Metabolism and Metabolic Pathways

Definition and Overview

Metabolism refers to the sum of all chemical reactions occurring within a cell. These reactions are organized into metabolic pathways, which are series of interconnected steps that transform molecules through enzyme-catalyzed reactions.

• Metabolic pathway: A sequence of chemical reactions, each catalyzed by a specific enzyme, leading to the transformation of a starting molecule to an end product.

• Benefits of metabolic pathways:

  • Efficient energy harvesting: Energy is captured in small, manageable amounts.

  • Minimization of energy lost as heat: Stepwise reactions reduce waste.

• Example: The breakdown of glucose () in cellular respiration is a metabolic pathway involving many steps and enzymes, maximizing energy extraction.

Types of Metabolic Reactions

Anabolism vs. Catabolism

Metabolic reactions are classified as either catabolic or anabolic:

• Catabolic pathways: Break down large molecules into smaller ones, releasing energy and increasing entropy.

• Anabolic pathways: Use energy to build larger molecules from smaller ones, decreasing entropy.

• Metabolism: The sum of both catabolic and anabolic processes in the cell.

• Example: Protein synthesis is anabolic; cellular respiration is catabolic.

Energy in Biological Reactions

Free Energy, Enthalpy, and Entropy

The direction and spontaneity of chemical reactions in cells are determined by changes in free energy (), enthalpy (), and entropy ():

• Free energy (): The energy available to do work in a system.

• Enthalpy (): The total energy of a system.

• Entropy (): The measure of disorder or randomness.

• Equation:

• : Temperature in Kelvin (K).

• Interpretation:

  • If is negative, the reaction is spontaneous (exergonic).

  • If is positive, the reaction is nonspontaneous (endergonic).

• Example: Cellular respiration is exergonic; photosynthesis is endergonic.

Coupled Reactions and Activated Carriers

Coupling Favorable and Unfavorable Reactions

Cells often couple exergonic (energy-releasing) reactions with endergonic (energy-requiring) reactions to drive essential processes.

• Activated carrier: A molecule that stores energy or chemical groups for transfer between reactions (e.g., ATP).

• ATP: The primary activated carrier in cells, used to power many cellular processes.

• Example: ATP hydrolysis (exergonic) is coupled to muscle contraction (endergonic).

Enzymes and Catalysis

Role and Mechanism

Enzymes are proteins that catalyze biochemical reactions by lowering the activation energy required for the reaction to proceed.

• Active site: The region on the enzyme where substrates bind and reactions occur.

• Enzyme-substrate complex: The temporary association between enzyme and substrate during catalysis.

• Specificity: Enzymes are highly specific for their substrates and reactions.

• Example: DNA polymerase catalyzes the synthesis of DNA.

Enzyme Functional Classes

Types of Enzyme Activities

• Hydrolases: Catalyze hydrolytic cleavage reactions (e.g., proteases, nucleases).

• Ligases: Join two molecules together (e.g., DNA ligase).

• Isomerases: Rearrange bonds within a single molecule.

• Polymerases: Catalyze polymerization reactions (e.g., DNA/RNA synthesis).

• Kinases: Add phosphate groups to molecules.

• Phosphatases: Remove phosphate groups from molecules.

Enzyme Regulation

Inhibition and Allosteric Control

Enzyme activity can be regulated by inhibitors and allosteric modulators:

• Competitive inhibition: Inhibitor binds to the active site, blocking substrate access.

• Non-competitive inhibition: Inhibitor binds elsewhere (allosteric site), changing enzyme shape and reducing activity.

• Allosteric site: A site on the enzyme other than the active site, where regulatory molecules bind.

• Allosteric regulation: Binding of a molecule at the allosteric site can stabilize the enzyme in active or inactive forms.

• Example: Feedback inhibition in metabolic pathways, where the end product inhibits an earlier enzyme.

Summary Table: Types of Enzyme Inhibition

Key Equations

• Free energy change:

• Pathway free energy: The total free energy change for a pathway is the sum of the free energy changes for each step.

• Example calculation: If is kJ, is kJ, then kJ.

Additional info:

• Some diagrams and images referenced are from "Essential Cell Biology, 4th ed. (© Garland Science 2014)" and illustrate concepts such as metabolic pathways, entropy, and enzyme mechanisms.

• Notes cover objectives for Chapter 8, which is typically focused on metabolism and enzyme regulation in introductory biology courses.