What is the fundamental difference between the Lock-and-Key and Induced-Fit hypotheses?

The Lock-and-Key model assumes both the enzyme and substrate are rigid and fit perfectly from the start. The Induced-Fit model suggests the active site is flexible and changes shape slightly upon substrate entry to create an ideal binding arrangement.

How do intracellular enzymes differ from extracellular enzymes in terms of function?

Intracellular enzymes, like catalase, operate within the cell to manage internal metabolic processes and protect against toxins. Extracellular enzymes, like amylase, are secreted to work outside the cell, primarily to break down large nutrients for absorption.

What is the difference between an anabolic and a catabolic reaction?

Anabolic reactions involve building larger, complex molecules from smaller units, which usually requires energy. Catabolic reactions involve breaking down large molecules into smaller, simpler ones, typically releasing energy in the process.

Why is it incorrect to say that an enzyme has been 'killed' by high temperatures?

Enzymes are non-living protein molecules, not organisms. High temperatures cause them to 'denature,' which means the kinetic energy breaks the internal bonds maintaining their 3D shape, rendering them non-functional.

What is a common mistake when interpreting a graph of reaction rate vs. substrate concentration?

Students often assume the rate continues to increase indefinitely; however, the rate eventually plateaus at a 'saturation point.' At this point, the enzyme concentration becomes the limiting factor because all active sites are occupied.

What error occurs if you calculate the rate of reaction using data from the end of an experiment?

The rate at the end is slower due to substrate depletion and possible product accumulation. To determine the true catalytic power, you must calculate the 'initial rate' using a tangent at the start of the reaction.

Define the term 'Active Site'.

The active site is a specific region on the surface of an enzyme, formed by its tertiary structure, where the substrate binds. Its unique shape and chemical environment are complementary to a specific substrate, enabling catalysis.

What is 'Activation Energy' ($E_a$)?

Activation energy is the minimum amount of energy required for a chemical reaction to occur. It represents the energy barrier that reactants must overcome to reach the transition state and transform into products.

What is meant by 'Denaturation'?

Denaturation is the permanent alteration of a protein's 3D structure due to external stress like heat or pH. In enzymes, this changes the shape of the active site so that it can no longer bind to its substrate.

How does temperature affect the frequency of successful collisions in an enzyme reaction?

Increasing temperature increases the kinetic energy of both enzymes and substrates, causing them to move faster. This leads to more frequent and more energetic collisions, increasing the likelihood of forming enzyme-substrate complexes.

Enzymes: Roles & Modes of Action | Pearson Edexcel International A-Level Biology

Revision Notes

International A-Level

Pearson Edexcel

Biology

2. Membranes, Proteins, DNA & Gene Expression

Enzymes: Roles & Modes of Action

Summary

Enzymes are biological catalysts, primarily globular proteins, that accelerate metabolic reactions by lowering activation energy. Their function is defined by a highly specific active site whose shape is determined by complex tertiary protein folding, allowing for precise substrate binding through models such as the Lock-and-Key and Induced-Fit hypotheses.

1. Definition & Core Concepts

Biological Catalysts: Enzymes are specialized proteins that increase the rate of chemical reactions within living organisms without being consumed or permanently altered in the process.
Globular Protein Structure: Most enzymes are globular proteins with complex tertiary structures; the specific folding of the polypeptide chain creates a unique 3D environment known as the active site.
The Active Site: This is a small, specific region of the enzyme where the substrate binds; its shape is complementary to the substrate, ensuring that only specific molecules can undergo catalysis.
Enzyme-Substrate Complex: When a substrate successfully collides with and binds to the active site in the correct orientation, it forms a temporary intermediate structure that facilitates the reaction.

2. Underlying Principles: Activation Energy

Activation energy graph showing two curves: a high blue curve representing the reaction without an enzyme and a lower red curve representing the reaction with an enzyme, illustrating the reduction in energy required.

3. Methods & Techniques: Models of Enzyme Action

Lock-and-Key Hypothesis: This early model suggests that the enzyme and substrate are rigid structures that fit together perfectly and precisely, much like a physical key fits into a specific lock.
Induced-Fit Hypothesis: A more modern refinement where the enzyme's active site is flexible; as the substrate enters, the active site undergoes a conformational change to wrap more tightly around the substrate.
Ideal Binding: The induced-fit model explains how the enzyme can maximize its catalytic power by molding itself to achieve the most effective binding arrangement for the reaction to occur.
Specificity: Both models emphasize that the unique shape of the active site, determined by the primary sequence of amino acids, ensures that only one specific substrate (or a small group of similar ones) can bind.

4. Key Distinctions: Intracellular vs. Extracellular

5. Exam Strategy & Tips