Lipid Metabolism Study Notes
Lipid Metabolism - Outline
II. Lipid Metabolism I & II: Biosynthesis of Fatty Acids and Triacylglycerols
Different Plasma/Tissue Lipids:
Free Fatty Acids (FFA), Monoglycerides (MAG), Diglycerides (DAG)
Triacylglycerols (TAG)
Cholesterol and Cholesterol Esters
Phospholipids and Sphingolipids
Other Intermediates
Sources of Plasma/Tissue Lipids:
Dietary lipids
Lipid synthesis in the body
Adipose tissue storage
Major Lipids in the Body:
Fats and oils: Triglycerides formed from the esterification of glycerol with three fatty acids.
Stored in adipocytes that make up adipose tissue.
Can originate from dietary intake, fat stores, or be synthesized primarily from excess carbohydrates and proteins.
III. Biological Roles of Lipids
Energy Source:
Triacylglycerols serve as an efficient energy store (9 kcal/g compared to 4 kcal/g for carbohydrates/proteins).
Hydration difference: 1 g water per g of fat vs. 4 g per g of carbohydrate or protein.
Structural Components:
Membrane formation via phospholipids and cholesterol.
Cofactors for Metabolism:
Vitamins and signaling molecules derived from lipids such as fatty acids, DAG, and sterols.
Hormones:
Lipid-derived steroid hormones include sex hormones and adrenocortical hormones.
IV. Fatty Acids
Predominantly composed of hydrogen and carbon with a carboxylic acid group.
Classification:
Saturated (single bonds) vs. Unsaturated (one or more double bonds).
Liquid at room temperature if unsaturated.
Lipogenesis: The process of synthesizing fatty acids from nonlipid precursors, primarily in the liver and adipose tissues, which requires:
Acetyl-CoA
NADPH
V. Lipogenesis – Pathways
Primary Requirement: Acetyl-CoA, NADPH, along with ATP and biotin in the cytosol.
Citrate Shuttle: Facilitates transport of acetyl-CoA from mitochondria to cytosol.
Key Metabolites involved:
Malate, Pyruvate, Oxaloacetate.
Enzyme Mechanisms:
Acetyl-CoA Carboxylase (ACC): Initial and controlling step in fatty acid synthesis.
Reaction Steps:
Carboxylation of biotin
Transfer of carboxyl group.
VI. Fatty Acid Synthase Complex
A single gene coding for seven enzyme activities, present in dimer forms.
Fatty Acid Synthesis Steps:
Reduction and addition of hydrogen from NADPH.
Removal of double bonds.
Cleavage into two-carbon units.
VII. Lipogenesis Cycle
1st Cycle: Requires Acetyl-CoA and Malonyl-CoA to form acyl chains.
Each cycle adds two carbons, utilizing NADPH.
VIII. Regulation of Lipogenesis
Hormonal Influence:
Insulin stimulates lipogenesis; glucagon has the opposite effect.
Enzymatic Regulation:
Through ACC activity, which is influenced by energy levels and substrate availability.
IX. Triacylglycerol Synthesis
Why TAGs Need to be Synthesized: Control free fatty acid levels, preventing excess accumulation in circulation.
Location of Synthesis:
Major sites include adipose tissue and the liver.*
Storage:
Stored primarily in adipose tissue.
X. Fatty Acid Activation
Process: Fatty Acids must be activated to Fatty Acyl-CoA before attaching to a glycerol backbone.
Key Enzyme: Fatty Acyl-CoA Synthetase.
Sources of Glycerol-3-Phosphate:
Reduction from dihydroxyacetone phosphate; phosphorylation (in liver).
XI. Lipoproteins
Functions: Transport lipids in the circulatory system.
Chylomicrons, VLDL, LDL, HDL classifications.
XII. Regulation of Lipoprotein Metabolism
Dietary and genetic factors: Affect both triacylglycerol and HDL levels in the plasma.
Clinical Relevance: Elevated levels of lipids linked to metabolic diseases.
Significance of Regulation
Understanding lipoprotein metabolism implications for conditions like obesity and diabetes.
XIII. Key Clinical Conditions
Dyslipidemia: Strong correlation with obesity, diabetes, cardiovascular diseases.
Fatty Liver Disease: Caused by excessive triglyceride synthesis or impaired lipoprotein export.
Recommended Readings
Stipanuk’s 4th Edition
Stipanuk’s Chapter 13, Chapter 14, Chapter 21, Chapter 24, Chapter 26
Harper’s Biochemistry chapters relevant for fatty acids and cholesterol metabolism.
I. Overview
Topics Covered:
Lipid Metabolism I & II: Focuses on the fundamental processes of lipid synthesis.
Biosynthesis of Fatty Acids: The creation of fatty acids from simpler precursors.
Biosynthesis of Triacylglycerols: The synthesis of storage lipids.
Lipid Metabolism III & IV: Explores the breakdown of lipids for energy.
Fatty Acid Oxidation: The catabolic pathway for fatty acids (beta-oxidation).
Ketogenesis: Production of ketone bodies during prolonged fasting or starvation.
Lipid Metabolism V & VI: Covers structural lipids and their transport.
Cholesterol Synthesis: The complex pathway for cholesterol production.
Lipoproteins: Particles responsible for transporting lipids in the bloodstream.
Lipid Metabolism VII: Addresses the control mechanisms for lipid transport and clinical implications.
Regulation of Lipoprotein Metabolism: How the body controls lipoprotein levels.
Clinical Relevance: Discusses the impact of lipid metabolism on health and disease.
II. Lipid Metabolism I & II: Biosynthesis of Fatty Acids and Triacylglycerols
Different Plasma/Tissue Lipids: These lipids circulate in the blood or are stored in tissues, each with distinct roles.
Free Fatty Acids (FFA): Unesterified fatty acids, important energy substrates, transported by albumin.
Monoglycerides (MAG), Diglycerides (DAG): Intermediates in triacylglycerol metabolism and signaling molecules.
Triacylglycerols (TAG): The primary form of energy storage, composed of a glycerol backbone esterified to three fatty acids.
Cholesterol and Cholesterol Esters: Essential components of cell membranes (cholesterol) and transported/stored in esterified form.
Phospholipids and Sphingolipids: Major structural components of biological membranes, involved in signaling.
Other Intermediates: Various lipid precursors and breakdown products involved in metabolic pathways.
Sources of Plasma/Tissue Lipids:
Dietary lipids: Absorbed from the gut, primarily as triacylglycerols, cholesterol, and phospholipids.
Lipid synthesis in the body: De novo synthesis from non-lipid precursors like carbohydrates and proteins, mainly in the liver and adipose tissue.
Adipose tissue storage: Mobilization of stored triacylglycerols from adipocytes to supply fatty acids to other tissues.
Major Lipids in the Body:
Fats and oils: Primarily triacylglycerols, formed from the esterification of one glycerol molecule with three fatty acid molecules.
These are stored predominantly in adipocytes, which make up adipose tissue, serving as the body's largest energy reserve.
Can originate from dietary intake, mobilization from existing fat stores, or be synthesized primarily from excess carbohydrates and proteins when energy intake exceeds expenditure.
III. Biological Roles of Lipids
Energy Source:
Triacylglycerols serve as a highly efficient and concentrated energy store, yielding about 9\ \text{kcal/g} upon oxidation, significantly more than carbohydrates or proteins (4\ \text{kcal/g}).
This high energy density is partly due to their low hydration: 1\ \text{g} of water is associated with 1\ \text{g} of fat, compared to 4\ \text{g} of water per 1\ \text{g} of carbohydrate or protein, making fat a much lighter storage form.
Structural Components:
Lipids are fundamental for membrane formation, with phospholipids forming the bilayer and cholesterol modulating membrane fluidity and stability in eukaryotic cells.
Cofactors for Metabolism:
Lipids and their derivatives can function as cofactors for enzymes or as signaling molecules.
Examples include fat-soluble vitamins (A, D, E, K), phosphatidylinositol-derived signaling molecules, diacylglycerol (DAG) as a second messenger, and sterols as precursors for other steroid compounds.
Hormones:
Lipid-derived steroid hormones, synthesized from cholesterol, include crucial regulatory molecules such as sex hormones (e.g., estrogen, testosterone) and adrenocortical hormones (e.g., cortisol, aldosterone) that regulate a wide range of physiological processes.
IV. Fatty Acids
Fatty acids are long hydrocarbon chains predominantly composed of hydrogen and carbon atoms, terminating with a carboxylic acid group (-\text{COOH}). This makes them amphipathic molecules.
Classification:
Saturated fatty acids: Contain only single bonds between carbon atoms in their hydrocarbon chain, leading to straighter, more rigid structures that pack tightly. They are typically solid at room temperature.
Unsaturated fatty acids: Possess one or more double bonds ($\text{C}=\text{C}$) in their hydrocarbon chain. Double bonds introduce kinks, preventing tight packing, which makes them generally liquid at room temperature. Monounsaturated fatty acids (MUFAs) have one double bond, while polyunsaturated fatty acids (PUFAs) have two or more.
Lipogenesis: The metabolic process of synthesizing fatty acids from nonlipid precursors, mainly excess carbohydrates and proteins. This process occurs primarily in the cytosol of liver and adipose tissues, and lactating mammary glands, and requires two key inputs:
Acetyl-CoA: The building block for the carbon backbone of fatty acids.
NADPH: The primary reducing agent, providing the electrons necessary for synthesis.
V. Lipogenesis – Pathways
Primary Requirements: The de novo synthesis of fatty acids in the cytosol fundamentally requires Acetyl-CoA as the carbon source and NADPH as the reductant, along with ATP for energy and biotin as a coenzyme.
Citrate Shuttle: Acetyl-CoA is produced in the mitochondrial matrix from pyruvate (via pyruvate dehydrogenase) or fatty acid oxidation. However, fatty acid synthesis occurs in the cytosol. The inner mitochondrial membrane is impermeable to Acetyl-CoA. Therefore, Acetyl-CoA condenses with oxaloacetate to form citrate, which readily exits the mitochondria via the citrate transporter. In the cytosol, citrate is cleaved back to Acetyl-CoA and oxaloacetate by ATP-citrate lyase.
Key Metabolites involved:
Malate: Formed from oxaloacetate in the cytosol; its conversion to pyruvate by malic enzyme is a major source of cytosolic NADPH for lipogenesis.
Pyruvate: The end product of glycolysis, which can be converted to Acetyl-CoA in the mitochondria.
Oxaloacetate: A citric acid cycle intermediate that combines with Acetyl-CoA to form citrate in the mitochondria and is regenerated in the cytosol from citrate cleavage.
Enzyme Mechanisms:
Acetyl-CoA Carboxylase (ACC): This is the rate-limiting and most tightly regulated enzyme in fatty acid synthesis. It catalyzes the irreversible carboxylation of Acetyl-CoA to Malonyl-CoA.
Reaction Steps: ACC is a biotin-dependent enzyme that functions in two main steps:
Carboxylation of biotin: Biotin is carboxylated at the expense of ATP, forming carboxybiotin.
Transfer of carboxyl group: The activated carboxyl group is then transferred from carboxybiotin to Acetyl-CoA, forming Malonyl-CoA.
VI. Fatty Acid Synthase Complex
The Fatty Acid Synthase (FAS) complex is a multifunctional enzyme system, typically a homodimer in animals, where each monomer is a single polypeptide chain containing seven distinct enzyme activities. It synthesizes saturated fatty acids, primarily palmitate (16\ \text{carbons}), using Malonyl-CoA and Acetyl-CoA as substrates and NADPH as the reducing power.
Fatty Acid Synthesis Steps (within the FAS complex): Fatty acid synthesis is a cyclical process of four reactions:
Condensation: Acetyl-CoA (or a growing acyl chain) condenses with Malonyl-CoA, releasing \text{CO}_2. This extends the carbon chain by two carbons.
Reduction: The \beta-keto group is reduced to a hydroxyl group by NADPH.
Dehydration: Water is removed, introducing a double bond.
Reduction: The double bond is reduced to a single bond by a second NADPH molecule.
These steps are repeated until the fatty acid chain reaches a desired length (e.g., palmitate, \text{C}16).
VII. Lipogenesis Cycle
1st Cycle: The initial priming step involves the loading of Acetyl-CoA onto a cysteine residue of the \beta-ketoacyl-ACP synthase (KS) subunit and Malonyl-CoA onto the acyl carrier protein (ACP) subunit of the FAS complex. These two-carbon units then condense.
Each cycle adds two carbons: Each subsequent cycle begins with the transfer of the growing acyl chain from ACP to KS, followed by the loading of a new Malonyl-CoA onto ACP. Repeating the four-step sequence (condensation, reduction, dehydration, reduction) elongates the fatty acid by two carbons per cycle, utilizing two molecules of NADPH for reduction.
VIII. Regulation of Lipogenesis
Hormonal Influence:
Insulin: Stimulates lipogenesis primarily by activating acetyl-CoA carboxylase (ACC) and increasing the synthesis of FAS and other lipogenic enzymes. High insulin levels (e.g., after a carbohydrate-rich meal) signal energy abundance and promote storage.
Glucagon, Epinephrine: Have the opposite effect, inhibiting lipogenesis by phosphorylating and inactivating ACC, thus promoting fatty acid oxidation and glucose utilization.
Enzymatic Regulation:
Primarily through the activity of Acetyl-CoA Carboxylase (ACC), which is subject to both allosteric and covalent modification.
Allosteric Regulation: Citrate is a powerful allosteric activator of ACC, promoting its polymerization into active filaments. Long-chain fatty acyl-CoA, the end product of synthesis, allosterically inhibits ACC.
Covalent Modification: ACC is regulated by phosphorylation/dephosphorylation. Phosphorylation (e.g., by AMP-activated protein kinase (AMPK) or protein kinase A) inactivates ACC, while dephosphorylation activates it.
Substrate Availability: The availability of Acetyl-CoA and NADPH also influences the rate of lipogenesis.
IX. Triacylglycerol Synthesis
Why TAGs Need to be Synthesized: Triacylglycerols (TAGs) are synthesized primarily to store excess energy and to manage high levels of free fatty acids (FFAs). Excess circulating FFAs can be detrimental, leading to lipotoxicity in various tissues, contributing to insulin resistance and cellular dysfunction. Converting them to TAGs for storage in adipose tissue prevents this.
Location of Synthesis: Major sites for TAG synthesis include adipose tissue (for storage) and the liver (for distribution to other tissues via VLDL).
Storage: TAGs are stored primarily in lipid droplets within adipocytes of adipose tissue, which acts as the body's largest and most efficient long-term energy reserve.
X. Fatty Acid Activation
Process: Before fatty acids can be used for triacylglycerol synthesis or oxidation, they must be activated. Activation involves converting the free fatty acid into a high-energy fatty acyl-CoA molecule. This high-energy thioester bond drives subsequent metabolic reactions.
Key Enzyme: Fatty Acyl-CoA Synthetase (also known as Acyl-CoA ligase or Thiokinase) catalyzes this activation. The reaction consumes ATP, forming an acyl-AMP intermediate, and then the AMP is replaced by Coenzyme A to form Fatty Acyl-CoA, releasing pyrophosphate (\text{PP}_i).
Sources of Glycerol-3-Phosphate: This molecule serves as the backbone for triacylglycerol synthesis. Its availability is crucial and depends on the tissue:
In most tissues (including adipose tissue): Glycerol-3-phosphate is primarily derived from dihydroxyacetone phosphate (DHAP), an intermediate of glycolysis, through reduction catalyzed by glycerol-3-phosphate dehydrogenase.
In the liver and kidney: Glycerol-3-phosphate can also be generated directly from free glycerol through phosphorylation by glycerol kinase, an enzyme largely absent in adipose tissue.
XI. Lipoproteins
Functions: Lipoproteins are complex particles with a hydrophobic core (containing triacylglycerols and cholesterol esters) and a hydrophilic outer shell (composed of phospholipids, unesterified cholesterol, and apolipoproteins). Their primary function is to transport hydrophobic lipids (which are insoluble in water) through the aqueous circulatory system to various tissues for storage or energy utilization.
Chylomicrons, VLDL, LDL, HDL classifications: These are the main classes, categorized by density, size, and apolipoprotein content, each with specific roles:
Chylomicrons: Transport dietary lipids (TAGs, cholesterol) from the intestines to peripheral tissues and the liver.
Very Low-Density Lipoproteins (VLDL): Carry endogenously synthesized TAGs and cholesterol from the liver to peripheral tissues.
Low-Density Lipoproteins (LDL): Primarily transport cholesterol from the liver to peripheral cells; often referred to as "bad cholesterol" due to its association with atherosclerosis when elevated.
High-Density Lipoproteins (HDL): Involved in reverse cholesterol transport, picking up excess cholesterol from peripheral tissues and delivering it back to the liver; often called "good cholesterol."
XII. Regulation of Lipoprotein Metabolism
Dietary and genetic factors: Both significantly influence plasma lipoprotein levels. High intake of saturated and trans fats can raise LDL, while monounsaturated and polyunsaturated fats tend to lower it. Genetic predispositions can affect a person's ability to clear or produce different lipoproteins, leading to conditions like familial hypercholesterolemia. These factors affect both triacylglycerol and HDL levels in the plasma.
Clinical Relevance: Elevated levels of certain lipids, particularly LDL-cholesterol and triacylglycerols (hyperlipidemia), are strongly linked to the development and progression of metabolic diseases, including atherosclerosis, cardiovascular disease, and pancreatitis.
Significance of Regulation: Understanding the intricate regulation of lipoprotein metabolism is critical for diagnosing, preventing, and treating conditions like obesity, diabetes mellitus, and cardiovascular diseases, as it provides targets for therapeutic interventions (e.g., statins to lower cholesterol).
XIII. Key Clinical Conditions
Dyslipidemia: An abnormal (unhealthy) amount of lipids (e.g., cholesterol and/or triglycerides) in the blood. It is a major risk factor for atherosclerosis and subsequent cardiovascular diseases (e.g., heart attack, stroke). There is a strong correlation between dyslipidemia and metabolic syndrome, obesity, and diabetes.
Fatty Liver Disease (Hepatic Steatosis): Characterized by excessive accumulation of triglycerides in liver cells. It can be caused by either excessive triglyceride synthesis (e.g., from overconsumption of carbohydrates and fats, leading to increased lipogenesis) or impaired lipoprotein export (the liver's inability to package and secrete lipids efficiently as VLDL). It can progress to more severe forms like non-alcoholic steatohepatitis (NASH) and cirrhosis, which can lead to liver failure and significantly increase the risk of hepatocellular carcinoma.
Study Guide Questions
Process of Lipogenesis and TAG Synthesis
Lipogenesis (Fatty Acid Synthesis):
Lipogenesis is the metabolic process of synthesizing fatty acids from nonlipid precursors, mainly excess carbohydrates and proteins. It occurs in the cytosol, primarily in the liver, adipose tissues, and lactating mammary glands.
It requires Acetyl-CoA, NADPH, along with ATP and biotin.
The process involves the Acetyl-CoA Carboxylase (ACC) enzyme catalyzing the initial and rate-limiting step, converting Acetyl-CoA to Malonyl-CoA. Subsequently, the Fatty Acid Synthase (FAS) complex, a multifunctional enzyme, takes over. It synthesizes saturated fatty acids (e.g., palmitate, 16extcarbons16extcarbons) through a cyclical process of four reactions:Condensation: Acetyl-CoA (or a growing acyl chain) condenses with Malonyl-CoA, extending the carbon chain by two carbons.
Reduction: A eta-keto group is reduced to a hydroxyl group by NADPH.
Dehydration: Water is removed, introducing a double bond.
Reduction: The double bond is reduced to a single bond by a second NADPH molecule.
Each cycle adds two carbons, utilizing NADPH, until the desired fatty acid length is reached.
Triacylglycerol (TAG) Synthesis:
TAGs are synthesized to store excess energy and prevent lipotoxicity from high levels of free fatty acids. This process occurs primarily in adipose tissue (for storage) and the liver (for distribution).
Before synthesis, fatty acids must be activated to Fatty Acyl-CoA by Fatty Acyl-CoA Synthetase, consuming ATP.
Glycerol-3-phosphate, the backbone for TAG synthesis, is derived from dihydroxyacetone phosphate (DHAP) in most tissues (including adipose tissue) via reduction, or directly from free glycerol in the liver and kidney via phosphorylation by glycerol kinase. The activated fatty acyl-CoAs then esterify with glycerol-3-phosphate to form TAGs.
Physiological Conditions, Tissues, and Cellular Locations for Fatty Acid and TAG Synthesis
Physiological Conditions:
Lipogenesis and TAG synthesis primarily occur in the fed state, when energy intake (especially from carbohydrates and proteins) exceeds immediate energy expenditure. This is signaled by high insulin levels, which stimulate lipogenesis.Tissues:
Fatty Acid (Lipogenesis) Synthesis: Primarily occurs in the liver and adipose tissues, as well as lactating mammary glands.
TAG Synthesis: Major sites include adipose tissue and the liver.
Cellular Location:
Fatty Acid (Lipogenesis) Synthesis: Occurs in the cytosol.
TAG Synthesis: Occurs in the liver and adipose tissue (the note does not specify a particular subcellular location like ER, but generally, it occurs on the endoplasmic reticulum).
Precursors of Fatty Acid Synthesis and TAG Synthesis
Fatty Acid Synthesis Precursors:
Acetyl-CoA: The carbon building block for the fatty acid chain.
NADPH: The primary reducing agent, providing electrons for synthesis.
ATP: For energy.
Biotin: As a coenzyme for certain steps.
TAG Synthesis Precursors:
Fatty Acyl-CoA: Activated fatty acids that provide the acyl chains.
Glycerol-3-Phosphate: The glycerol backbone molecule.
Key Reaction in Lipogenesis: Acetyl-CoA to Malonyl-CoA
The key reaction is the irreversible carboxylation of Acetyl-CoA to Malonyl-CoA. This is the rate-limiting and most tightly regulated step in fatty acid synthesis.
Substrate: Acetyl-CoA
Product: Malonyl-CoA
Enzyme: Acetyl-CoA Carboxylase (ACC)
Vitamin Involved: Biotin
Other Requirements: ATP (for carboxylation of biotin)
The reaction proceeds in two main steps catalyzed by ACC:
Carboxylation of biotin at the expense of ATP, forming carboxybiotin.
Transfer of the activated carboxyl group from carboxybiotin to Acetyl-CoA, forming Malonyl-CoA.
Acetyl-CoA Transport Across the Inner Mitochondrial Membrane and Role of Citrate Shuttle
The inner mitochondrial membrane is impermeable to Acetyl-CoA, which is produced in the mitochondrial matrix. To transport Acetyl-CoA to the cytosol for fatty acid synthesis, the Citrate Shuttle is utilized:
Process:
In the mitochondria, Acetyl-CoA condenses with oxaloacetate to form citrate.
Citrate readily exits the mitochondria into the cytosol via the citrate transporter.
In the cytosol, citrate is cleaved back to Acetyl-CoA and oxaloacetate by ATP-citrate lyase.
Purpose/Role of Citrate Shuttle:
The primary purpose of this process is to facilitate the transport of Acetyl-CoA from the mitochondria to the cytosol, making Acetyl-CoA available for de novo fatty acid synthesis. Additionally, the conversion of cytosolic oxaloacetate (derived from citrate) to malate and then to pyruvate by malic enzyme is a major source of cytosolic NADPH, which is critically required as a reducing agent for lipogenesis.