Ch 1 · 3.1 Mode of action of enzymes · AS & A Level Biology 9700

Topic 1 of 2 in Enzymes — AS & A Level Biology 9700 — 77 lessons.

Introduction

Enzymes catalyse essentially every metabolic reaction in a cell, and their catalytic power comes directly from their tertiary structure as globular proteins. Building on the levels of protein structure, this lesson explains how the active site forms an enzyme-substrate complex, why this lowers activation energy, how the induced-fit hypothesis refines the older lock-and-key model, and how the same catalytic principle applies both inside and outside the cell. It ends with the two standard practicals for following a reaction, using catalase and amylase. (Syllabus section 3.1.)

Key ideas — Enzymes as globular protein catalysts

Enzymes are globular proteins that act as biological catalysts: they increase the rate of specific metabolic reactions without being used up or permanently changed themselves. Their compact, soluble tertiary structure folds so that a small region, the active site, is exposed on the surface in a precise three-dimensional shape.

🧑‍🏫 Teacher's explanation

Think of an enzyme as a highly specialized tool, like a specific shaped key, designed to do one job very quickly. In your body, most chemical reactions are too slow to sustain life on their own. Enzymes act as biological catalysts, meaning they speed up these reactions by lowering the 'energy barrier' required to get the reaction started.

📖 Definitions

🔍 Worked example — Easy

A student uses a colorimeter to measure the progress of an enzyme-catalyzed reaction. The absorbance of the solution decreases from 0.85 to 0.25 over 5 minutes. Calculate the rate of decrease in absorbance per minute.

🔍 Worked example — Standard

Compare the lock-and-key hypothesis with the induced-fit hypothesis regarding the shape of the active site.

🔍 Worked example — Exam-style

Explain why an increase in temperature from 20circC to 40circC increases the rate of an enzyme-catalyzed reaction, but an increase from 60circC to 80circC decreases it.

⚠️ Common errors

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🎯 Exam question

(a) Describe the relationship between substrate concentration and the rate of reaction up to a concentration of 0.08 mol/dm^-3. [2] (b) Explain why the rate of reaction remains constant between 0.16 mol/dm^-3 and 0.32 mol/dm^-3. [2] [4 marks]

Key ideas — Active site, enzyme-substrate complex and activation energy

The active site is a region on the enzyme's surface, formed by the folding of the tertiary (and sometimes quaternary) structure, into which a specific substrate molecule can bind. When the substrate binds, an enzyme-substrate complex (ESC) forms, held together briefly by bonds such as hydrogen and ionic bonds between the substrate and R groups lining the active site. Formation of the ESC strains particular bonds within the substrate and orients reacting groups precisely, lowering the activation energy needed for the reaction to proceed; the products then form and leave the active site, which is free to bind another substrate molecule.

🧑‍🏫 Teacher's explanation

Think of an enzyme as a specialized tool, like a specific wrench designed for one exact bolt. The enzyme doesn't just 'hold' the substrate; it actively helps the reaction happen by making it easier to start.

📖 Definitions

🔍 Worked example — Easy

State the difference between an intracellular enzyme and an extracellular enzyme, providing one example of each.

🔍 Worked example — Standard

Explain how the formation of an enzyme-substrate complex (ESC) leads to an increase in the rate of reaction.

🔍 Worked example — Exam-style

A researcher uses a colorimeter to measure the progress of an enzyme-catalyzed reaction where the substrate is colorless and the product is blue. Describe how the colorimeter can be used to determine the rate of reaction.

⚠️ Common errors

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🎯 Exam question

Key ideas — Enzyme specificity

Because the active site has a shape (and chemical environment) complementary to only one substrate, or a small group of closely related substrates, each enzyme is specific: it catalyses only the reaction of the substrate that fits, and has no catalytic effect on molecules of a different shape.

🧑‍🏫 Teacher's explanation

Think of an enzyme as a highly specialized tool, like a specific key designed for a single lock. Even if another key looks similar, if the teeth don't match perfectly, the lock won't turn. In biology, enzymes are large, complex, folded proteins called globular proteins. Their 'function' depends entirely on a tiny, specifically shaped pocket called the active site.

📖 Definitions

🔍 Worked example — Easy

An enzyme-catalysed reaction shows a rate of 0.5 μ mol min^-1 at 37^° C. If the concentration of substrate is doubled, the rate increases to 0.9 μ mol min^-1. Calculate the percentage increase in the rate of reaction.

🔍 Worked example — Standard

Explain why a change in the primary structure of an enzyme (the sequence of amino acids) might lead to a loss of catalytic activity.

🔍 Worked example — Exam-style

A student uses a colorimeter to measure the rate of an enzyme-controlled reaction where a clear substrate turns blue as it reacts. The absorbance increases from 0.10 to 0.50 in 4 minutes. Calculate the rate of absorbance increase per minute.

⚠️ Common errors

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🎯 Exam question

Key ideas — Lock-and-key and induced-fit hypotheses

The lock-and-key hypothesis proposes that the active site is a rigid shape that already exactly matches the substrate, like a key fitting a lock, with no change in enzyme shape on binding. The induced-fit hypothesis instead proposes that the active site is flexible: it is only roughly complementary to the substrate before binding, and the presence of the substrate causes the active site to change shape slightly, moulding itself around the substrate to form a tighter fit. Induced fit better explains why binding also strains substrate bonds and lowers activation energy, and is the model supported by current evidence.

🧑‍🏫 Teacher's explanation

Think of an enzyme as a specialized tool designed to do a specific job. In the old 'lock-and-key' model, imagine a rigid metal key fitting into a rigid padlock; if the key isn't the exact shape, nothing happens. However, biology is rarely that rigid. The 'induced-fit' model is more like a high-tech memory foam glove. When you put your hand (the substrate) into the glove (the enzyme), the glove slightly changes shape to wrap tightly around your fingers. This tight hug is crucial because it puts physical stress on the substrate, making it easier for the chemical reaction to occur.

📖 Definitions

🔍 Worked example — Easy

A student is asked to distinguish between the lock-and-key and induced-fit hypotheses. Describe the primary difference regarding the shape of the active site.

🔍 Worked example — Standard

Explain how the formation of an enzyme-substrate complex leads to an increase in the rate of reaction.

🔍 Worked example — Exam-style

An enzyme-catalyzed reaction shows a rate of 0.5 mol L^-1 s^-1 at 37^° C. If the activation energy is lowered by 40 kJ/mol due to the enzyme, calculate the percentage reduction in activation energy.

⚠️ Common errors

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🎯 Exam question

Key ideas — Intracellular and extracellular enzymes

Intracellular enzymes act within the cell that made them. Catalase is an intracellular enzyme found in peroxisomes and the cytoplasm that rapidly breaks down hydrogen peroxide, a toxic by-product of metabolism, into water and oxygen, protecting the cell from oxidative damage. Extracellular enzymes are synthesised inside a cell but secreted to act outside it. Amylase is an extracellular enzyme, secreted by the salivary glands and pancreas into the mouth and small intestine, where it hydrolyses starch into maltose outside the cells that produced it, since large starch molecules cannot cross the cell surface membrane to be digested internally.

🧑‍🏫 Teacher's explanation

Think of enzymes as biological tools. Some tools are used inside a workshop to build things (intracellular), while others are sent out to a construction site to work on materials that are too big to bring inside the workshop (extracellular).

📖 Definitions

🔍 Worked example — Easy

A student is investigating the effect of temperature on the rate of reaction of amylase. If the rate of reaction at 20^° C is 0.5 units/min and at 40^° C it is 2.5 units/min, calculate the factor by which the rate increased.

🔍 Worked example — Standard

Explain why amylase is considered an extracellular enzyme in the human digestive system.

🔍 Worked example — Exam-style

Compare the lock-and-key hypothesis with the induced-fit hypothesis regarding enzyme specificity.

⚠️ Common errors

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🎯 Exam question

(a) Explain how the colorimeter can be used to measure the progress of this reaction. [2] (b) State why measuring the disappearance of a substrate is an alternative method to measuring the formation of a product. [1] [3 marks]

Key ideas — Following the progress of a reaction

The rate of an enzyme-catalysed reaction is followed by measuring either the appearance of a product or the disappearance of a substrate at regular time intervals. Plot the amount against time; the initial rate is the gradient of a tangent drawn at time zero, because the rate falls as substrate is used up.

🧑‍🏫 Teacher's explanation

Think of an enzyme-catalyzed reaction like a factory assembly line. The substrate is the raw material, and the product is the finished good. To see how fast the factory is working, you can either count how many finished goods come off the belt every minute, or you can count how quickly the pile of raw materials is shrinking.

📖 Definitions

🔍 Worked example — Easy

In an experiment measuring the formation of oxygen using catalase, the volume of gas produced (V) at time (t) is given by the data below. Calculate the rate of reaction at t = 30 s if the volume increases from 2.0 cm^3 at t = 0 to 5.0 cm^3 at t = 30 s. Assume a linear increase for this interval.

🔍 Worked example — Standard

A student uses amylase to digest starch. They use iodine to test for the presence of starch. At t = 120 s, the iodine turns blue-black. At t = 150 s, the iodine remains orange. Calculate the rate of disappearance of starch using the 'end point' method (Rate = 1/t).

🔍 Worked example — Exam-style

A graph shows the volume of oxygen produced by catalase over time. The curve is steep at t=0 and flattens out as t increases. Explain why the rate of reaction decreases over time.

⚠️ Common errors

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🎯 Exam question

(a) Describe how the student could use a colorimeter to measure the rate of a different enzyme-catalyzed reaction that produces a colored product. [3]

Key ideas — Using a colorimeter

When a reaction involves a colour change, a colorimeter follows it continuously without stopping the reaction. It shines light of one colour (chosen with a filter of the complementary colour) through a cuvette and records absorbance (or transmission).

🧑‍🏫 Teacher's explanation

Imagine you are watching a crowd of people walk through a narrow doorway. If you want to know how fast they are moving, you could try to guess by looking at the crowd every minute, but you might be wrong because human eyes are subjective. Instead, you could place a sensor at the door that counts every single person passing through. A colorimeter is that sensor, but for chemistry.

Ch 1 · 3.1 Mode of action of enzymes · AS & A Level Biology 9700

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