peroxisomal β-oxidation. 8) Odd-Chain Fatty Acid Catabolism Overview of Odd-Chain Fatty Acid Catabolism Odd-chain fatty acids are unique in that their catabolism results in the production of propionyl-CoA, a 3-carbon molecule. This process is crucial for integrating odd-chain fatty acid metabolism into the TCA cycle. Conversion of Propionyl-CoA to Succinyl-CoA The conversion of propionyl-CoA to succinyl-CoA involves three key enzymatic steps: 1. Propionyl-CoA Carboxylase Reaction: Propionyl-CoA is converted to (S)-methylmalonyl-CoA. Requirements: This reaction requires ATP and CO2. 2. Methylmalonyl-CoA Racemase Reaction: The (S)-methylmalonyl-CoA is converted to (R)-methylmalonyl-CoA. 3. Methylmalonyl-CoA Mutase Reaction: The (R)-methylmalonyl-CoA is transformed into succinyl-CoA. Requirements: This step requires vitamin B12 in the form of 5'-deoxyadenosylcobalamin. Note: The mechanism is similar to that of heme rings, but it utilizes Co3+ instead of Fe2+. The overall reaction can be summarized as follows: Propionyl-CoA → (S)-methylmalonyl-CoA → (R)-methylmalonyl-CoA → Succinyl-CoA This flowchart illustrates the conversion of propionyl-CoA to succinyl-CoA and its subsequent processing. Integration into the TCA Cycle Succinyl-CoA is not directly consumed by the TCA cycle. Instead, it undergoes further transformations: Succinyl-CoA is converted into malate with the involvement of NADP+. Malate can then be converted into pyruvate, releasing CO2 in the process. This pathway allows for the integration of odd-chain fatty acid catabolism into the TCA cycle, ultimately contributing to energy production. 9) Ketogenesis in the Liver Overview of Ketogenesis Ketogenesis is a metabolic process that occurs in the liver and serves as a crucial source of fuel for certain tissues during periods of low carbohydrate availability, such as fasting or starvation. During ketogenesis, acetyl-CoA derived from the β-oxidation of fatty acids is converted into ketone bodies, which include: Acetone Acetoacetate β-Hydroxybutyrate This process is particularly important during absolute starvation, as the liver utilizes oxaloacetate for gluconeogenesis, preventing acetyl-CoA from being converted into glucose. This is a key point to remember, as acetyl-CoA cannot be transformed into glucose directly. Steps of Ketogenesis The process of ketogenesis involves several key steps: Step 1: Formation of Acetoacetyl-CoA 1. Two molecules of Acetyl-CoA combine to form Acetoacetyl-CoA. Step 2: Conversion to HMG-CoA 1. HMG-CoA Synthase catalyzes the conversion of Acetoacetyl-CoA into HMG-CoA (β-Hydroxy-β-methylglutaryl-CoA) by adding another Acetyl-CoA and releasing water (H2O). Step 3: Formation of Acetoacetate 1. HMG-CoA Lyase then removes one Acetyl-CoA from HMG-CoA to produce Acetoacetate. Step 4: Conversion of Acetoacetate From Acetoacetate, two possible pathways can occur: 1. Decarboxylation of Acetoacetate results in the formation of Acetone. 2. Reduction of Acetoacetate using NADH leads to the production of β-Hydroxybutyrate. Visual Representation of Ketogenesis To better understand the process of ketogenesis, refer to the following biochemical diagrams: This diagram illustrates the initial steps of ketogenesis, showing how two molecules of Acetyl-CoA combine to form Acetoacetyl-CoA and the subsequent conversion to HMG-CoA. This diagram outlines the conversion of Acetoacetyl-CoA to Acetoacetate and its further transformation into Acetone and β-Hydroxybutyrate. 10) Ketolysis in Peripheral Tissues Ketolysis in Peripheral Tissues Ketolysis is the process by which ketone bodies are utilized as an energy source in peripheral tissues such as the brain, heart, and muscles. During prolonged fasting or in states of ketosis, ketone bodies become a significant fuel source, especially for the brain, which can utilize up to 60% of its energy from ketone bodies during these times. Mechanism of Ketolysis The process of ketolysis involves the conversion of ketone bodies back into acetyl-CoA, which can then enter the citric acid cycle for ATP production. The key steps in ketolysis are as follows: 1. Conversion of D-β-Hydroxybutyrate to Acetoacetate: This reaction is catalyzed by the enzyme β-hydroxybutyrate dehydrogenase, producing NADH and H⁺ in the process. 2. Conversion of Acetoacetate to Acetoacetyl-CoA: This step involves the enzyme 3-ketoacyl-CoA transferase, which uses Succinyl-CoA as a cofactor. This enzyme is crucial for the ketolysis process and is absent in the liver. 3. Splitting of Acetoacetyl-CoA into Two Acetyl-CoA Molecules: The enzyme thiolase catalyzes this reaction, allowing the acetyl-CoA to enter the citric acid cycle for energy production. Why the Liver Cannot Perform Ketolysis The liver is unable to undergo ketolysis due to the absence of the enzyme 3-ketoacyl-CoA transferase. This enzyme is essential for the conversion of acetoacetate to acetoacetyl-CoA, which is a critical step in the ketolysis pathway. Instead, the liver primarily focuses on ketogenesis, the process of producing ketone bodies from fatty acids, especially during periods of low carbohydrate availability. In summary, while peripheral tissues can efficiently utilize ketone bodies for energy through ketolysis, the liver lacks the necessary enzymatic machinery to perform this process, thus playing a different role in energy metabolism. 11) Hormonal Regulation of β-Oxidation Hormonal Regulation of β-Oxidation The regulation of β-oxidation is significantly influenced by hormones, particularly glucagon and epinephrine. These hormones are released when the body is low on fuel, signaling the need to mobilize energy stores. Role of Glucagon and Epinephrine 1. Activation of Hormone-Sensitive Lipase: ○ Glucagon and epinephrine activate hormone-sensitive lipase in adipose tissues. This enzyme is crucial for breaking down triglycerides into free fatty acids, which can then enter the bloodstream. 2. Proteolysis Activation: ○ In addition to lipolysis, these hormones also stimulate proteolysis in muscle tissues, providing amino acids that can be used for energy or gluconeogenesis.  3. Gluconeogenesis in the Liver: ○ The liver responds to glucagon and epinephrine by activating gluconeogenesis, which is the process of generating glucose from non-carbohydrate sources. The ATP produced during this process is essential for energy metabolism. 4. Production of Ketone Bodies: ○ The breakdown of fatty acids leads to the formation of Acetyl-CoA, which can be converted into ketone bodies. This is particularly important during periods of fasting or low carbohydrate intake, as ketone bodies serve as an alternative energy source for various tissues, including the brain. Insulin's Role It is important to note that the effects of glucagon and epinephrine are reversed by insulin. Insulin promotes the storage of energy and inhibits the processes activated by glucagon and epinephrine, thus maintaining energy homeostasis. Visual Representation The following flowchart illustrates the process of β-oxidation activation in the context of hormonal control: This flowchart outlines the steps from triglyceride breakdown in adipose tissue to the production of ATP and ketone bodies in the liver mitochondria, emphasizing the role of glucagon and epinephrine in these processes. Summary of Key Points Glucagon and epinephrine are key hormones that activate β-oxidation by promoting lipolysis and gluconeogenesis. They lead to the production of free fatty acids and Acetyl-CoA, which are essential for energy production and ketogenesis. The actions of these hormones are counteracted by insulin, which promotes energy storage.