Energy and Enzymes: Foundations of Cellular Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions occurring within cells, enabling life by facilitating energy acquisition, utilization, and molecular transformations. Enzymes are biological catalysts that direct metabolic pathways, determining which reactions occur and when.

• Metabolic pathways: Ordered series of chemical reactions, each catalyzed by a specific enzyme, that build up (anabolic) or break down (catabolic) molecules.

• Enzymes: Proteins that accelerate reaction rates and regulate cellular activity.

8.1 What Happens to Energy in Chemical Reactions?

Types of Energy

Energy is fundamental to all biological processes and exists in two primary forms:

• Kinetic energy: Energy of motion; in cells, this is often thermal energy (movement of molecules).

• Potential energy: Energy stored in position or configuration; in biology, this is often chemical energy stored in bonds.

Potential Energy in Molecules

The potential energy of a molecule is determined by the configuration and position of shared electrons in covalent bonds:

• Longer, weaker bonds with equally shared electrons (nonpolar) have higher potential energy.

• Shorter, stronger bonds with unequally shared electrons (polar) have lower potential energy.

Energy Transformations in Chemical Reactions

Chemical reactions transform energy:

• Products typically have shorter, stronger covalent bonds than reactants, resulting in decreased potential energy.

• The difference in energy is transformed into kinetic energy, thermal energy, 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 affecting pressure and volume.

Exothermic and Endothermic Reactions

• Exothermic: Reactions release heat; is negative, products have less potential energy than reactants.

• Endothermic: Reactions absorb heat; is positive, products have more potential energy than reactants.

Entropy and the Second Law of Thermodynamics

Entropy (S) measures disorder. During chemical reactions, if products are less ordered than reactants, entropy increases ( is positive). The second law 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:

• Equation:

• Spontaneous reactions: (exergonic)

• Nonspontaneous reactions: (endergonic)

• At equilibrium:

Temperature and Concentration Affect Reaction Rates

Reaction rates depend on temperature and concentration:

• Higher concentrations increase collision frequency.

• Higher temperatures increase kinetic energy and reaction rate.

8.2 Nonspontaneous Reactions May Be Driven Using Chemical Energy

Energetic Coupling

Energetic coupling allows exergonic reactions to drive endergonic reactions, often through electron or phosphate group transfer.

Redox Reactions

Reduction–oxidation (redox) reactions involve electron transfer:

• Oxidation: Loss of electrons (exergonic)

• Reduction: Gain of electrons (endergonic)

• Redox reactions always occur together, coupling energy release and uptake.

Electron Carriers

Electron carriers such as FAD and NAD+ accept electrons and protons, forming FADH2 and NADH, which can donate electrons to other molecules.

ATP Transfers Energy via Phosphate Groups

Structure and Function of ATP

Adenosine triphosphate (ATP) is the energy currency of cells, storing potential energy in bonds between three negatively charged phosphate groups.

ATP Hydrolysis

ATP hydrolysis releases free energy:

• ATP reacts with water, breaking the bond between the outermost phosphate group.

• Produces ADP and inorganic phosphate; highly exergonic, releasing 7.3 kcal/mol.

ATP Drives Endergonic Reactions

Cells use ATP energy for cellular activities by phosphorylation, which adds a phosphate group to target molecules, increasing their potential energy and coupling exergonic and endergonic reactions.

8.3 How Enzymes Work

Activation Energy and Transition State

Activation energy is the kinetic energy required to strain chemical bonds and reach the transition state, an intermediate with high free energy.

Enzyme Function and Substrate Binding

Enzymes are catalysts that bring substrates together in precise orientations, facilitating reactions. Substrates bind to the enzyme's active site, often causing an induced fit (conformational change).

Enzymes Lower Activation Energy

Enzymes decrease the activation energy required for reactions by destabilizing substrate bonds and facilitating the transition state.

Three-Step Model of Enzyme Action

• Initiation: Substrates bind to active site in specific orientation.

• Transition state facilitation: Active site interactions lower activation energy.

• Termination: Products are released; enzyme is unchanged.

Limits of Catalysis: Saturation Kinetics

Enzyme-catalyzed reactions show saturation kinetics: reaction rate increases with substrate concentration but plateaus when all enzyme molecules are occupied.

Enzyme Helpers: Cofactors, Coenzymes, and Prosthetic Groups

• Cofactors: Inorganic ions (e.g., Zn2+, Mg2+, Fe2+) that reversibly 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 and function are sensitive to environmental factors:

• Temperature: Affects folding, movement, and kinetic energy.

• pH: Affects shape, reactivity, and charge on acidic/basic groups.

• Interactions with other molecules and modifications of primary structure also affect activity.

Enzyme Regulation

Enzymes are regulated by molecules that change their structure or ability to bind substrates, activating or inactivating their function.

Noncovalent Regulation: Competitive and Allosteric

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

• Allosteric regulation: Molecule binds elsewhere, changing enzyme shape and activity (can activate or inhibit).

Covalent Regulation: Phosphorylation

• Regulation may involve covalent modifications, such as phosphorylation, which changes enzyme shape and activity.

• Irreversible changes often result from peptide bond cleavage; phosphorylation is a common reversible modification.

8.5 Enzymes Can Work Together in Metabolic Pathways

Metabolic Pathways and Regulation

Metabolic pathways are series of enzyme-catalyzed reactions that build or break down biological molecules. Regulation is often achieved through feedback inhibition, where the final product inhibits an enzyme early in the pathway.

Evolution of Metabolic Pathways

Metabolic pathways evolve through retro-evolution and patchwork evolution, allowing cells to adapt to changing substrate availability and environmental conditions.

Catabolic vs. Anabolic Pathways

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

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

Additional info: These notes expand on brief points with academic context, definitions, and examples to provide a comprehensive, self-contained study guide for General Biology students.