Introduction: The Chemistry of Life in Motion
Welcome to Lesson 8. Metabolism is the sum of all chemical reactions that occur in an organism. These reactions would happen far too slowly to sustain life without the help of biological catalysts called enzymes. In this lesson, we will explore how enzymes function, what factors affect their activity, and how cells manage energy through metabolic pathways, with a focus on the central role of ATP.
Part 1: Enzymes - The Agents of Metabolism
1.1 Enzyme Action and the Active Site
Enzymes are protein catalysts that speed up biochemical reactions without being consumed. The reactant that an enzyme acts on is called the substrate. The substrate binds to a specific region on the enzyme called the active site. This specificity is often described by the induced-fit model, where the binding of the substrate induces a conformational change in the active site, leading to a tighter, more precise fit that facilitates the reaction.
Diagram: The Induced-Fit Model of Enzyme Action
1.2 Lowering the Activation Energy Barrier
All chemical reactions require an initial input of energy to break existing bonds, known as the activation energy (Ea). Enzymes speed up reactions by lowering this energy barrier, making it easier for the reaction to proceed. They do not change the overall free energy change ($\Delta G$) of the reaction.
Diagram: Effect of Enzyme on Activation Energy
1.3 Enzyme Kinetics and Inhibition
The rate of an enzyme-catalyzed reaction is influenced by factors like temperature, pH, and substrate concentration. As substrate concentration increases, the reaction rate increases until the enzyme becomes saturated, at which point the rate reaches its maximum ($V_{max}$).
Enzyme inhibitors are molecules that bind to an enzyme and decrease its activity.
- Competitive Inhibition: The inhibitor resembles the substrate and competes for the same active site. It can be overcome by increasing substrate concentration.
- Non-competitive Inhibition: The inhibitor binds to a different site on the enzyme (the allosteric site), causing the active site to change shape and become less effective.
Diagram: Competitive vs. Non-competitive Inhibition
Part 2: Energy and Metabolism
2.1 Metabolic Pathways: Catabolism and Anabolism
Metabolism is the totality of an organism's chemical reactions. A metabolic pathway begins with a specific molecule and ends with a product, with each step catalyzed by a specific enzyme.
- Catabolic Pathways (Catabolism): Release energy by breaking down complex molecules into simpler compounds (e.g., cellular respiration). These are exergonic reactions ($\Delta G < 0$).
- Anabolic Pathways (Anabolism): Consume energy to build complex molecules from simpler ones (e.g., synthesis of a protein). These are endergonic reactions ($\Delta G > 0$).
2.2 ATP: The Cell's Energy Currency
Adenosine triphosphate (ATP) is the main energy-shuttling molecule in cells. It consists of adenine, a ribose sugar, and a chain of three phosphate groups. The energy is stored in the high-energy bonds between the phosphate groups. When a phosphate group is removed by hydrolysis (ATP → ADP + Pi), energy is released. This energy is then used to power cellular work, such as muscle contraction or active transport. ATP is regenerated from ADP and phosphate during cellular respiration.
Diagram: The ATP-ADP Cycle
Interactive Practice Quiz
Test your understanding of enzymes and metabolism. Choose the best answer for each question (A-E) and then submit to see your results.