3.13 · Enzymes
Mode of Action of Enzymes — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
3.1 Mode of Action of Enzymes — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how enzymes catalyse reactions, the models of enzyme action and how enzyme activity is measured.Globular proteins — the polypeptide folds into a precise tertiary structure held by …
- Active site
- Specificity
- Enzyme–substrate complex
- Catabolic reactions
- Anabolic reactions
- Lock-and-key
Part 1 of 9Enzymes and the active site
Globular proteins
- Globular proteins the polypeptide folds into a precise tertiary structure held by hydrogen, ionic and disulfide bonds and hydrophobic interactions.
Active site
- Active site only a few amino acids form it, and their R groups interact with the substrate.
Specificity
- Specificity only a substrate with a complementary shape and charge distribution binds.
Enzyme–substrate complex
- Enzyme–substrate complex binding forms the complex, then an enzyme–product complex, before the products leave.
Part 2 of 9Lowering activation energy
Mechanism
- Mechanism enzymes hold substrates in positions that favour bonding, or strain bonds in the substrate.
Effect
- Effect reactions proceed rapidly at body temperature.
Catabolic reactions
- Catabolic reactions a single substrate is broken into smaller products, e.g. hydrolysis and respiration.
Anabolic reactions
- Anabolic reactions two or more substrates join to make one product, e.g. protein synthesis and photosynthesis.
Part 3 of 9Models of enzyme action
Lock-and-key
- Lock-and-key the active site is rigid and already exactly complementary.
Induced fit
- Induced fit the flexible active site moulds around the substrate, which explains how bonds are strained and why some enzymes bind related substrates.
Part 4 of 9Induced-fit mechanism
1
Substrate collides with the enzyme in the correct orientation.
2
The substrate enters the active site, which changes shape to fit closely.
3
In the enzyme–substrate complex, bonds in the substrate are strained, lowering activation energy.
4
The reaction occurs, forming the enzyme–product complex.
5
The products are released and the enzyme is unchanged and reused.
Part 5 of 9Denaturation
Cause
- Cause high temperature or extreme pH breaks the hydrogen and ionic bonds of the tertiary structure.
Effect
- Effect the active site changes shape, the substrate cannot bind, and no complexes form.
Permanence
- Permanence denaturation is usually permanent.
Part 6 of 9Intracellular and extracellular enzymes
Intracellular
- Intracellular catalase, and enzymes of respiration and DNA replication.
Extracellular
- Extracellular amylase, trypsin, and enzymes secreted by fungi and bacteria for external digestion.
Part 7 of 9Measuring reaction progress
Product appearance
- Product appearance e.g. the volume of oxygen from catalase.
Substrate disappearance
- Substrate disappearance e.g. starch tested with iodine at regular intervals.
Progress curve
- Progress curve it is steepest at the start and flattens as substrate is used up.
Initial rate
- Initial rate draw a tangent at t = 0 and calculate its gradient, because later substrate concentrations differ.
Part 8 of 9Colorimetry and calibration
Colorimeter
- Colorimeter measures absorbance or transmission of light through a solution.
Starch–amylase
- Starch–amylase as starch is broken down, absorbance falls and transmission rises.
Filter
- Filter use a red filter, which is complementary to the blue-black colour.
Blank
- Blank zero the instrument with a blank before readings.
Calibration curve
- Calibration curve make a stock starch solution, dilute it by the same factor each time, measure each with iodine and plot a curve to read unknowns.
Part 9 of 9More from the specification
More from the specification
Catalytic active site function
- Catalytic active site function precise structural complementarity between enzymes and transition states lowers activation energy to fulfill biological catalytic function.
Enzyme-substrate complex
- Enzyme-substrate complex the substrate binds to the active site, lowering activation energy so products are formed faster
Induced fit
- Induced fit the active site changes shape slightly to fit the substrate more closely, a refinement of the lock and key hypothesis
Hydrogen peroxide
- Hydrogen peroxide catalase in cells breaks it down into water and oxygen, and oxygen volume measures the reaction rate
Specificity
- Specificity the tertiary structure gives a specific active site shape, so each enzyme catalyses one chemical reaction
Hypothesis
- A testable prediction with a scientific explanation.
Hydrogen peroxide
- A toxic by-product of metabolism.
Enzyme-substrate complex
- The temporary structure formed when a substrate binds to an enzyme's active site.
Induced-fit hypothesis
- The active site changes shape to fit the substrate.
Lock-and-key hypothesis
- Rigid enzyme and substrate fit exactly.
Key terms
| Term | Meaning |
|---|---|
| Enzyme–substrate complex | The temporary structure formed when a substrate binds to an active site. |
| Anabolic reaction | A reaction building larger molecules from smaller ones. |
| Catabolic reaction | A reaction breaking larger molecules into smaller ones. |
| Denaturation | Loss of the specific 3D shape of a protein. |
| Initial rate | The rate at the start of a reaction. |
| Colorimeter | An instrument measuring light absorbed by a solution. |
| Calibration curve | A graph of known concentrations against readings. |
Avoid these mistakes
1
Saying enzymes are used up in reactions — they are unchanged.
2
Using lock-and-key as the current model — induced fit is accepted.
3
Saying the active site is formed by the whole enzyme — only a few amino acids form it.
4
Comparing rates over long times — use the initial rate because substrate varies later.
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