24.3 Lipid Metabolism
3 min read•june 18, 2024
Lipid metabolism is crucial for energy production and storage in the body. It involves breaking down fats for fuel and creating new fats when energy is abundant. This process helps maintain energy balance and provides alternative fuel sources during fasting.
Lipids play diverse roles beyond energy, including hormone production and cell membrane structure. Understanding lipid metabolism sheds light on how the body adapts to different nutritional states and maintains overall health.
Lipid Metabolism
Energy extraction from fats
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- are the primary form of stored fat in the body composed of three attached to a backbone (e.g., saturated fats, unsaturated fats)
- breaks down into free fatty acids and glycerol in adipose tissue stimulated by hormones such as and (adrenaline)
- process breaks down fatty acids to generate energy in the of cells
- Fatty acids are converted into which enters the citric acid cycle (Krebs cycle)
- Each cycle of beta-oxidation produces and used in the electron transport chain to generate ATP (cellular energy currency)
Ketogenesis in energy production
- produces from fatty acids in the liver when glucose availability is limited during fasting or low-carbohydrate diets (ketogenic diets)
- Acetyl-CoA from beta-oxidation is converted into ketone bodies , , and
- Ketone bodies serve as an alternative energy source for tissues, especially the brain which cannot directly utilize fatty acids for energy
- Ketone bodies can cross the blood-brain barrier and provide energy for the brain during glucose scarcity (e.g., during prolonged fasting)
Ketone body utilization in tissues
- Ketone bodies are released from the liver and transported through the bloodstream to target tissues (e.g., brain, heart, skeletal muscle)
- In target tissues, ketone bodies are converted back into acetyl-CoA:
- Acetoacetate is converted into acetoacetyl-CoA by the enzyme (SCOT)
- Acetoacetyl-CoA is then split into two molecules of acetyl-CoA by the enzyme
- Acetyl-CoA enters the citric acid cycle to generate NADH and FADH2 for ATP production via the electron transport chain
- Ketone body utilization maintains energy homeostasis during periods of glucose scarcity by providing an alternative energy source for the brain and other tissues and sparing glucose for tissues that rely on it as their primary energy source
Lipogenesis process and regulation
- synthesizes fatty acids and triglycerides from excess carbohydrates primarily in the liver and adipose tissue to store excess energy as triglycerides for future use
- Key steps of :
- Acetyl-CoA is produced from excess glucose through glycolysis and the pyruvate dehydrogenase complex
- Acetyl-CoA is converted into malonyl-CoA by the enzyme (ACC)
- (FAS) catalyzes the formation of (a 16-carbon fatty acid) from malonyl-CoA and acetyl-CoA
- Palmitic acid undergoes elongation and desaturation to form various fatty acids (e.g., , )
- Fatty acids are esterified with glycerol to form triglycerides
- Regulation of lipogenesis:
- stimulates lipogenesis by activating ACC and FAS
- and inhibit lipogenesis by inactivating ACC and FAS
- Dietary factors such as high-carbohydrate intake can promote lipogenesis
Lipid Transport and Metabolism
- are complex particles that transport lipids through the bloodstream
- is a crucial component of cell membranes and serves as a precursor for steroid hormones
- , synthesized from cholesterol in the liver, aid in the digestion and absorption of dietary fats
- are essential components of cell membranes, contributing to membrane fluidity and function
Key Terms to Review (45)
Acetoacetate: Acetoacetate is a ketone body produced during the breakdown of fatty acids and certain amino acids when the body is in a state of starvation, uncontrolled diabetes, or other conditions that cause an imbalance in energy metabolism. It is an important intermediate in various metabolic pathways, particularly related to lipid and protein metabolism, as well as the overall metabolic states of the body.
Acetone: Acetone is a colorless, volatile liquid organic compound with the chemical formula C3H6O, commonly known as a ketone. In lipid metabolism, acetone is significant as it is one of the three main ketone bodies produced during the process of ketogenesis, which occurs when the body breaks down fatty acids for energy in situations of low carbohydrate availability, such as fasting or prolonged exercise.
Acetyl-CoA: Acetyl-CoA is a key molecule in cellular metabolism, serving as a central hub that connects the breakdown of carbohydrates, fats, and some amino acids to the production of energy through the citric acid cycle. It is the entry point for these nutrients into the final common pathway of cellular respiration.
Acetyl-CoA Carboxylase: Acetyl-CoA carboxylase is a key enzyme that catalyzes the conversion of acetyl-CoA to malonyl-CoA, a critical step in the regulation of lipid metabolism. This enzyme serves as a control point in the biosynthesis of fatty acids, playing a central role in the overall lipid metabolic pathways.
ATP (Adenosine Triphosphate): ATP, or adenosine triphosphate, is the primary energy currency of the cell. It is a high-energy molecule that stores and transports the chemical energy needed to power a wide variety of cellular processes, from muscle contraction to protein synthesis. ATP is central to the functions of human life, chemical bonds, chemical reactions, organic compounds, cellular organelles, protein synthesis, muscle contraction, respiration, metabolism, and fluid balance.
ATP synthase: ATP synthase is an enzyme complex embedded in the mitochondrial membrane that facilitates the synthesis of ATP (adenosine triphosphate), the primary energy carrier in cells, from ADP (adenosine diphosphate) and inorganic phosphate during the process of oxidative phosphorylation within carbohydrate metabolism. It acts as a molecular generator, converting an electrochemical gradient into energy stored in the form of ATP.
Beta (β)-hydroxybutyrate: Beta (β)-hydroxybutyrate is a type of ketone body produced by the liver from fatty acids during periods of low food intake, carbohydrate restrictive diets, prolonged intense exercise, or in untreated type 1 diabetes. It serves as an important energy source for the brain and muscles when glucose levels are low.
Beta (β)-oxidation: Beta (β)-oxidation is the metabolic process by which fatty acid molecules are broken down in the mitochondria and peroxisomes of cells to generate acetyl-CoA, NADH, and FADH₂. These products are then used to produce ATP, the energy currency of the cell, through further metabolic pathways.
Beta-Hydroxybutyrate: Beta-hydroxybutyrate (BHB) is a ketone body produced by the liver during periods of low carbohydrate availability or fasting. It serves as an alternative energy source for the body when glucose is scarce, playing a crucial role in lipid metabolism and the metabolic states of the body.
Beta-Oxidation: Beta-oxidation is the process by which fatty acids are broken down in the mitochondria of cells to generate acetyl-CoA, which can then enter the citric acid cycle to produce ATP, the primary energy currency of the cell. This catabolic pathway is a crucial component of lipid metabolism and overall energy production within the human body.
Bile Acids: Bile acids are a group of steroid-based compounds synthesized in the liver from cholesterol. They play a crucial role in the digestion and absorption of fats, as well as the regulation of lipid metabolism in the body.
Cholesterol: Cholesterol is a waxy, fat-like substance found in the body that serves important functions, such as aiding in the production of hormones, vitamin D, and bile acids. It is an essential organic compound that is vital for human functioning, but can also contribute to health issues if present in excess.
Chylomicrons: Chylomicrons are a type of lipoprotein that transports dietary lipids from the intestines to other locations in the body. They are essentially spherical particles that encapsulate triglycerides, cholesterol, and other fats absorbed from food, making them soluble in the blood and lymphatic system for transport.
Diacylglycerol (DAG): Diacylglycerol (DAG) is a lipid composed of two fatty acid chains covalently bonded to a glycerol molecule. It acts as a secondary messenger in various signaling pathways, particularly in the endocrine system where it influences hormone action and cellular responses.
Epinephrine: Epinephrine, also known as adrenaline, is a hormone and neurotransmitter produced by the adrenal glands that plays a crucial role in the body's fight-or-flight response by increasing heart rate, muscle strength, blood pressure, and sugar metabolism. It prepares the body for rapid action in situations perceived as stressful or dangerous.
Epinephrine: Epinephrine, also known as adrenaline, is a hormone and neurotransmitter produced by the adrenal glands. It plays a crucial role in the body's stress response, preparing the body to handle emergency situations by increasing heart rate, blood pressure, and blood sugar levels.
FADH2: FADH2, or flavin adenine dinucleotide, is a coenzyme that plays a crucial role in cellular respiration and energy production within the body. It is a key component of the electron transport chain, a series of reactions that generate the majority of the cell's energy in the form of ATP.
Fatty acid oxidation: Fatty acid oxidation is the metabolic process where fatty acids are broken down to produce energy. This occurs in the mitochondria of cells, primarily in the liver and muscle tissues.
Fatty acid synthase: Fatty acid synthase is a multi-enzyme complex that plays a crucial role in the biosynthesis of fatty acids, primarily converting acetyl-CoA and malonyl-CoA into long-chain fatty acids. This process is vital for lipid metabolism, as fatty acids serve as important energy sources and building blocks for various lipids in the body. Fatty acid synthase not only catalyzes the elongation of fatty acid chains but also facilitates the reduction and dehydration steps necessary to create fully saturated fatty acids.
Fatty Acids: Fatty acids are long-chain carboxylic acids that are the building blocks of lipids, a class of organic compounds essential for human functioning. They play a vital role in various physiological processes, including energy production, cell membrane structure, and hormone synthesis.
Glucagon: Glucagon is a hormone produced by the alpha cells in the pancreas that raises blood glucose levels by promoting the conversion of stored glycogen to glucose in the liver. It plays a critical role in glucose homeostasis, especially during periods of fasting or low blood sugar.
Glucagon: Glucagon is a hormone produced by the alpha cells of the pancreatic islets. As a key regulator of glucose metabolism, glucagon plays a crucial role in the endocrine system, carbohydrate metabolism, and overall metabolic states of the body.
Glycerol: Glycerol, also known as glycerin, is a simple sugar alcohol that is an essential component of lipids, particularly triglycerides, which are the main form of fat stored in the human body. Glycerol plays a crucial role in both the structure and metabolism of these important organic compounds.
Hydroxymethylglutaryl CoA (HMG CoA): Hydroxymethylglutaryl CoA (HMG CoA) is a molecule involved in the synthesis and degradation of ketone bodies and cholesterol within the body. It plays a crucial role in the metabolic pathway that converts acetyl-CoA to mevalonate, which is a precursor for cholesterol synthesis.
Insulin: Insulin is a hormone produced by the pancreas that plays a crucial role in regulating blood glucose levels and facilitating the metabolism of carbohydrates, fats, and proteins in the body. It is essential for maintaining homeostasis, supporting the functions of human life, and ensuring the proper utilization of organic compounds necessary for human functioning.
Insulin-like growth factors (IGFs): Insulin-like Growth Factors are proteins with a high similarity to insulin that play a crucial role in childhood growth and continue to have anabolic effects in adults. They are produced by the liver upon stimulation by growth hormone (GH) and act on various tissues, contributing to growth and development.
Ketogenesis: Ketogenesis is the metabolic process by which ketone bodies are produced from fatty acids in the liver, primarily during periods of fasting or low carbohydrate intake. This process plays a critical role in providing an alternative energy source for the brain and other tissues when glucose levels are low, linking lipid metabolism and energy production.
Ketone bodies: Ketone bodies are three water-soluble molecules (acetoacetate, beta-hydroxybutyrate, and acetone) that are produced by the liver from fatty acids during periods of low food intake (fasting), carbohydrate restrictive diets, starvation, or prolonged intense exercise. They serve as an alternative energy source for the brain and other tissues when glucose is not readily available.
Lipogenesis: Lipogenesis is the metabolic process by which acetyl-CoA is converted into fatty acids in the body, primarily occurring in the liver and adipose (fat) tissue. This process is critical for energy storage and cell membrane formation.
Lipogenesis: Lipogenesis is the metabolic process by which excess carbohydrates and proteins are converted into fat molecules, primarily triglycerides, for storage in adipose tissue. This process is crucial for maintaining energy balance and regulating lipid metabolism within the body.
Lipolysis: Lipolysis is the metabolic process by which triglycerides in fat cells are broken down into glycerol and free fatty acids, providing energy for the body. It occurs in the adipose tissue and is regulated by hormones such as insulin and adrenaline.
Lipolysis: Lipolysis is the process of breaking down stored fat, or triglycerides, into free fatty acids and glycerol. This process is crucial for providing the body with an alternative energy source when glucose is not readily available, and it occurs primarily in adipose tissue under the regulation of the autonomic nervous system and endocrine hormones.
Lipoproteins: Lipoproteins are complex molecules composed of lipids and proteins that transport lipids, such as cholesterol and triglycerides, through the bloodstream. They play a crucial role in the metabolism and transport of fats within the body.
Mitochondria: Mitochondria are double-membrane-bound organelles found in the cytoplasm of eukaryotic cells, often referred to as the powerhouse of the cell due to their role in producing adenosine triphosphate (ATP) through cellular respiration. These organelles play a crucial role in energy metabolism, converting nutrients into usable energy while also being involved in other important cellular functions such as apoptosis and calcium homeostasis.
Monoglyceride molecules: Monoglyceride molecules are a type of fat molecule consisting of a single fatty acid chain attached to a glycerol backbone. They are important intermediates in the digestion and absorption of dietary fats in the body.
NADH: NADH, or nicotinamide adenine dinucleotide (reduced), is a coenzyme that plays a crucial role in cellular metabolism. It is the reduced form of NAD+, which is an essential cofactor in numerous metabolic reactions, including those involved in energy production, carbohydrate and lipid metabolism, and cellular signaling.
Oleic Acid: Oleic acid is a monounsaturated fatty acid that is a common component of various plant and animal fats and oils. It is an important molecule in the context of lipid metabolism, playing a role in energy storage and cell membrane structure.
Palmitic Acid: Palmitic acid is a saturated fatty acid that is commonly found in animal and plant fats. It is the most abundant saturated fatty acid in the human body and plays a crucial role in lipid metabolism.
Pancreatic lipases: Pancreatic lipases are enzymes produced by the pancreas that break down dietary fats into smaller molecules, such as fatty acids and glycerol, so they can be absorbed by the intestines. They play a crucial role in the digestive process by enabling the body to utilize fat from foods.
Phospholipids: Phospholipids are a class of lipids that are essential components of cell membranes, playing a crucial role in maintaining the structure and function of cells. They are amphipathic molecules, containing both hydrophilic (water-loving) and hydrophobic (water-fearing) regions, which allows them to form the characteristic bilayer structure of cell membranes.
Stearic acid: Stearic acid is a saturated fatty acid with the chemical formula C18H36O2, commonly found in animal and plant fats. It plays a vital role in lipid metabolism as it is a significant component of triglycerides and phospholipids, contributing to cellular structure and energy storage.
Succinyl-CoA:3-ketoacid CoA transferase: Succinyl-CoA:3-ketoacid CoA transferase is an enzyme that plays a crucial role in the metabolism of fatty acids and ketone bodies. It catalyzes the transfer of the CoA group from succinyl-CoA to acetoacetate, an important step in the breakdown and utilization of ketone bodies as an alternative energy source when glucose is scarce.
Thiolase: Thiolase is an enzyme that plays a crucial role in lipid metabolism, specifically in the breakdown and synthesis of fatty acids. It catalyzes the reversible cleavage of carbon-carbon bonds in long-chain fatty acid molecules, enabling the degradation and subsequent energy production from these lipids.
Triglycerides: Triglycerides are a type of fat found in the blood, used by the body as a major source of energy and stored in fat cells for later use. They are formed from three fatty acid molecules attached to a glycerol backbone.
Triglycerides: Triglycerides are a type of lipid, or fat, that are the main form of fat stored in the body. They are composed of three fatty acid molecules attached to a glycerol backbone and serve as an important energy source for the body. Triglycerides are essential for human life, as they are involved in various physiological processes related to organic compounds, connective tissue, digestion, and lipid metabolism.