Energy and Enzymes: An Introduction to Metabolism

Overview of Chapter 8

This chapter explores how enzymes use energy to drive the chemistry of life, focusing on the principles of metabolism, energy transformations, and enzyme function. The chapter is structured around five main questions:

• What happens to energy in chemical reactions?

• Can chemical energy drive nonspontaneous reactions?

• How do enzymes help speed chemical reaction rates?

• What factors affect enzyme function?

• How do enzymes work together in metabolic pathways?

8.1 What Happens to Energy in Chemical Reactions?

Types of Energy

Energy is essential for cellular activities and exists in two primary forms:

• Kinetic energy: The energy of motion, including thermal energy (energy of molecules moving).

• Potential energy: Energy stored in position or configuration, such as chemical energy stored in chemical bonds.

• Energy can be transformed from one type to another during cellular processes.

Chemical Bonds and Potential Energy

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

• Longer, weaker bonds with equally shared electrons have high potential energy.

• Shorter, stronger bonds with unequally shared electrons have low potential energy.

Energy Transformations in Chemical Reactions

Chemical reactions involve changes in potential energy:

• Products formed typically have shorter, stronger covalent bonds than reactants.

• Potential energy decreases, and the difference is transformed into kinetic energy, thermal energy, or light.

First Law of Thermodynamics

The first law of thermodynamics states that energy is conserved; it cannot be created or destroyed, only transferred or transformed.

Enthalpy and Exothermic/Endothermic Reactions

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

• Exothermic reactions: Release heat, ΔH is negative, products have less potential energy than reactants.

• Endothermic reactions: Absorb heat, ΔH is positive, products have higher potential energy than reactants.

Entropy and the Second Law of Thermodynamics

• Entropy (S): Measure of disorder in a system.

• When products are less ordered than reactants, entropy increases (ΔS is positive).

• The second law of thermodynamics states that total entropy always increases in a system.

Gibbs Free Energy and Reaction Spontaneity

Gibbs free energy (G) determines whether a reaction is spontaneous or requires added energy:

• Standard free-energy change equation:

• ΔG < 0: Reaction is spontaneous (exergonic).

• ΔG > 0: Reaction is nonspontaneous (endergonic).

• ΔG = 0: Reaction is at equilibrium.

Temperature and Concentration Affect Reaction Rates

• Higher concentrations and temperatures increase the rate of chemical reactions.

• Substances must collide in specific orientations for reactions to proceed.

• Iodine clock experiment demonstrates that reaction rates increase with higher temperature and concentration.

8.2 Nonspontaneous Reactions May Be Driven Using Chemical Energy

Energetic Coupling

Energetic coupling allows chemical energy released from exergonic reactions to drive endergonic reactions:

• Occurs via transfer of electrons or phosphate groups.

Redox Reactions

Reduction–oxidation (redox) reactions involve electron transfer:

• Oxidation: Loss of electrons (exergonic).

• Reduction: Gain of electrons (endergonic).

• Redox reactions always occur together and represent energetic coupling.

Electron Carriers

• FAD accepts two electrons and two protons to form FADH2.

• NAD+ accepts two electrons and one proton to form NADH.

• These carriers donate electrons to other molecules, facilitating energy transfer.

ATP Transfers Energy via Phosphate Groups

Adenosine triphosphate (ATP) is the energy currency for cells:

• ATP contains three negatively charged phosphate groups, creating high potential energy.

• Hydrolysis of ATP releases free energy, which is used for cellular activities.

ATP Hydrolysis and Phosphorylation

• ATP hydrolysis is highly exergonic, releasing 7.3 kcal/mol.

• Phosphorylation adds a phosphate group to a target molecule, increasing its potential energy and coupling exergonic and endergonic reactions.

8.3 How Enzymes Work

Activation Energy and Transition State

Enzymes lower the activation energy required for reactions:

• Activation energy (Ea): Kinetic energy needed to strain chemical bonds and reach the transition state.

• Transition state: Intermediate point with high free energy between breaking old bonds and forming new ones.

Enzyme Function and Substrate Binding

• Enzymes are catalysts that bring substrates together in precise orientations.

• Substrates bind to the enzyme's active site, often causing a conformational change called induced fit.

Enzyme Catalysis Steps

• Initiation: Substrates are oriented as they bind to the active site.

• Transition state facilitation: Interactions lower activation energy.

• Termination: Products are released from the enzyme.

Saturation Kinetics

• Enzyme-catalyzed reactions increase linearly at low substrate concentrations, then plateau at high concentrations.

• All enzymes show "saturation kinetics"—active sites cannot accept substrates any faster once saturated.

Enzyme Helpers

• Cofactors: Inorganic ions (e.g., Zn2+, Mg2+, Fe2+) that interact with enzymes.

• Coenzymes: Organic molecules (e.g., NADH, FADH2) that interact with enzymes.

• Prosthetic groups: Non-amino acid atoms or molecules permanently attached to proteins.

8.4 What Factors Affect Enzyme Function?

Environmental Effects on Enzyme Activity

• Enzyme structure is sensitive to temperature, pH, and interactions with other molecules.

• Temperature affects folding, movement, and kinetic energy.

• pH affects enzyme shape, reactivity, and charge on acidic/basic groups.

• Enzymes have optimal temperature and pH for activity.

Regulation of Enzyme Activity

• Regulatory molecules can activate or inactivate enzymes by changing their structure or substrate binding ability.

• Regulation can be reversible (noncovalent) or involve covalent modifications.

Noncovalent Regulation

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

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

Covalent Modifications

• Changes to enzyme's primary structure can be reversible or irreversible.

• Phosphorylation is a common reversible modification, changing enzyme shape and activity.

8.5 Enzymes Can Work Together in Metabolic Pathways

Metabolic Pathways

• Metabolic pathways are series of reactions, each catalyzed by a different enzyme, that build or break down biological molecules.

Regulation of Metabolic Pathways

• Feedback inhibition: Final product of pathway inhibits an enzyme earlier in the pathway, preventing depletion of initial substrate.

Evolution of Metabolic Pathways

• Metabolic pathways evolve through retro-evolution and patchwork evolution.

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

• Anabolic pathways: Use energy and carbon to synthesize molecules.

Additional info: These notes expand on brief points from the original materials, providing definitions, examples, and context for key concepts in metabolism and enzyme function. All images included are directly relevant to the adjacent explanations, visually reinforcing the concepts discussed.