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

The Lock-and-Key model assumes the active site is a rigid, pre-formed shape that perfectly fits the substrate. In contrast, the Induced-Fit model suggests the active site is flexible and changes its conformation to fit the substrate more tightly upon binding.

How do intracellular enzymes differ from extracellular enzymes in terms of their functional location?

Intracellular enzymes are produced and function entirely within the cell (e.g., enzymes for DNA replication), while extracellular enzymes are secreted by the cell to catalyze reactions in the external environment (e.g., digestive enzymes in the gut).

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

Anabolic reactions build complex molecules from simpler substrates by forming new chemical bonds, whereas catabolic reactions break down complex molecules into simpler products by straining and breaking existing bonds.

Why is it incorrect to say that enzymes are 'used up' in a chemical reaction?

Enzymes are catalysts, meaning they participate in the reaction to lower activation energy but emerge chemically unchanged at the end. This allows a single enzyme molecule to be reused multiple times for subsequent reactions.

What is a common misconception regarding enzymes and the total energy change ($\Delta G$) of a reaction?

Many students mistakenly believe enzymes change the total energy released or absorbed by a reaction. In reality, enzymes only lower the activation energy barrier and have no effect on the initial energy of reactants or the final energy of products.

Why is it wrong to assume that any substrate can bind to any enzyme's active site?

Enzymes exhibit high specificity due to their tertiary structure. Only a substrate with a shape and chemical properties complementary to the specific active site can successfully bind and form an enzyme-substrate complex.

Define 'Activation Energy' ($E_a$) in the context of enzyme action.

Activation energy is the minimum amount of energy required for substrate molecules to reach a transition state where bonds can be broken or formed to create products.

What is the 'Active Site' of an enzyme?

The active site is a specific, three-dimensional region of an enzyme, formed by its tertiary protein structure, where the substrate binds and the catalytic reaction takes place.

What does the term 'Denaturation' mean for an enzyme?

Denaturation is the process where an enzyme's tertiary structure is disrupted (often by heat or pH), causing the active site to lose its specific shape and rendering the enzyme unable to bind to its substrate.

How does the formation of an Enzyme-Substrate Complex (ESC) facilitate a reaction?

The ESC brings substrates into the optimal orientation and puts physical or chemical stress on their bonds. This destabilizes the reactants, making it easier for them to reach the transition state with less energy.

Enzyme Action | Cambridge International Examinations AS-Level Biology

Revision Notes

AS-Level

Cambridge International Examinations

Biology

3. Enzymes

Enzyme Action

Summary

Enzymes are specialized globular proteins that function as biological catalysts, accelerating metabolic reactions by lowering the activation energy required for a reaction to proceed. Their function is defined by a highly specific active site that interacts with substrate molecules through mechanisms like the lock-and-key and induced-fit models, facilitating both the breakdown (catabolism) and synthesis (anabolism) of molecules without being consumed in the process.

1. Definition & Core Concepts

Biological Catalysts: Enzymes are proteins that increase the rate of chemical reactions within living organisms by providing an alternative reaction pathway with lower energy requirements.
Globular Protein Structure: The catalytic ability of an enzyme is derived from its complex tertiary structure, which folds to create a unique three-dimensional shape.
The Active Site: This is a specific region or 'pocket' on the enzyme's surface where the substrate binds; its shape and chemical environment are precisely matched to a specific substrate molecule.
Enzyme-Substrate Complex (ESC): When a substrate successfully collides with and binds to the active site, a temporary ESC is formed, allowing the reaction to occur more readily.

2. Underlying Principles: Activation Energy

Energy profile diagram showing the reduction of activation energy by an enzyme. The red curve represents the uncatalyzed reaction with a high energy peak, while the green curve represents the catalyzed reaction with a significantly lower energy peak.

3. Mechanisms of Enzyme-Substrate Interaction

Lock-and-Key Hypothesis: This model proposes that the enzyme and substrate possess rigid, complementary shapes that fit together perfectly from the start, much like a key fits into a specific lock.
Induced-Fit Hypothesis: A more modern refinement suggesting that the active site is flexible; as the substrate approaches, the enzyme undergoes a conformational change to wrap more tightly around the substrate.
Catalytic Environment: Once bound, the enzyme may use R-groups of amino acids in the active site to create an environment (e.g., specific pH or charge) that promotes the reaction.
Product Release: After the reaction is complete, the products no longer fit the active site and are released, leaving the enzyme unchanged and ready to catalyze another reaction.

4. Types of Enzymatic Reactions

5. Key Distinctions

6. Exam Strategy & Tips