BCHM4320/6320 Metabolism: Comprehensive Metabolic Programs and Regulation Study Guide
Hormonal Coordination of Metabolic Programs
Metabolic Programming Strategy: Tissues coordinate different metabolic functions based on the needs of the body as a whole, specifically regulated by hormone release under specific physiological conditions.
Epinephrine: * Metabolic Program: Promotes a program of energy Mobilization. * Activation Condition: Released under Fight-or-Flight conditions.
Glucagon: * Metabolic Program: Promotes a program of energy Mobilization. * Activation Condition: Released under Fasting conditions.
Insulin: * Metabolic Program: Promotes a program of energy Storage. * Activation Condition: Released under Fed conditions.
Fuel Metabolism During Extended Fasting
Extended Fasting Adaptation: During fasting periods exceeding 4 days, the body adapts to supply adequate fuels to all tissues through specific circulating fuels.
Circulating Fuels and Their Sources (> 4 days fasting): 1. Ketones: Produced by the Liver. 2. Glucose: Produced by the Liver. 3. Fatty Acids: Produced by the Adipose tissue.
Tissue-Specific Fuel Utilization Table:
Tissue | Extended > 4-Day Fast Fuel | Resting Non-Fasting/Non-Fed Fuel |
|---|---|---|
Brain | Ketones | Glucose |
Red Blood Cells | Glucose | Glucose |
Adipose | Ketones | Fatty Acids |
Muscle | Ketones | Fatty Acids |
Liver | Fatty Acids | Fatty Acids |
Principles of Anabolic and Catabolic Pathways
Anabolic vs. Catabolic Functions: The major difference between these functions is energy utilization.
Anabolic Reactions: These processes consume energy to synthesize more complex molecules from simpler precursors.
Catabolic Reactions: These processes break down complex molecules into simpler ones to release energy.
Metabolic Fates of Pyruvate
Central Metabolite Status: Pyruvate is a central participant in multiple pathways. The specific pathways where Pyruvate is a direct participant include: * Glycolysis (Breakdown) * Gluconeogenesis (Synthesis/As a substrate via oxaloacetate or direct participant depending on context) * Fermentation (e.g., Lactic acid fermentation) * Cori Cycle (Interconversion of lactate and glucose using pyruvate as an intermediate) * PDH / Entry into the TCA Cycle (Conversion to Acetyl-CoA) * Amino Acid Metabolism (Specifically Alanine Aminotransferase reactions) * TCA Anaplerosis (Specifically Pyruvate Carboxylase converting pyruvate to Oxaloacetate)
Delivery of Reducing Equivalents to the Electron Transport Chain (ETC)
General Mechanism: Reducing equivalents (, ) produced in various pathways are delivered to Oxidative Phosphorylation () via specific mechanisms.
Mechanism Key: * A: Directly to Complex I * B: Directly to Complex II * C: Directly to Q (Ubiquinone) * D: ETF (Electron Transfer Flavoprotein) Relay to Q * E: Malate-Aspartate (Mal-Asp.) Shuttle to Complex I * F: Mitochondrial Glycerol-3-Phosphate Dehydrogenase (Gly3PDH) to Q
Pathway/Enzyme Specifics: * Brain Glycolysis (GAPDH): F (Mitochondrial Gly3PDH to Q). Note: This requires the Glycerol-3-Phosphate shuttle to pass equivalents from the cytosol to the mitochondrial counterpart. * Liver Pyruvate Dehydrogenase: A (Directly to Complex I). * TCA Cycle (Succinate Dehydrogenase - SDH): B or C (Directly to Complex II or Q). * TCA Cycle (Malate Dehydrogenase - MDH): A (Directly to Complex I). Note: This enzyme is already located in the mitochondrial matrix. * Amino Acid Breakdown (Glutamate Semialdehyde Dehydrogenase): A (Directly to Complex I). * Ketone Breakdown ($\beta$-Hydroxybutyrate Dehydrogenase): A (Directly to Complex I). * Lipolysis (Cytosolic Gly3PDH): F (Mitochondrial Gly3PDH to Q). * $\beta$-Oxidation ($\beta$-Hydroxyacyl-CoA Dehydrogenase): A (Directly to Complex I). * $\beta$-Oxidation (Acyl-CoA Dehydrogenase - ACD): D (ETF Relay to Q). Note: This enzyme funnels its equivalents through the ETF relay.
Tissue Protein Breakdown and Nitrogen Carriers
Functions of Endogenous Protein Breakdown: * Turnover of old or defective endogenous proteins. * Provision of carbon precursors for Gluconeogenesis (Most critical for survival during fasting to support RBCs). * Provision of carbon precursors for Ketogenesis. * Note: While it provides energy to muscle and disposes of nitrogen to prevent ammonia build-up, these are metabolic consequences rather than the primary "survival" functions compared to gluconeogenesis.
Importance for Survival: In prolonged fasting (> 4 days), providing carbon precursors for gluconeogenesis is the most important function because Red Blood Cells (RBCs) have an absolute requirement for glucose. Failure to produce glucose results in rapid death.
Histidine Breakdown and Nitrogen Transport: * Histidine Aminotransferase (HisAT): Produces the $\alpha$-keto acid Imidazolepyruvate (ImPyr). * Carrier Mechanism: Amino groups from Histidine () are funnelled into two primary carriers for transport to the liver: 1. Glutamine 2. Alanine * Pathway Paradigm: Amino acid aminotransferases hand off amino groups to collect them into Glutamate (). is then converted into one of the two nitrogen carriers.
The Urea Cycle and Nitrogen Disposal
Critical Function: The cycle is essential for the disposal of toxic nitrogen (ammonia).
Sources of Free Ammonia in the Liver: * Major Sources for CPS I: Glutaminase and Glutamate Dehydrogenase (). * Other Direct Deaminases: Serine Dehydratase, Glycine Cleavage System, D-Amino Acid Oxidase, Asparaginase, Histidine Ammonia Lyase, Phenylalanine Ammonia Lyase, Threonine Dehydratase. * Note: Aminotransferases (transaminases) like Alanine Aminotransferase do not liberate free ammonia; they transfer the amino group from one carbon backbone to another.
Urea Cycle Mechanics: * The cycle originates in the Mitochondria. * Most enzymes in the cycle are irreversible. * It eliminates 2 Nitrogens per molecule of Urea. * The energy cost of the cycle is partially offset by recycling Fumarate (linking it to the TCA cycle). * Calculation Correction: The cycle (including CPS I) consumes a total of 3 ATP equivalents/Urea (often cited as 4 high energy bonds because counts as 2 equivalents).
Intermediates and Nitrogen Flow: * Arginosuccinate: An intermediate containing four nitrogens. * Nitrogen Origins for Urea: One nitrogen enters via free ammonia (Carbamoyl Phosphate) and the other enters from Aspartate.
Clinical Correlation: Hyperammonemia and High Glutamate
Case Presentation: Infant with moderately elevated blood ammonia and high levels of Glutamate () in both blood and liver.
Differential Diagnosis: Reduced activity of Glutamate Dehydrogenase (GLU DH). * Mechanism: is a major pathway in the liver to liberate from Glutamate to feed into the Urea Cycle via CPS I. Without it, accumulates. * Secondary Path: Aspartate Aminotransferase () can still function to degrade some GLU, funneling nitrogen into the cycle at the Arginosuccinate Synthetase step, which prevents ammonia levels from becoming catastrophically high, though they remain elevated.
Energy Yield and Carbon Skeletons of Amino Acids
Determination of Fate: * Glucogenic: Amino acids that can be broken down into Pyruvate or TCA intermediates to serve as precursors for gluconeogenesis. * Ketogenic: Amino acids that break down into Acetyl-CoA or Acetoacetyl-CoA to serve as precursors for ketogenesis.
Energy Yield Hierarchy (High to Low): 1. ALA (Alanine): Yields . * Calculation: Pyruvate $\rightarrow$ Acetyl-CoA (yields $1 \, NADH$). TCA cycle yields $3 \, NADH, 1 \, GTP, 1 \, FADH_2$. . 2. GLU (Glutamate): Yields . * Calculation: yields $1 \, NADH$. TCA cycle from $\alpha$-KG yields $2 \, NADH, 1 \, GTP, 1 \, FADH_2$. . 3. THR (Threonine via dehydratase): Yields . * Calculation: Threonine $\rightarrow$ Propionyl-CoA (yields $1 \, NADH$). Propionyl-CoA $\rightarrow$ Succinyl-CoA (costs $1 \, ATP$). Succinyl-CoA through TCA yields $1 \, GTP, 1 \, FADH_2, 1 \, NADH$. . 4. MET (Methionine): Yields . * Calculation: Costs $1 \, ATP$ to reach Homocysteine. Conversion to Propionyl-CoA yields $1 \, NADH$. Propionyl-CoA $\rightarrow$ Succinyl-CoA costs $1 \, ATP$. Succinyl-CoA through TCA yields $1 \, GTP, 1 \, FADH_2, 1 \, NADH$. .
Standard Calculations: Assumes $1 \, NADH = 2.5 \, ATP$ and $1 \, FADH_2 = 1.5 \, ATP$.
Management of Amino Acid Metabolic Disorders (PKU Model)
Disease Paradigm: In a genetic deficiency of enzyme "EX" for breaking down amino acid "X": 1. Dietary Supplementation: Provide the products of the missing reaction "EX". 2. Pharmacological Intervention: Use drugs to degrade the accumulating substrate "X". 3. Dietary Restriction: Implement a Low Protein Diet to limit the intake of substrate "X" and prevent the build-up of toxic by-products.
Ketone Bodies: Synthesis and Utilization
Functions of Ketone Synthesis in the Liver: 1. Recycling CoA: Enables continued energy production via fatty acid $\beta$-oxidation, which requires significant amounts of Coenyme A. 2. Alternative Fuel: Generates fuel for Extra-Hepatic Tissues () to conserve fatty acids for the liver’s own energy needs.
Ketone Utilization Facts: * Occurs in the Brain and Skeletal Muscle. * Does not occur in the liver (Liver lacks specific enzymes like $\beta$-ketoacyl-CoA transferase). * Occurs in the Mitochondria. * Occurs in untreated diabetes (Ketoacidosis). * Cannot occur under anaerobic conditions.
Clinical Scenario: Mutation in $\beta$-Hydroxybutyrate Dehydrogenase: * Symptoms: Sweety-smelling breath, extreme fatigue, nausea, rapid breathing (signaling acidosis). * Findings: Elevated $\beta$-Hydroxybutyrate in blood/liver, but Acetoacetate is nearly normal. * Diagnosis: Mutant Liver $\beta$-Hydroxybutyrate Dehydrogenase.
Lipid Mobilization (Lipolysis)
Regulation of Lipolysis: * Inhibited by: Insulin. * Stimulated by: Glucagon and Epinephrine. * Location: Starts inside Lipid Droplets and occurs mostly in Adipose and Liver tissues.
Perilipin Mutation Scenario: * A mutation in Perilipin that mimics its phosphorylated state leads to constant association of active Hormone-Sensitive Lipase (HSL) to the lipid droplet. * Consequences: * Higher Adipocyte Lipolysis Rate (elevated regardless of insulin/glucagon). * Lower Adipocyte Lipid Droplet Size/Number (storage is depleted). * Higher Blood [Fatty Acids] due to constant release. * Lower Post-Meal NET TAG Synthesis (insulin-stimulated synthesis is counteracted by constant lipolysis).
Metabolic Fate of Glycerol: * In Resting/Fasting Muscle: Enters Glycolysis $\rightarrow$ TCA $\rightarrow$ OXPHOS (). * In Resting/Fasting Liver: Enters Gluconeogenesis to produce Glucose ().
Fatty Acid $\beta$-Oxidation
Case Study: Arachidonic Acid (): * Acetyl-CoA Units Produced: 10. * Rounds of $\beta$-Oxidation: 9. * Reducing Equivalents (NET): * $7 \, NADH$ * $7 \, FADH_2$ * Note: The presence of double bonds requires isomerases and reductases (consuming ), which can skip standard -producing steps.
Regulation: $\beta$-oxidation is decreased by: * High [Insulin]. * High cellular [Malonyl-CoA] (inhibits CPT1). * Low cellular [free Coenzyme A].
Skeletal Muscle Mutant (Enoyl-CoA Hydratase deficiency): * Accumulates a $\beta$-hydroxy fatty acid intermediate on the carbon that would have been hydrated.
Fatty Acid Synthesis
Synthesis Facts: * Stimulated by Insulin action on ATP-Citrate Lyase and Acetyl-CoA Carboxylase (ACC). * Occurs in the Cytoplasm. * Is an Energy-Consuming Pathway.
Cost Analysis: Decanoic Acid (): * Acetyl-CoAs needed: 5. * Rounds of synthesis: 4. * NET ATP Cost: 29 .
Regulation (ACC Mutation): * A mutation in Liver ACC preventing phosphorylation by PKA means ACC cannot be inactivated during fasting. * Result: * Higher ACC Activity and Higher Malonyl-CoA. * Lower Fatty Acid $\beta$-Oxidation (due to Malonyl-CoA inhibition). * Higher Liver [Lipid Droplet Size/#] due to continued synthesis and blocked oxidation. * Lower Fasting Blood [Glucose] (due to altered metabolic reliance).
Cholesterol Synthesis
Key Trivia: * Related Enzymes: Thiolase and HMG-CoA Synthase (similar to ketone synthesis); Thiolase (similar to $\beta$-oxidation). * Substrate Requirement: 18 Acetyl-CoAs are required to synthesize one Cholesterol molecule. * Cellular Location: Begins in the Cytosol, ends in the ER membrane. * Major Regulatory Point: HMG-CoA Reductase.
SREBP Regulation: * SREBP (Sterol Regulatory Element-Binding Protein) must be cleaved by Golgi proteases to enter the nucleus and activate gene expression. * Mutant Effect: If SREBP lacks the site for the second Golgi protease, it remains stuck in the membrane. This leads to Lower liver cholesterol biosynthesis and uptake due to failed activation of genetic expression.