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.