3.1.13 · Enzymes
Enzymes — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 7 parts Lesson 3 of 17 Unit quiz
Lesson summary
3.1.1 Enzymes — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe enzymes as globular protein catalysts and give examples of intracellular and extracellular enzymes.Biological catalysts — they function in living systems and speed up reactions without being used…
- Globular proteins
- Active site
- Metabolic pathways
- Essential for life
- Denaturation
- Specificity
Part 1 of 5What enzymes are
Biological catalysts
- Biological catalysts they function in living systems and speed up reactions without being used up or changed.
Globular proteins
- Globular proteins compact proteins with a specific shape.
Active site
- Active site the part of the enzyme where the substrate binds.
Part 2 of 5Importance of enzymes
1Metabolic pathways
- Metabolic pathways enzymes control pathways as cascades of reactions.
2Essential for life
- Essential for life virtually every metabolic reaction is catalysed by an enzyme.
Part 3 of 5Intracellular and extracellular enzymes
| Feature | Intracellular | Extracellular |
|---|---|---|
| Where it acts | Inside the cell | Outside the cell after secretion |
| Example | Catalase | Amylase |
| Function | Breaks down toxic hydrogen peroxide to water and oxygen | Digests starch to sugars in the gut |
| Reason | H2O2 is a harmful by-product of metabolism | Food macromolecules are too large to enter cells |
Part 4 of 5Enzymes as proteins
Denaturation
- Denaturation anything that denatures a protein, such as extreme heat or pH, also denatures an enzyme.
Specificity
- Specificity each enzyme acts on a specific substrate because of the shape of its active site.
Part 5 of 5More from the specification
More from the specification
Enzyme Action and the Induced Fit Hypothesis
- Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy without being consumed;
- the induced fit hypothesis states that the active site undergoes conformational changes upon substrate binding to form a stable enzyme-substrate complex.
Catalytic Mechanism and Activation Energy Reduction
- By binding substrate molecules in an optimal spatial orientation and straining specific covalent bonds, an enzyme provides an alternative reaction pathway that significantly reduces the activation energy required for the transition state.
Quantitative Enzyme Assays Using a Colorimeter
- Reaction rates are determined experimentally by measuring changes in light absorbance or percentage transmission of a reaction solution over time using a calibrated colorimeter;
- for example, monitoring the disappearance of starch in an amylase-starch-iodine solution.
Lock-and-key hypothesis
- Rigid enzyme and substrate fit exactly.
Iodine solution
- Orange-brown reagent that turns blue-black with starch.
Complex
- Two or more molecules joined, such as an antibody and antigen.
Hypothesis
- A testable prediction with a scientific explanation.
Colorimeter
- An instrument that measures light absorbed by a solution.
Transmission
- The amount of light passing through a sample.
Absorbance
- A measure of the light absorbed by a sample.
Activation energy
- The energy needed for a substrate to become unstable enough to react.
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.
Key terms
| Term | Meaning |
|---|---|
| Catalyst | A substance that speeds up a reaction without being used up. |
| Enzyme | A biological catalyst, a globular protein. |
| Active site | The region of an enzyme where the substrate binds. |
| Metabolic pathway | A sequence of enzyme-controlled reactions. |
| Intracellular enzyme | An enzyme working inside the cell. |
| Extracellular enzyme | An enzyme working outside the cell. |
Avoid these mistakes
1
Forgetting that enzymes are proteins — heat and extreme pH denature them.
2
Saying enzymes are consumed in the reaction — they are unchanged.
3
Saying all enzymes work outside cells — many are intracellular.
4
Saying catalase makes hydrogen peroxide — it breaks it down.
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