Biology 1 - Lesson 6: Enzymes and Metabolism
Overview
Enzymes are biological catalysts that speed up chemical reactions without being consumed in the process. Almost every metabolic pathway in living organisms relies on enzymes to regulate the flow of molecules and energy. In this lesson, we’ll explore the fundamental concepts of metabolism, how enzymes lower activation energy, and the regulatory mechanisms that keep cellular processes balanced.
1. Introduction to Metabolism
1.1 Metabolic Pathways
Definition: A metabolic pathway is a series of interconnected biochemical reactions that convert a substrate molecule (such as glucose) step-by-step through intermediates to a final product.
Catabolic vs. Anabolic Pathways
Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).
Anabolic Pathways: Synthesize complex molecules from simpler ones, requiring an input of energy (e.g., protein synthesis).
1.2 Energy and Life
ATP (Adenosine Triphosphate)
The primary energy “currency” of cells.
Energy is stored in the high-energy phosphate bonds; breaking these bonds releases usable energy.
Thermodynamics in Biology
First Law: Energy is conserved (cannot be created or destroyed).
Second Law: Entropy (disorder) tends to increase; cells maintain organization by continuously using energy.
2. Enzymes as Biological Catalysts
2.1 Activation Energy
Definition: The initial energy barrier that must be overcome for a reaction to proceed.
Enzymes’ Role: Enzymes lower the activation energy, allowing reactions to occur more rapidly under physiological conditions (mild temperature, neutral pH, etc.).
2.2 Structure and Specificity
Active Site: A specialized region on the enzyme where the substrate binds.
Substrate Specificity
Lock-and-Key Model: The enzyme’s active site fits a specific substrate like a lock fits a single key.
Induced Fit Model: The enzyme undergoes a slight change in shape to more snugly fit the substrate.
Active Site: A specialized region on the enzyme where the substrate binds.
Substrate Specificity
- Lock-and-Key Model: The enzyme’s active site fits a specific substrate like a lock fits a single key.
- Induced Fit Model: The enzyme undergoes a slight change in shape to more snugly fit the substrate.
[Diagram: Comparison of Lock-and-Key vs. Induced Fit Models]
2.3 Enzyme-Substrate Complex
Binding: The substrate molecules bind to the enzyme’s active site.
Transition State Formation: The enzyme stabilizes the transition state, lowering activation energy.
Product Release: The enzyme returns to its original form, ready to catalyze another reaction.
3. Factors Affecting Enzyme Activity
3.1 Temperature and pH
Temperature:
Moderate increases in temperature generally accelerate reaction rates, but very high temperatures can denature (unfold) the enzyme.
Each enzyme has an optimal temperature (e.g., ~37°C for many human enzymes).
pH:
pH affects the charges of amino acids in the active site.
Most enzymes have an optimal pH range (e.g., pepsin in the stomach at ~pH 2).
3.2 Cofactors and Coenzymes
Definition: Non-protein molecules that help the enzyme function properly.
Metal Ions: Such as Fe²⁺, Mg²⁺, Zn²⁺ serve structural or catalytic roles.
Organic Molecules (Coenzymes): Derived often from vitamins (e.g., NAD⁺ from niacin).
3.3 Enzyme Inhibitors
Competitive Inhibition: Inhibitor competes with substrate for the active site.
Effect: Can be overcome by increasing substrate concentration.
Noncompetitive Inhibition: Inhibitor binds to another site (allosteric site), changing the enzyme’s shape.
Effect: Cannot be overcome by adding more substrate.
[Diagram: Competitive vs. Noncompetitive Inhibition Sites]
Competitive Inhibition
- Inhibitor competes with substrate for the active site.
- Effect: Can be overcome by increasing substrate concentration.
Noncompetitive Inhibition
- Inhibitor binds to another site (allosteric site), changing the enzyme’s shape.
- Effect: Cannot be overcome by adding more substrate.
[Diagram: Competitive vs. Noncompetitive Inhibition Sites]
4. Regulation of Metabolic Pathways
4.1 Allosteric Regulation
Allosteric Sites: Some enzymes have additional binding sites apart from the active site.
Effectors:
Activators: Stabilize the enzyme’s active conformation.
Inhibitors: Stabilize the enzyme’s inactive conformation.
4.2 Feedback Inhibition
Definition: The end product of a metabolic pathway inhibits an earlier step, preventing overproduction.
Example: ATP often serves as a feedback inhibitor in glycolysis when energy is abundant.
4.3 Enzyme Cascades and Phosphorylation
Phosphorylation: Addition of phosphate groups by kinases can turn enzymes on or off.
Signal Amplification: A small number of activated molecules can rapidly trigger multiple downstream responses.
5. Real-Life Applications
5.1 Medical Relevance
Drug Design: Many drugs are enzyme inhibitors or modulators, e.g., ACE inhibitors for hypertension.
Diagnostic Enzymes: Elevated levels of certain enzymes in blood tests (e.g., troponin in myocardial infarction) indicate tissue damage or disease.
5.2 Industrial and Research Uses
Biotechnology: Enzymes are used in biocatalysis (e.g., making biofuels, pharmaceuticals).
Food Industry: Enzymes like proteases tenderize meat; amylases break down starches in brewing.
5.3 Agricultural Connections
Pest Control: Enzyme inhibitors targeting insects’ metabolic pathways.
Crop Yield: Genetic engineering to optimize enzymes for growth in harsh climates.
6. Exercise: Investigating Enzyme Activity with Household Materials
Objective
Observe how temperature affects enzyme activity using an easily obtainable enzyme—catalase from potato or liver tissue.
Materials
Fresh potato or a piece of raw liver (high in catalase)
Hydrogen peroxide (3% from a pharmacy)
Three small containers or test tubes
Water bath or cups of cold and warm water
Thermometer
Procedure
Prepare Tissue Samples
Cut the potato or liver into three small pieces of equal size.
Label Containers
“Cold” (~4–10°C), “Room Temperature” (~20–25°C), and “Hot” (~60°C or higher, but do not exceed 70°C to avoid injury).
Add Hydrogen Peroxide
Place each tissue sample in its container. Immediately add ~5 mL of hydrogen peroxide.
Observe
Bubbles indicate oxygen release from the breakdown of hydrogen peroxide (2H₂O₂ → 2H₂O + O₂).
Compare bubble formation across different temperatures.
Analysis
Note that at colder temperatures, enzyme activity is slower.
At very high temperatures, the enzyme may denature, reducing or stopping bubble formation.
7. Additional Learning Components
7.1 Historical Anecdote: Eduard Buchner’s Fermentation Experiment
In the late 19th century, Eduard Buchner discovered that yeast extracts (without any living cells) could ferment sugars into alcohol. This groundbreaking work won him the Nobel Prize in Chemistry (1907) and demonstrated that enzymes, not intact cells, carried out fermentation.
7.2 Researcher Spotlight: Maud Leonora Menten
Maud Menten, co-creator of the famous Michaelis-Menten equation, made fundamental contributions to enzyme kinetics. Her work laid the foundation for quantitatively describing how enzyme activity relates to substrate concentration.
7.3 Advanced Reading Suggestions
“Lehninger Principles of Biochemistry” (Nelson & Cox) – In-depth coverage of enzyme mechanisms and kinetics.
Research Articles: Look for papers in Nature Chemical Biology or Biochemistry on recent developments in enzyme engineering and drug-target design.
7.4 Notable Breakthrough: Directed Evolution of Enzymes
Frances Arnold won the Nobel Prize in Chemistry (2018) for pioneering methods of “directed evolution,” engineering enzymes with novel or improved functions. This approach revolutionized industrial processes from drug synthesis to biofuel production.
7.5 Interactive Concept
Several online simulations allow students to vary substrate concentration, enzyme concentration, and temperature to visually observe changes in reaction rates in real time. These can be found on reputable educational websites and in virtual lab software.
8. Recall Questions
Activation Energy: Define activation energy. How do enzymes affect it?
Enzyme Specificity: Compare the lock-and-key model with the induced fit model. Which better explains enzyme flexibility?
Temperature and pH: Why might an enzyme function poorly at a temperature or pH outside its optimum range?
Inhibition: Differentiate between competitive and noncompetitive inhibitors. Give one example of each in biological or medical contexts.
Feedback Inhibition: How does feedback inhibition prevent the overaccumulation of products in metabolic pathways?
Use these questions to test your understanding of enzyme structure, function, and regulation within the broader context of metabolism.
