Chapter 8: An Introduction to Metabolism

8.1 Metabolism and Energy Transformations

Metabolism encompasses all chemical reactions within an organism, transforming matter and energy to sustain life. These reactions are organized into metabolic pathways, where each step is catalyzed by a specific enzyme.

• Metabolic Pathway: A series of chemical reactions that convert a starting molecule into a product through enzyme-catalyzed steps.

• Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).

• Anabolic Pathways: Build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

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

8.1 Forms of Energy and Thermodynamics

Cells must transform energy to perform work. Energy exists in various forms, including kinetic, potential, and chemical energy. Organisms are open systems that exchange energy and matter with their surroundings, obeying the laws of thermodynamics.

• First Law of Thermodynamics: Energy can be transferred and transformed, but not created or destroyed.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

• Heat: The least useful form of energy for biological work.

8.2 Free Energy, Spontaneity, and Equilibrium

Free energy (G) is the portion of a system's energy available to do work. The change in free energy (ΔG) predicts whether a process is spontaneous (can occur without energy input) or nonspontaneous (requires energy input).

• Spontaneous Process: Increases entropy; ΔG < 0; system becomes more stable.

• Nonspontaneous Process: Decreases entropy; ΔG ≥ 0; requires energy input.

• Equation:

• Stability: Higher G = less stable; lower G = more stable.

Exergonic vs. Endergonic Reactions

• Exergonic Reaction: Releases free energy; spontaneous; ΔG < 0.

• Endergonic Reaction: Absorbs free energy; nonspontaneous; ΔG > 0.

Equilibrium in Biological Systems

• In a closed system, reactions reach equilibrium (ΔG = 0) and can do no work.

• Living cells are open systems, never reaching equilibrium, allowing continuous work.

8.3 ATP: The Cell's Energy Currency

ATP (adenosine triphosphate) stores and releases energy for cellular work. Hydrolysis of ATP's terminal phosphate yields ADP, inorganic phosphate (Pi), and energy.

• ATP Hydrolysis:

• Energy is released due to the formation of products with lower free energy and the relief of repulsion among phosphate groups.

Energy Coupling and the ATP Cycle

• Energy Coupling: ATP hydrolysis (exergonic) drives endergonic reactions by transferring a phosphate group (phosphorylation).

• ATP Cycle: ATP is regenerated from ADP and Pi using energy from catabolic pathways.

8.4 Enzymes and Activation Energy

Enzymes are biological catalysts that speed up reactions by lowering the activation energy (Ea) barrier, without being consumed or altering ΔG.

• Activation Energy (Ea): The energy required to initiate a reaction by destabilizing bonds in reactants.

• Enzyme Specificity: Each enzyme acts on a specific substrate, forming an enzyme–substrate complex.

How Enzymes Work

• Enzymes lower Ea but do not affect ΔG.

• They provide a lower-energy pathway for the reaction.

Enzyme Structure and Function

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

• Induced Fit: The enzyme changes shape to better fit the substrate upon binding.

The Catalytic Cycle

• Substrates bind to the active site.

• The enzyme lowers Ea and converts substrates to products.

• Products are released, and the enzyme is free to catalyze another reaction.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature; too high or too low reduces activity.

• pH: Each enzyme has an optimal pH, matching its environment (e.g., pepsin in the stomach, trypsin in the intestine).

Cofactors and Enzyme Inhibition

• Cofactors: Nonprotein helpers required by many enzymes. Can be inorganic (e.g., Zn2+, Mg2+) or organic (coenzymes, often derived from vitamins).

• Competitive Inhibitors: Resemble the substrate and compete for the active site.

• Noncompetitive Inhibitors: Bind elsewhere, changing enzyme shape and reducing activity.

8.5 Regulation of Metabolic Pathways

Cells regulate metabolism by controlling enzyme activity through allosteric regulation and feedback inhibition.

• Allosteric Regulation: Regulatory molecules bind to a site other than the active site, stabilizing the enzyme in active or inactive forms.

• Feedback Inhibition: The end product of a pathway inhibits an early enzyme, preventing overproduction and conserving resources.

Summary: Enzymes and Activation Energy

Enzymes lower the activation energy required for reactions, allowing them to proceed rapidly at cellular temperatures, but do not alter the overall free energy change (ΔG) of the reaction.