Metabolism: Chemical Processes of Life

Chapter 8: Metabolism

Introduction to Metabolism

Metabolism encompasses all chemical processes that occur within living organisms to sustain life. These processes involve the transformation of matter and energy, enabling cells to grow, reproduce, maintain their structures, and respond to environmental changes.

• Metabolism: The sum total of all chemical reactions in an organism.

• Metabolic Pathways: Series of chemical reactions that either build up or break down molecules.

• Energy: The capacity to do work; required for all cellular activities.

Key Learning Objectives

• Define key terms related to metabolism.

• Understand major metabolic pathways and their interconnections.

• Explain the function of enzymes and enzyme inhibitors.

• Distinguish between catabolism and anabolism in biochemical reactions.

Types of Metabolic Pathways

Catabolism and Anabolism

Metabolic pathways are classified into two main types based on their function and energy requirements:

• Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy. Example: Cellular respiration.

• Anabolic Pathways: Build complex molecules from simpler ones, consuming energy. Example: Synthesis of proteins from amino acids.

Example: The breakdown of glucose during glycolysis is a catabolic process, while the synthesis of fatty acids is anabolic.

Overview of Major Metabolic Pathways

Central Pathways and Interconnections

Metabolic pathways are highly interconnected, allowing cells to efficiently manage energy and resources. The following diagram summarizes the flow of carbon and energy through major pathways:

• Glycolysis: Converts glucose to pyruvate, generating ATP and NADH.

• Citric Acid Cycle (Krebs Cycle): Oxidizes acetyl-CoA to CO2, producing NADH, FADH2, and ATP.

• Electron Transport Chain: Uses electrons from NADH and FADH2 to generate ATP via oxidative phosphorylation.

• Lipogenesis: Synthesizes fatty acids from acetyl-CoA.

• Gluconeogenesis: Generates glucose from non-carbohydrate precursors.

Example: Pyruvate produced in glycolysis can enter the citric acid cycle or be converted to lactate under anaerobic conditions.

Energy and Thermodynamics in Metabolism

Forms of Energy

Energy exists in various forms and is essential for cellular work:

• Kinetic Energy: Energy of motion.

• Potential Energy: Stored energy due to position or structure.

• Chemical Energy: Energy stored in chemical bonds.

Thermodynamic Laws

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

Example: During cellular respiration, chemical energy in glucose is converted to ATP and heat.

Free Energy and Spontaneity

The change in free energy ($\Delta G$) determines whether a reaction is spontaneous:

• Exergonic Reaction: Releases free energy ($\Delta G < 0$); spontaneous.

• Endergonic Reaction: Absorbs free energy ($\Delta G > 0$); nonspontaneous.

Equation:

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

Where $\Delta H$ is change in enthalpy, $T$ is temperature in Kelvin, and $\Delta S$ is change in entropy.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

ATP (adenosine triphosphate) is the primary energy carrier in cells. It consists of ribose, adenine, and three phosphate groups.

• Energy is released when the terminal phosphate bond is broken, forming ADP (adenosine diphosphate) and inorganic phosphate ($P_i$).

• ATP hydrolysis powers cellular work by coupling exergonic and endergonic reactions.

Equation:

$\text{ATP} \rightarrow \text{ADP} + P_i$

Types of Cellular Work Powered by ATP

• Chemical Work: Driving endergonic reactions.

• Transport Work: Moving substances across membranes.

• Mechanical Work: Muscle contraction, movement of organelles.

Enzymes and Catalysis

Role of Enzymes

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy ($E_a$) barrier.

• Catalyst: Substance that increases reaction rate without being consumed.

• Enzyme: Protein catalyst specific to a substrate.

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

Activation Energy and Reaction Progress

• Activation energy ($E_a$) is the initial energy required to start a reaction.

• Enzymes lower $E_a$, allowing reactions to proceed faster at lower temperatures.

Enzyme-Substrate Complex

• Substrate: Reactant acted upon by an enzyme.

• Active Site: Region on the enzyme where the substrate binds.

• Enzyme-Substrate Complex: Temporary association during catalysis.

Enzymes facilitate reactions by orienting substrates, straining bonds, providing a favorable microenvironment, or forming temporary covalent bonds.

Enzyme Inhibition and Regulation

Types of Enzyme Inhibitors

• Competitive Inhibitors: Bind to the active site, competing with the substrate.

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

Examples: Toxins, poisons, pesticides, antibiotics.

Feedback Inhibition

Feedback inhibition prevents a cell from wasting resources by synthesizing more product than needed. The end product of a pathway inhibits an earlier step.

Factors Affecting Enzyme Activity

Environmental Influences

• Temperature: Each enzyme has an optimal temperature for activity.

• pH: Each enzyme has an optimal pH range.

• Chemicals: Specific molecules can enhance or inhibit enzyme function.

Localization of Enzymes Within the Cell

Enzymes are often compartmentalized within organelles to bring order to metabolic pathways. For example, enzymes for cellular respiration are located in mitochondria.

Summary Table: Catabolism vs. Anabolism

Pathway Type	Description	Energy Change	Example
Catabolism	Breakdown of complex molecules into simpler ones	Releases energy	Cellular respiration
Anabolism	Synthesis of complex molecules from simpler ones	Consumes energy	Protein synthesis

Summary Table: Types of Enzyme Inhibition

Inhibitor Type	Binding Site	Effect on Enzyme	Example
Competitive	Active site	Blocks substrate binding	Malathion (insecticide)
Noncompetitive	Allosteric site	Changes enzyme shape	Heavy metals

Metabolic Pathways Map

The provided diagrams illustrate the complexity and interconnectivity of metabolic pathways, including carbohydrate, lipid, and amino acid metabolism. Key intermediates such as pyruvate, acetyl-CoA, and citric acid cycle components serve as central hubs for metabolic flux.

Additional info: The diagrams also show nucleotide and amino acid biosynthesis, highlighting the integration of metabolism across macromolecule classes.