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 maintain life. These reactions are organized into metabolic pathways, which are sequences of enzymatically catalyzed steps that build up (anabolic) or break down (catabolic) molecules.

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy.

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy.

• Enzymes: Biological catalysts that speed up chemical reactions by lowering activation energy and determining which reactions occur and when.

Types of Energy in Biological Systems

Kinetic and Potential Energy

Energy exists in different forms and can be transformed from one type to another during metabolic processes.

• Kinetic energy: The energy of motion. In biological systems, this includes thermal energy (energy of molecules moving).

• Potential energy: Stored energy due to position or configuration. Chemical energy is a form of potential energy stored in chemical bonds.

Energy Transformations in Chemical Reactions

• 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 (nonpolar) have higher potential energy.

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

Thermodynamics in Biology

First Law of Thermodynamics

• Energy is conserved: it cannot be created or destroyed, only transferred or transformed.

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

Second Law of Thermodynamics and Entropy

• Entropy (S): A measure of disorder in a system. During chemical reactions, if products are less ordered than reactants, entropy increases (ΔS is positive).

• The total entropy of a system always increases over time.

Free Energy and Spontaneity

• Gibbs free energy (G): Determines whether a reaction is spontaneous.

• The change in free energy (ΔG) is calculated as:

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

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

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

• If $\Delta G = 0$, the reaction is at equilibrium.

Factors Affecting Reaction Rates

Temperature and Concentration

• Higher concentrations and temperatures increase the rate of chemical reactions by increasing the frequency and energy of molecular collisions.

• Even spontaneous reactions may require activation energy to proceed.

Energetic Coupling in Cells

Coupling Exergonic and Endergonic Reactions

• Cells use energetic coupling to drive endergonic reactions using the energy released from exergonic reactions.

• This coupling occurs via transfer of electrons (redox reactions) or phosphate groups (e.g., ATP hydrolysis).

Redox Reactions

Oxidation and Reduction

• Oxidation: Loss of electrons (exergonic).

• Reduction: Gain of electrons (endergonic).

• Redox reactions always occur in pairs (one molecule is oxidized, another is reduced).

• Electron carriers such as FAD and NAD+ play key roles in transferring electrons and energy in cells.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

• Adenosine triphosphate (ATP): A ribonucleotide with three phosphate groups, used as the primary energy carrier in cells.

• The bonds between phosphate groups are high in potential energy due to repulsion between negatively charged phosphates.

ATP Hydrolysis

• Hydrolysis of ATP releases energy:

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Standard free energy change: $\Delta G = -7.3$ kcal/mol (highly exergonic).

• Cells use this released energy to drive endergonic reactions via phosphorylation (adding a phosphate group to a substrate).

Enzymes and Catalysis

How Enzymes Work

• Enzymes lower the activation energy required for reactions to proceed by stabilizing the transition state.

• Activation energy (Ea): The energy required to reach the transition state.

• Enzymes are highly specific, binding substrates at their active site and often undergoing an induced fit upon substrate binding.

Mechanism of Enzyme Action

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

2. Transition state facilitation: Interactions between substrate and active site lower activation energy.

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

Enzyme Kinetics

• At low substrate concentrations, reaction rate increases linearly with substrate concentration.

• At high substrate concentrations, the rate plateaus (saturation kinetics) because all enzyme active sites are occupied.

Enzyme Regulation

Factors Affecting Enzyme Activity

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

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

Regulation by Molecules

• Competitive inhibition: A molecule competes with the substrate for the active site.

• Allosteric regulation: A molecule binds at a site other than the active site, causing a conformational change that affects enzyme activity (can activate or inhibit).

• Covalent modification: Enzyme activity can be regulated by covalent changes, such as phosphorylation, which can activate or deactivate the enzyme.

Metabolic Pathways and Their Regulation

Organization and Evolution of Metabolic Pathways

• Metabolic pathways are sequences of enzyme-catalyzed reactions that build or break down molecules.

• Pathways can evolve by recruiting new enzymes or through retro-evolution (backward evolution of steps).

Feedback Inhibition

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

Key Terms Table

Term	Definition
Metabolism	All chemical reactions in a living organism
Enzyme	Protein catalyst that speeds up reactions
Catabolic pathway	Breaks down molecules, releases energy
Anabolic pathway	Builds molecules, consumes energy
ATP	Primary energy carrier in cells
Redox reaction	Reaction involving electron transfer
Activation energy	Energy needed to start a reaction
Feedback inhibition	End product inhibits pathway enzyme

Example: ATP Hydrolysis Coupling

Cells use the energy released from ATP hydrolysis to drive endergonic reactions, such as the synthesis of macromolecules or active transport across membranes.

Summary

• Metabolism is the sum of all chemical reactions in a cell, organized into pathways regulated by enzymes.

• Energy transformations follow the laws of thermodynamics, with ATP serving as the main energy currency.

• Enzymes lower activation energy, are highly specific, and are regulated by various mechanisms to ensure proper metabolic control.