Chapter 8: An Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all the chemical reactions that occur within a living organism, enabling it to maintain life, grow, and reproduce. These reactions are organized into metabolic pathways, each catalyzed by specific enzymes.

• Metabolism: The totality of an organism’s chemical reactions.

• Metabolic pathway: A series of chemical reactions that transform a starting molecule into a product, each step catalyzed by a specific enzyme.

• Catabolic pathways: "Downhill" reactions that break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic pathways: "Uphill" reactions that build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

• Bioenergetics: The study of how energy flows through living organisms.

Forms of Energy in Biological Systems

Energy is the capacity to cause change or do work. Living cells transform energy from one form to another to perform life’s functions.

• Kinetic energy: Energy of motion (e.g., movement of molecules).

• Thermal energy: Kinetic energy associated with random movement of atoms or molecules; transferred as heat.

• Light energy: Energy in the form of light, used in photosynthesis.

• Potential energy: Stored energy due to location or structure.

• Chemical energy: Potential energy available for release in a chemical reaction (e.g., energy stored in glucose).

Example: The chemical energy in food molecules is converted to kinetic energy for movement and thermal energy (heat).

Thermodynamics and Biological Systems

Thermodynamics is the study of energy transformations. Biological systems obey the laws of thermodynamics.

The First Law of Thermodynamics

• Law: Energy can be transferred and transformed, but cannot be created or destroyed.

• Also called: The principle of conservation of energy.

• Application: Plants convert light energy to chemical energy; animals convert chemical energy to kinetic and heat energy.

The Second Law of Thermodynamics

• Law: Every energy transfer or transformation increases the entropy (disorder) of the universe.

• Implication: Some energy is always lost as heat, making it unavailable to do work.

• Biological relevance: Living organisms increase the disorder of their surroundings through metabolism.

Example: When a plant uses sunlight to make organic molecules, some energy is lost as heat, increasing entropy.

Free Energy and Spontaneity of Reactions

Free energy (Gibbs free energy) determines whether a reaction can occur spontaneously.

• Free energy (G): The portion of a system’s energy that can perform work at constant temperature and pressure.

• Change in free energy (ΔG): Indicates the spontaneity of a process.

Equation:

• = change in free energy

• = change in enthalpy (total energy)

• = change in entropy

• = temperature in Kelvin

• Spontaneous process: (negative); can occur without energy input.

• Nonspontaneous process: ; requires energy input.

Example: Cellular respiration is a spontaneous, exergonic process ().

Exergonic and Endergonic Reactions

Chemical reactions in metabolism are classified by their free energy changes.

• Exergonic reaction: Proceeds with a net release of free energy; is negative; spontaneous.

• Endergonic reaction: Absorbs free energy from surroundings; is positive; nonspontaneous.

Example: Synthesis of glucose (photosynthesis) is endergonic; breakdown of glucose (cellular respiration) is exergonic.

ATP and Energy Coupling

ATP (adenosine triphosphate) is the cell’s main energy currency, coupling exergonic and endergonic reactions.

• ATP structure: Adenine, ribose, and three phosphate groups.

• ATP hydrolysis: Releases energy by breaking the terminal phosphate bond.

• Phosphorylation: Transfer of a phosphate group from ATP to another molecule, making it more reactive.

• ATP cycle: ATP is regenerated by adding a phosphate to ADP, using energy from catabolic reactions.

Equation for ATP hydrolysis:

Example: ATP powers muscle contraction, active transport, and biosynthesis.

Enzymes and Activation Energy

Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy barriers.

• Enzyme: A macromolecule (usually a protein) that catalyzes a specific reaction.

• Activation energy (Ea): The initial energy needed to start a reaction.

• Substrate: The reactant an enzyme acts on.

• Active site: The region on the enzyme where the substrate binds.

• Induced fit: The enzyme changes shape slightly to fit the substrate more snugly.

How enzymes lower Ea:

• Orienting substrates correctly

• Straining substrate bonds

• Providing a favorable microenvironment

• Directly participating in the reaction

Example: Sucrase catalyzes the hydrolysis of sucrose into glucose and fructose.

Factors Affecting Enzyme Activity

Enzyme activity is influenced by environmental conditions and the presence of specific molecules.

• Temperature: Each enzyme has an optimal temperature; activity decreases above or below this point.

• pH: Each enzyme has an optimal pH; extreme pH can denature the enzyme.

• Cofactors: Nonprotein helpers (inorganic ions or organic coenzymes) required for enzyme function.

Example: Human enzymes typically function best at 37°C and neutral pH, while stomach enzymes like pepsin work best at pH 2.

Enzyme Inhibition and Regulation

Enzyme activity can be regulated by inhibitors and allosteric mechanisms.

• Competitive inhibitors: Resemble the substrate and bind to the active site, blocking substrate binding. Can be overcome by increasing substrate concentration.

• Noncompetitive inhibitors: Bind to another part of the enzyme, causing a shape change that reduces activity.

• Allosteric regulation: Regulatory molecules bind to a site other than the active site, affecting enzyme activity (can inhibit or activate).

• Cooperativity: Substrate binding to one active site stabilizes the active form for other sites (amplifies response).

• Feedback inhibition: The end product of a metabolic pathway inhibits an earlier step, preventing overproduction.

Example: Isoleucine inhibits the first enzyme in its biosynthetic pathway when present in excess (feedback inhibition).

Organization of Metabolic Pathways

Cells organize enzymes and metabolic pathways to increase efficiency and regulation.

• Enzymes may form multienzyme complexes.

• Some enzymes are fixed in specific locations within the cell (e.g., mitochondria).

• Compartmentalization allows for regulation and separation of metabolic processes.

Example: Enzymes for cellular respiration are located in the mitochondria.

Summary Table: Types of Enzyme Inhibition