13 Overview of Metabolism

Overview of Metabolism

  • General Concept: Focus on storage and transfer of energy in living organisms.

Enzyme Catalysis

  • Essential Role of Enzymes:

    • Catalyze nearly all reactions in living cells.

    • Most enzymes are proteins (some are nucleic acids).

    • Speed biological reactions, channel specific products, and prevent unwanted side products.

    • Highly regulated activities of enzymes.

Components of Metabolism

  • Metabolism: Encompasses all enzyme-catalyzed reactions in living cells.

    • Divided into:

      • Catabolism: breakdown

        • Converts complex biomolecules into simpler products (e.g., CO₂, H₂O, urea).

        • Involves oxidation and energy release.

        • Examples: Carbon containing molecules (Carbohydrates, proteins, lipids) release energy; nitrogenous (purines and pyrimidines) biomolecules do not.

      • Anabolism: formation

        • Synthesizes complex biomolecules from simpler ones.

        • Examples: Biosynthesis of proteins, lipids, nucleic acids.

        • Involves reduction and requires significant energy input.

Central Themes in Metabolism

  • Core Ideas:

    • Catabolic pathways produce energy-rich intermediates (ATP, NADH, NADPH).

    • ATP generated during catabolism of organic compounds.

    • NADH derived from oxidative catabolism; used for ATP production in oxidative phosphorylation.

    • NADPH is produced in the pentose phosphate pathway and used in anabolic reactions.

    • All biomolecules synthesized from a few simple building blocks.

    • Catabolic and anabolic pathways related yet distinct.

Summary of Pathways

Catabolic Pathways

  • Utilize reduced organic compounds (lipids, carbohydrates, amino acids).

  • Yield oxidized products (pyruvate, acetylCoA, CO₂).

  • Use oxygen as an oxidizing agent.

  • Focus on net energy production.

  • Energy released as heat.

  • ATP produced through substrate-level phosphorylation and used to synthesize NADH and NADPH.

Anabolic Pathways

  • Synthesize complex products (lipids, carbohydrates, proteins) from simple materials.

  • Yield more reduced end products than starting materials.

  • NADPH acts as reducing agent.

  • ATP powers thermodynamically unfavorable reactions, with heat released.

ATP and Energy Storage

  • ATP captures energy released from oxidizing organic compounds.

  • Energy from catabolic pathways used to form ATP's phosphoanhydride bonds.

  • Hydrolysis of these bonds energizes reactions.

Reaction Dynamics of ATP

  • Hydrolysis: Releases energy for biochemical reactions.

  • Synthesis: Requires energy coupling from other reactions.

Coupled Reactions

  • Thermodynamic Principle: Unfavorable reactions can proceed when linked with favorable reactions.

Glucose Phosphorylation Example

  • Reaction: Glucose + Pi → Glucose-6-P is unfavorable; ATP hydrolysis makes it favorable.

  • Catalyst: Hexokinase drives the first glycolysis step.

NADH and NADPH Roles

  • NAD+ is reduced to NADH during catabolism; collects electrons released.

    • NADH: Stored energy, oxidized in aerobic cells to produce ATP.

  • NADPH: Used as reducing power in anabolic pathways.

NAD Structure and Function

  • NAD+ acts as an electron acceptor in oxidation reactions.

  • NADP+ has an extra phosphate and functions in reduction reactions.

Metabolic Processes

  • Oxidation-Reduction in Metabolism: Catabolism oxidizes compounds to produce energy; anabolism reduces oxidized compounds to synthesize biomolecules.

Energy Production in Organisms

  • Complete combustion of glucose releases approximately 687 kcal.

  • Living organisms capture some energy as chemical energy, formatting about 30% efficiency in ATP synthesis.

Human Energy Reserves

  • Carbohydrates: 1500 kcal (glycogen).

  • Fats: 135,000 kcal (triacylglycerols).

  • Proteins: 25,000 kcal (scavenged during starvation).

Daily Energy Usage

  • Average human uses 1500-2500 kcal daily; marathon runners may burn significantly more.

  • Energy sources: carbohydrates, fats, proteins, and oxygen; output as CO₂, water, and urea.

Catabolic Pathways Overview

  • Glycolysis: Converts glucose to pyruvate; aerobic conversion to acetylCoA.

  • Fat Metabolism: Triacylglycerols hydrolyzed to fatty acids and glycerol; enter glycolysis or oxidized to acetylCoA.

  • Protein Metabolism: Amino acids converted to pyruvate, acetylCoA, or TCA cycle intermediates.

  • TCA Cycle: Central oxidation point; produces NADH as primary energy-rich product and supports oxidative phosphorylation.

Organ-Specific Metabolism

Metabolic Profiles of Organs

  • Adipose Tissue: Fat storehouse (triacylglycerols).

  • Liver: Interconverts nutrients, stores carbohydrates, manages fat resources, releases energy-rich metabolites.

  • Brain: Relies on glucose for metabolism and nerve transmission; dependent on blood glucose levels.

  • Muscle: Utilizes fats and carbohydrates for exertion; stores 75% of glycogen.

Regulation of Metabolic Activity

  • Metabolic regulation through:

    • Allosteric control by metabolites.

    • Covalent modification of proteins.

    • Gene expression adjustments.

    • Compartmentalization for specific metabolic functions in cells.

Energy Sources and Athletic Performance

  • ATP and phosphocreatine levels vary significantly; high energy phosphate transfer sustains ATP availability, particularly during intense exercise.

During a 100 meter race, the primary energy source is phosphocreatine, which provides quick energy through the ATP-PC system. For a 1000 meter race, the body continues to utilize phosphocreatine but also gradually shifts to anaerobic glycolysis, relying on carbohydrates for energy production. In a marathon, the main sources of energy are carbohydrates and fats, with a significant reliance on aerobic metabolism. These energy sources enable endurance through longer races, highlighting the body's ability to oxidize substrates for ATP production.