Chapter 8: Energy and Enzymes – An Introduction to Metabolism (General Biology Study Notes)

Energy and Enzymes: An Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions that occur within living organisms to sustain life. These reactions are organized into metabolic pathways, which are sequences of enzymatically catalyzed steps that build up (anabolic) or break down (catabolic) molecules.

• Metabolic Pathways: Ordered series of chemical reactions, each catalyzed by a specific enzyme.

• Anabolic Pathways: Build complex molecules from simpler ones; require energy input.

• Catabolic Pathways: Break down complex molecules into simpler ones; release energy.

• Enzymes: Biological catalysts that direct which reactions occur and when, helping cells acquire and use energy efficiently.

Types of Energy in Biological Systems

Energy is essential for cellular activities and exists in different forms within biological systems.

• Kinetic Energy: Energy of motion; includes thermal energy (energy of molecules moving).

• Potential Energy: Energy stored due to position or configuration; includes chemical energy (energy stored in chemical bonds).

• Energy Transformation: Energy can be converted from one form to another, such as from potential to kinetic energy during chemical reactions.

Chemical Bonds and Potential Energy

The amount of potential energy in a covalent bond depends on the position of shared electrons relative to the nuclei of bonded atoms.

• Nonpolar Bonds: Electrons are shared equally; longer, weaker bonds with high potential energy.

• Polar Bonds: Electrons are shared unequally; shorter, stronger bonds with low potential energy.

• Energy Change in Reactions: Products often have shorter, stronger bonds than reactants, resulting in a decrease in potential energy and release of kinetic energy.

Thermodynamics in Biology

Thermodynamics governs energy changes in biological reactions.

• First Law of Thermodynamics: Energy is conserved; it cannot be created or destroyed, only transferred or transformed.

• Enthalpy (H): Total energy in a molecule, including potential energy in bonds and kinetic energy affecting pressure and volume.

• Exothermic Reactions: Release heat; $riangle H$ is negative.

• Endothermic Reactions: Absorb heat; $riangle H$ is positive.

Entropy and the Second Law of Thermodynamics

Entropy (S) measures the amount of disorder in a system. The second law states that total entropy always increases in a system.

• Entropy Increase: When products of a reaction are less ordered than reactants, $riangle S$ is positive.

• Disorder: More disordered states are favored energetically.

Gibbs Free Energy and Reaction Spontaneity

Gibbs free energy (G) determines whether a reaction is spontaneous or requires energy input.

• Standard Free-Energy Change Equation:

$riangle G = riangle H - T riangle S$

• Spontaneous Reactions: $riangle G < 0$ (exergonic)

• Nonspontaneous Reactions: $riangle G > 0$ (endergonic)

• Equilibrium: $riangle G = 0$

Factors Affecting Reaction Rates

Even spontaneous reactions may not occur rapidly. Reaction rates depend on temperature and concentration.

• Collision Theory: Reactants must collide in the correct orientation and with sufficient energy to break and form bonds.

• Temperature: Higher temperatures increase kinetic energy and reaction rates.

• Concentration: Higher concentrations increase the frequency of collisions and reaction rates.

Energetic Coupling in Cells

Cells couple exergonic and endergonic reactions to drive nonspontaneous processes.

• Energetic Coupling: Energy released from one reaction is used to drive another.

• Mechanisms: Electron transfer (redox reactions) and phosphate group transfer (e.g., ATP hydrolysis).

Redox Reactions

Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules.

• Oxidation: Loss of electrons; exergonic.

• Reduction: Gain of electrons; endergonic.

• Electron Carriers: Molecules like FAD and NAD+ accept electrons and protons, forming FADH2 and NADH, which have reducing power.

ATP: The Energy Currency of the Cell

Adenosine triphosphate (ATP) stores and provides energy for most cellular activities.

• Structure: Three negatively charged phosphate groups linked together, creating high potential energy due to repulsion.

• Hydrolysis: ATP reacts with water, breaking the bond between the outermost phosphate group and releasing energy.

• Energy Released: Approximately 7.3 kcal/mol per ATP hydrolyzed.

ATP and Phosphorylation

ATP drives endergonic reactions by transferring a phosphate group to a target molecule (phosphorylation), increasing its potential energy and making it more reactive.

• Phosphorylation: Coupling exergonic ATP hydrolysis to endergonic cellular reactions.

Enzyme Function and Catalysis

Enzymes lower the activation energy required for reactions, increasing reaction rates.

• Activation Energy: Minimum energy required to initiate a chemical reaction.

• Transition State: High-energy intermediate state during a reaction.

• Enzyme Catalysis Steps:

  1. Initiation: Substrates bind to the enzyme's active site in precise orientation.

  2. Transition State Facilitation: Enzyme stabilizes the transition state, lowering activation energy.

  3. Termination: Products are released, and the enzyme is free to catalyze another reaction.

Enzyme Saturation and Kinetics

Enzyme-catalyzed reactions exhibit saturation kinetics.

• Low Substrate Concentration: Reaction rate increases linearly.

• High Substrate Concentration: Reaction rate plateaus as all active sites are occupied.

• Saturation Kinetics: Maximum speed is reached when enzyme is saturated with substrate.

Enzyme Cofactors and Coenzymes

Some enzymes require additional molecules for activity.

• Cofactors: Inorganic ions (e.g., Zn2+, Mg2+) that interact reversibly with enzymes.

• Coenzymes: Organic molecules (e.g., NADH, FADH2) that assist enzyme function.

• Prosthetic Groups: Non-amino acid molecules permanently attached to enzymes.

Factors Affecting Enzyme Activity

Enzyme structure and function are sensitive to environmental conditions.

• Temperature: Influences enzyme folding, movement, and reactivity.

• pH: Affects charge and shape of enzyme, impacting activity.

• Interactions: With other molecules or modifications to primary structure can alter activity.

Enzyme Regulation

Cells regulate enzyme activity to control metabolic pathways.

• Noncovalent Regulation:

  • Competitive Inhibition: Molecule competes with substrate for active site.

  • Allosteric Regulation: Molecule binds elsewhere, changing enzyme shape and activity.

• Covalent Modification:

  • Changes enzyme's primary structure; can be reversible (e.g., phosphorylation) or irreversible (e.g., peptide bond cleavage).

  • Phosphorylation: Most common reversible modification; alters enzyme shape and activity.

Metabolic Pathways and Feedback Inhibition

Metabolic pathways are regulated by feedback inhibition, where the end product inhibits an enzyme earlier in the pathway.

• Feedback Inhibition: Prevents overproduction of products and conserves resources.

• Pathway Evolution: Enzymes evolve to create new metabolic pathways as substrates decline or new needs arise.

• Catabolic Pathways: Break down molecules for energy and carbon building blocks.

• Anabolic Pathways: Use energy and carbon building blocks to synthesize molecules.

Summary Table: Key Concepts in Energy and Enzymes

Concept	Definition	Example/Application
Metabolic Pathway	Series of enzyme-catalyzed reactions	Glycolysis, Citric Acid Cycle
Enzyme	Biological catalyst	Hexokinase in glycolysis
ATP	Energy currency of the cell	Drives muscle contraction
Redox Reaction	Electron transfer between molecules	NAD+ + 2e- + H+ → NADH
Feedback Inhibition	End product inhibits pathway enzyme	Threonine biosynthesis regulation

Additional info: These notes expand on the original slides by providing full academic context, definitions, and examples for each concept, ensuring a self-contained study guide suitable for exam preparation in General Biology.