Overview
This lecture explains the progress of enzyme-catalyzed reactions, how to monitor them, the shape of product vs. time graphs, and the significance of the initial rate of reaction.
Enzyme-Catalyzed Reaction Basics
- In biology, reactants are called substrates, which are converted to products in the presence of enzymes.
- Enzymes reduce activation energy, making substrate conversion faster and easier.
- Example: Maltose (a disaccharide) is broken down by the enzyme maltase into glucose.
Monitoring Reaction Progress
- You cannot visually observe enzyme reactions directly, even with microscopes.
- Progress is detected by changes such as substrate disappearance or product appearance.
- If substrate is colored and product is colorless, fading color indicates substrate loss.
- If substrate is colorless and product is colored, color development indicates product formation.
- Hydrogen peroxide (H₂O₂) broken down by catalase produces water and oxygen; oxygen bubbles show product formation.
Measuring Product Formation Over Time
- Enzyme (E) and substrate (S) form an enzyme-substrate complex (ES), leading to product (P) formation.
- At reaction start (0 seconds), only substrates and enzymes are present; products appear as reaction proceeds.
- As time passes, substrates decrease and products increase until all substrates are used up.
- Graphing product amount vs. time gives a curve, not a straight line, due to decreasing substrate availability.
Understanding the Curved Product-Time Graph
- The initial reaction phase is steep because substrate concentration is highest, so enzyme-substrate collisions are frequent.
- As substrates decrease, formation of the ES complex slows, making the graph less steep.
- The reaction stops when all substrates are converted, and no further product is formed.
Initial Rate of Reaction
- The steepest part of the curve (at the beginning) is the initial rate of reaction.
- To calculate initial rate, measure product formed per unit time at the steepest slope.
- The initial rate enables fair comparison between different experiments, regardless of total substrate or product amounts.
Comparing Experiments
- For equal enzyme amounts, more substrates mean the reaction lasts longer and more product is formed.
- Initial rate comparison is done using the steepest part of each product-time graph.
- More substrate usually means a higher initial rate, but there are limits to this effect.
Key Terms & Definitions
- Substrate — The molecule an enzyme acts on in a biological reaction.
- Enzyme — A biological catalyst that speeds up reactions by lowering activation energy.
- Product — The molecule(s) formed from the substrate during an enzyme-catalyzed reaction.
- Activation Energy — Minimum energy needed to start a chemical reaction.
- Enzyme-Substrate Complex (ES Complex) — Temporary molecule formed when an enzyme binds its substrate.
- Initial Rate of Reaction — The maximum rate at which products are formed at the start of the reaction.
Action Items / Next Steps
- Review the concept of initial rate of reaction and practice calculating it from product-time graphs.
- Prepare for discussion on limits to increasing initial rate by adding more substrate (in the next lecture).