Metabolism: An Introduction

Metabolic Pathways

Metabolism encompasses all chemical processes in an organism that maintain life. These processes are organized into metabolic pathways, where a starting molecule is converted into a product through a series of steps, each catalyzed by a specific enzyme.

• Catabolic pathways: Break down complex molecules into simpler ones, releasing energy.

• Anabolic pathways: Build complex molecules from simpler ones, consuming energy.

• ATP: Adenosine triphosphate acts as the energy currency, linking catabolic and anabolic reactions.

Energy in Biological Systems

Kinetic and Potential Energy

Energy is the capacity to do work. It exists in two main forms:

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

• Potential energy: Stored energy (e.g., a cyclist at the top of a hill).

Thermodynamics in Biology

Biological systems obey the laws of thermodynamics:

• First Law: Energy can be transferred and transformed, but cannot be created or destroyed (conservation of energy).

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

Free Energy and Spontaneous Reactions

Gibbs Free Energy (G) is the energy available to do work. The change in free energy () determines whether a reaction is spontaneous:

• Exergonic reactions: Release energy, are spontaneous ().

• Endergonic reactions: Require energy input, are nonspontaneous ().

ATP and Cellular Work

Types of Cellular Work

Cells perform three main types of work, powered by ATP:

• Transport work: Pumping substances across membranes.

• Mechanical work: Movement of structures like flagella or motor proteins.

• Chemical work: Driving anabolic reactions (e.g., protein synthesis).

Structure and Function of ATP

ATP is a nucleotide composed of adenine, ribose, and three phosphate groups. It stores energy in the bond of the third phosphate group.

• Belongs to the class of nucleic acids.

• Hydrolysis of ATP releases energy for cellular work.

The ATP Cycle

ATP is continuously regenerated from ADP and inorganic phosphate through cellular respiration and used for cellular work.

• Exergonic reactions (catabolism) provide energy to form ATP.

• ATP hydrolysis powers endergonic reactions (anabolism).

Enzymes and Metabolic Regulation

Enzyme Function and Specificity

Enzymes are biological catalysts, usually proteins, that speed up reactions by lowering activation energy without being consumed.

• Each enzyme is specific to its substrate due to the shape of its active site.

• The induced-fit model describes how substrate binding changes the enzyme's shape for optimal catalysis.

Activation Energy and Enzyme Action

Enzymes lower the activation energy () required for reactions, making them proceed faster.

• They do not alter the overall free energy change ().

Factors Affecting Enzyme Activity

Enzyme activity is influenced by:

• Substrate concentration: Higher concentration increases reaction rate until saturation.

• Temperature and pH: Each enzyme has optimal ranges; extremes can denature the enzyme.

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

• Inhibitors: Competitive inhibitors bind the active site; noncompetitive inhibitors bind elsewhere, altering enzyme shape.

Regulation of Enzyme Activity

Cells regulate metabolism through:

• Allosteric regulation: Effectors bind to sites other than the active site, changing enzyme activity.

• Cooperativity: Binding of one substrate enhances binding of others (e.g., oxygen binding to hemoglobin).

• Feedback inhibition: End products inhibit pathway enzymes to prevent waste.

Summary Table: Catabolic vs. Anabolic Pathways

Key Terms and Concepts

• Metabolism: All chemical reactions in an organism.

• Enzyme: Protein catalyst for biochemical reactions.

• ATP: Main energy carrier in cells.

• Activation energy: Energy required to start a reaction.

• Allosteric regulation: Regulation of enzyme activity by binding at a site other than the active site.

• Feedback inhibition: End product inhibits an earlier step in the pathway.

Additional info: These notes expand on brief points from slides and textbook images, providing academic context and definitions for clarity.