Chapter 8: Energy and Enzymes – An Introduction to Metabolism

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 ordered series of chemical reactions that build up or break down molecules. Enzymes play a critical role in directing which reactions occur and when, enabling cells to acquire and use energy efficiently.

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

• Enzymes: Biological catalysts that speed up chemical reactions by lowering activation energy.

8.1 What Happens to Energy in Chemical Reactions?

Types of Energy

Energy in biological systems exists in two main forms: kinetic and potential energy. The transformation of energy is fundamental to all chemical reactions in cells.

• Kinetic energy: Energy of motion (e.g., thermal energy from moving molecules).

• Potential energy: Energy stored in position or configuration (e.g., chemical energy in bonds).

• Energy can be transformed from one type to another 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. Longer, weaker bonds with equally shared electrons have higher potential energy, while shorter, stronger bonds with unequally shared electrons have lower potential energy.

Energy Transformations in Chemical Reactions

• Products of chemical reactions often have shorter, stronger covalent bonds than reactants, resulting in a decrease in potential energy.

• The difference in energy is transformed into kinetic energy, often released as heat or light.

Thermodynamics in Biology

• 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 effects on pressure and volume.

• Exothermic reactions: Release heat; products have less potential energy than reactants ($\Delta H$ is negative).

• Endothermic reactions: Absorb heat; products have more potential energy than reactants ($\Delta H$ is positive).

• Entropy (S): Measure of disorder; increases when products are less ordered than reactants ($\Delta S$ is positive).

• Second law of thermodynamics: Total entropy always increases in a system.

Gibbs Free Energy and Spontaneity

Gibbs free energy ($G$) determines whether a reaction is spontaneous (exergonic) or requires energy input (endergonic). The standard free-energy change equation is:

$\Delta G = \Delta H - T\Delta S$

• If $\Delta G < 0$, the reaction is spontaneous (exergonic).

• If $\Delta G > 0$, the reaction is nonspontaneous (endergonic).

Temperature and Concentration Effects

Even spontaneous reactions may not occur quickly. Reaction rates depend on the frequency and orientation of molecular collisions, which are increased by higher concentrations and temperatures.

• Higher concentrations increase collision frequency.

• Higher temperatures increase kinetic energy and reaction rates.

8.2 Nonspontaneous Reactions May Be Driven Using Chemical Energy

Energetic Coupling

Cells drive nonspontaneous (endergonic) reactions by coupling them with exergonic reactions. This energetic coupling occurs via the transfer of electrons (redox reactions) or phosphate groups.

Redox Reactions

• Oxidation: Loss of electrons (exergonic).

• Reduction: Gain of electrons (endergonic).

• Redox reactions always occur together and represent energetic coupling of two half-reactions.

• Electrons may be transferred alone or with protons (as hydrogen atoms).

Electron Carriers

• FAD (Flavin adenine dinucleotide): Accepts two electrons and two protons to form FADH2.

• NAD+ (Nicotinamide adenine dinucleotide): Accepts two electrons and one proton to form NADH.

• These carriers have reducing power and readily donate electrons to other molecules.

ATP: The Energy Currency

• ATP (Adenosine triphosphate): Provides energy for most cellular activities; contains three negatively charged phosphate groups whose repulsion stores high potential energy.

• ATP hydrolysis releases free energy ($\Delta G = -7.3$ kcal/mol), which can be used to drive endergonic reactions via phosphorylation.

• Phosphorylation: Addition of a phosphate group to a molecule, increasing its potential energy and enabling coupling of exergonic and endergonic reactions.

8.3 How Enzymes Work

Activation Energy and Transition State

Enzymes lower the activation energy required for reactions by stabilizing the transition state, an intermediate point where old bonds are breaking and new ones are forming.

• Activation energy: Kinetic energy required to initiate a reaction.

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

Enzyme Function and Specificity

• Enzymes are highly specific catalysts, bringing substrates together in precise orientations at the active site.

• Substrate binding often induces a conformational change in the enzyme (induced fit).

• Enzyme catalysis involves three steps: initiation, transition state facilitation, and termination.

Limits of Catalysis

The rate of enzyme-catalyzed reactions increases with substrate concentration but eventually plateaus when all active sites are occupied (saturation kinetics).

• At high substrate concentrations, reaction rate reaches a maximum (Vmax).

Cofactors, Coenzymes, and Prosthetic Groups

• Cofactors: Inorganic ions that interact reversibly with enzymes.

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

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

8.4 What Factors Affect Enzyme Function?

Environmental Effects

Enzyme structure and function are sensitive to environmental conditions such as temperature and pH, which affect protein folding, substrate binding, and catalytic activity.

• Each enzyme has an optimal temperature and pH for maximal activity.

Regulation of Enzyme Activity

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

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

• Allosteric regulation: Molecule binds at a site other than the active site, causing a conformational change.

• Covalent modifications: Changes to enzyme structure (e.g., phosphorylation) can reversibly or irreversibly activate or deactivate enzymes.

8.5 Enzymes Can Work Together in Metabolic Pathways

Organization and Regulation of Pathways

Metabolic pathways consist of a series of enzyme-catalyzed reactions that build or break down biological molecules. These pathways are tightly regulated to ensure cellular efficiency and homeostasis.

• Feedback inhibition: The final product of a pathway inhibits an enzyme earlier in the pathway, preventing overproduction and conserving resources.

• Catabolic pathways break down molecules for energy and building blocks; anabolic pathways use energy to synthesize molecules.

Evolution of Metabolic Pathways

• Metabolic pathways evolve through retro-evolution (backward process) and patchwork evolution (recruitment of new enzymes).

• Bioremediation uses engineered pathways to clean up pollutants.