Chapter 8: An Introduction to Metabolism – Study Notes

Introduction to Metabolism

Overview of Metabolism

Metabolism encompasses all chemical reactions that occur within an organism. These reactions are organized into metabolic pathways, where a specific molecule is altered in a series of defined steps, resulting in a final product. Each step is catalyzed by a specific enzyme.

• 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., synthesis of proteins from amino acids).

Example: The breakdown of glucose in cellular respiration is a catabolic pathway, while the synthesis of DNA is anabolic.

Energy and Life

Introduction to Energy

Energy is the capacity to perform work. In biological systems, energy exists in various forms and can be transformed from one form to another.

• Potential energy: Stored energy due to position or structure (e.g., water behind a dam).

• Kinetic energy: Energy of motion (e.g., a moving car, heat).

Example: Chemical energy stored in bonds is a form of potential energy; when bonds break, kinetic energy is released.

Thermodynamics in Biological Systems

Thermodynamics is the study of energy transformations. Biological systems are open systems that exchange energy and matter with their surroundings.

• First Law of Thermodynamics: Energy can be transferred and transformed, but cannot be created or destroyed. $\text{\Delta E}_{\text{universe}} = 0$

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe. $\Delta S_{\text{universe}} > 0$

Example: Plants convert solar energy to chemical energy, but some energy is lost as heat, increasing entropy.

Chemical Reactions and Energy

Chemical Reactions

Chemical reactions involve the making and breaking of chemical bonds, leading to changes in matter.

• Reactants: Starting materials in a chemical reaction.

• Products: Substances formed as a result of a chemical reaction.

Types of Chemical Reactions

• Exergonic reactions: Release energy; products have less free energy than reactants. $\Delta G < 0$

• Endergonic reactions: Require energy input; products have more free energy than reactants. $\Delta G > 0$

Example: Cellular respiration is exergonic; photosynthesis is endergonic.

ATP: The Energy Currency of the Cell

Structure and Function of ATP

Adenosine triphosphate (ATP) is the main energy currency in cells. It consists of an adenine base, a ribose sugar, and three phosphate groups.

• Energy is released when ATP is hydrolyzed to ADP (adenosine diphosphate) and inorganic phosphate ($P_i$): $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + P_i + \text{energy}$

Example: ATP hydrolysis powers cellular work such as muscle contraction and active transport.

Energy Coupling

Cells use energy coupling to drive endergonic reactions by pairing them with exergonic reactions, often using ATP hydrolysis.

• ATP hydrolysis provides energy to phosphorylate other molecules, making them more reactive.

Example: The synthesis of glutamine from glutamic acid and ammonia is driven by ATP hydrolysis.

Enzymes and Metabolic Pathways

Enzyme Structure and Function

Enzymes are biological catalysts that speed up chemical reactions without being consumed. They lower the activation energy required for reactions to proceed.

• Substrate: The reactant that an enzyme acts upon.

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

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

Factors Affecting Enzyme Activity

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

• pH: Each enzyme has an optimal pH range.

• Substrate concentration: Increasing substrate increases reaction rate up to a point.

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

Enzyme Activation Energy

Enzymes lower the activation energy ($E_a$) needed for reactions to proceed, allowing reactions to occur more rapidly at lower temperatures.

• Transition state: A high-energy, unstable state during a reaction.

Example: The enzyme catalase speeds up the breakdown of hydrogen peroxide into water and oxygen.

Enzyme Binding and Regulation

• Enzyme-substrate complex: Temporary complex formed when an enzyme binds its substrate.

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins) required for enzyme activity.

Enzyme Inhibition

• Competitive inhibitors: Bind to the active site, blocking substrate binding.

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

Example: Many drugs and toxins act as enzyme inhibitors.

Regulation of Metabolic Pathways

Feedback Inhibition

Metabolic pathways are regulated by feedback inhibition, where the end product inhibits an enzyme involved earlier in the pathway, preventing overproduction.

• Negative feedback: End product inhibits pathway.

• Positive feedback: End product stimulates pathway (less common in metabolism).

Example: Isoleucine synthesis from threonine is regulated by feedback inhibition.

Summary Table: Types of Energy and Reactions

Type	Description	Example
Potential Energy	Stored energy due to position or structure	Water behind a dam
Kinetic Energy	Energy of motion	Rolling ball
Exergonic Reaction	Releases energy ($\Delta G < 0$)	Cellular respiration
Endergonic Reaction	Requires energy input ($\Delta G > 0$)	Photosynthesis

Key Equations

• First Law of Thermodynamics: $\Delta E = q + w$

• Gibbs Free Energy: $\Delta G = \Delta H - T\Delta S$

• ATP Hydrolysis: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + P_i + \text{energy}$

Additional info:

• Enzyme activity can be regulated by allosteric regulation, covalent modification, and compartmentalization within cells.

• Metabolic pathways are highly integrated and regulated to maintain homeostasis.