Enzymes

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Notes

What are Enzymes?

Enzymes are **biological catalysts** that speed up chemical reactions without being changed or used up.
They are **proteins** made of **amino acids**.
Enzymes are necessary for life — they maintain metabolic reaction speeds at a rate that sustains life.
Without digestive enzymes, digesting a meal would take 2–3 weeks; with enzymes it takes ~4 hours.

Enzyme Action & Specificity

Each enzyme has an **active site** with a specific **3D shape** complementary to its substrate.
This is the **lock and key hypothesis**: only a substrate of the correct shape fits the active site.
When substrate binds, an **enzyme-substrate complex** forms; the reaction occurs and **products** are released.
The enzyme is **unchanged** and can catalyse further reactions.
Enzymes and substrates **collide randomly** in solution; a successful collision forms the complex.

Effect of Temperature

Enzymes have an **optimum temperature** (e.g. 37 °C in the human body).
From 0 °C to optimum, increasing temperature gives molecules more **kinetic energy**, increasing collisions and rate of reaction.
Above the optimum, **bonds holding the enzyme's shape break** — the enzyme **denatures** (active site changes shape).
Denaturation is **irreversible**; substrate can no longer fit the active site, so activity stops.
Low temperatures **do not denature** enzymes; they only slow the reaction.

Effect of pH

Most enzymes have an optimum pH near **7**, but some work best at acidic pH (e.g. **pepsin** in stomach, pH 2) or alkaline pH (e.g. in duodenum, pH 8–9).
Extreme pH breaks bonds in the protein, **denaturing** the enzyme and altering the active site.
Moving away from optimum pH reduces activity; far from optimum, denaturation occurs and activity stops.

Enzyme Investigations

**Temperature investigation**: Heat starch solution to set temp, add amylase, sample every minute into iodine. Time until iodine stays orange-brown (no starch) measures activity.
**pH investigation**: Mix amylase, buffer (pH), and starch; sample every 10 s into iodine. Shorter time to orange-brown = faster reaction at that pH.
**Iodine test**: Iodine turns **blue-black** in presence of starch; orange-brown means starch is fully digested.
Control variables: temperature, pH, enzyme/substrate concentration.

Key Terms

**Catalyst**: substance that speeds up a reaction without being changed.
**Substrate**: molecule(s) that bind to the enzyme's active site.
**Active site**: region on the enzyme where the reaction occurs.
**Denaturation**: irreversible loss of enzyme shape (active site) due to heat or extreme pH.
**Optimum**: condition (temperature or pH) at which an enzyme works fastest.

Lock and key model: substrate fits into the enzyme's active site to form an enzyme-substrate complex.

Graph showing rate of reaction vs temperature: rises to optimum then falls sharply as enzyme denatures.

Graph showing rate of reaction vs pH: peaks at optimum pH, declines sharply at extremes due to denaturation.

Practice questions

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1.Which of the following statements about enzymes is correct?
Easy
- AEnzymes are used up during chemical reactions.
- BEnzymes are biological catalysts that speed up reactions without being changed.
- CEnzymes are made of carbohydrates.
- DEnzymes increase the activation energy of reactions.
2.All enzymes are composed of which type of biological molecule?
Easy
- ACarbohydrates
- BLipids
- CProteins
- DNucleic acids
3.The diagram shows the lock and key model of enzyme action. Which part of the enzyme is represented by the 'lock'?
Medium
- AThe substrate
- BThe active site
- CThe product
- DThe enzyme-substrate complex
4.A student investigates the effect of temperature on the activity of amylase. Which statement describes what happens to the enzyme at 80°C?
Medium
- AThe enzyme works faster due to increased kinetic energy.
- BThe enzyme is denatured and its active site changes shape.
- CThe enzyme is reversibly inhibited.
- DThe enzyme remains unchanged but the substrate is destroyed.
5.In an experiment to investigate the effect of pH on amylase activity, a student uses iodine solution as an indicator. What colour change indicates that starch is still present?
Hard
- AOrange-brown to blue-black
- BBlue-black to orange-brown
- COrange-brown to colourless
- DBlue-black to colourless
6.What is the optimum pH for the enzyme pepsin, which is secreted in the stomach?
Medium
- ApH 2
- BpH 7
- CpH 8
- DpH 10
7.The graph shows the effect of temperature on the rate of an enzyme-controlled reaction. Which statement explains the shape of the graph between 0°C and the optimum temperature?
Hard
- AEnzyme molecules gain kinetic energy and collide more frequently with substrate.
- BEnzyme molecules are denatured gradually.
- CSubstrate molecules are broken down faster by heat alone.
- DThe active site becomes more complementary to the substrate.
8.Which of the following elements are all enzymes composed of?
Easy
- ACarbon, hydrogen, oxygen, nitrogen
- BCarbon, hydrogen, oxygen, nitrogen, sulfur
- CCarbon, hydrogen, oxygen
- DCarbon, hydrogen, oxygen, nitrogen, phosphorus

Slides

What are enzymes?

Enzymes are biological catalysts that speed up chemical reactions without being changed or used up.
They are proteins made in living cells.
Enzymes are essential for life – they keep metabolic reactions at a rate that can sustain life.
For example, without digestive enzymes, digesting a meal would take 2–3 weeks; with enzymes, it takes about 4 hours.

Which of the following best describes an enzyme?

A protein that is used up in reactions
A biological catalyst that speeds up reactions without being changed
A carbohydrate that provides energy
A lipid that stores energy

How enzymes work – lock and key model

Each enzyme has an active site – a groove or pocket where the substrate binds.
The active site has a specific shape that is complementary to the shape of the substrate.
This is called the lock and key model: the enzyme (lock) only fits one substrate (key).
When the substrate binds, an enzyme-substrate complex forms, the reaction occurs, and products are released.

The part of the enzyme where the substrate binds is called the ____.

Enzyme specificity

Enzymes are specific – each enzyme only catalyses one type of reaction.
This is because the active site is complementary in shape to only one substrate.
If the substrate shape does not match the active site, the reaction cannot occur.

Why is an enzyme specific to one substrate?

Because enzymes are made of amino acids
Because the active site has a shape complementary to the substrate
Because enzymes work at high temperatures
Because enzymes are used up in reactions

Factors affecting enzyme activity – temperature

Enzymes have an optimum temperature – the temperature at which they work fastest (e.g., 37°C in the human body).
As temperature increases from 0°C to optimum, enzyme activity increases because molecules have more kinetic energy and collide more often.
Above the optimum, bonds holding the enzyme's shape break – the enzyme denatures and the active site changes shape.
Denaturation is irreversible – the enzyme stops working.

Which of these are true about temperature and enzymes?

Low temperatures denature enzymes.
At optimum temperature, enzyme activity is highest.
Denaturation is reversible.
High temperatures break bonds in the enzyme.

Factors affecting enzyme activity – pH

Each enzyme has an optimum pH (e.g., most enzymes work best at pH 7).
Enzymes in the stomach (e.g., pepsin) have an optimum pH of about 2.
Enzymes in the duodenum (e.g., pancreatic enzymes) have an optimum pH of about 8–9.
If pH is too high or too low, bonds in the enzyme break, the active site changes shape, and the enzyme denatures.

What happens to an enzyme if the pH is far from its optimum?

It works faster
It denatures and stops working
It changes substrate
It becomes more specific

Investigating enzyme activity – temperature experiment

Starch solution is heated to a set temperature, then amylase is added.
Every minute, a drop of the mixture is added to iodine solution in a spotting tile.
Iodine turns blue-black in the presence of starch; when it stays orange-brown, all starch has been digested.
The time taken for the iodine to stop turning blue-black is recorded – shorter time means faster enzyme activity.

In the temperature experiment, iodine solution is used to test for the presence of ____.

Investigating enzyme activity – pH experiment

Buffer solutions are used to maintain different pH values.
Amylase, buffer, and starch are mixed, and a drop is added to iodine every 10 seconds.
The time until iodine remains orange-brown indicates how quickly starch is digested.
The shorter the time, the better the enzyme works at that pH.

In the pH experiment, what does a shorter time for iodine to stay orange-brown indicate?

The enzyme is denatured
The enzyme is working faster
The pH is too high
The substrate is not present

Extended: Detailed enzyme action

Enzymes and substrates move randomly in solution.
When an enzyme and its complementary substrate collide, the substrate fits into the active site, forming an enzyme-substrate complex.
The reaction occurs, products are released, and the enzyme is unchanged and can be reused.
This random collision and binding is why increasing temperature (more kinetic energy) increases the rate of reaction.

The complex formed when a substrate binds to an enzyme's active site is called the ____.

Summary

Enzymes are biological catalysts that speed up reactions without being changed.
They have an active site complementary to their substrate (lock and key).
Temperature and pH affect enzyme activity; extremes cause denaturation.
Enzyme activity can be investigated using starch and iodine.

Which statements about enzymes are correct?

Enzymes are used up in reactions.
The active site shape is complementary to the substrate.
Denaturation is reversible.
Enzymes are proteins.

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