PL - Enzymes

Question 1

What are enzymes?

Answer 1

Biological catalysts.

Question 2

What do enzymes do?

Answer 2

Speed up chemical reactions by acting as biological catalysts.

They catalyse many essential reactions in the bodies of living organisms.

Question 3

What type of molecule are enzymes?

Answer 3

Proteins.

Question 4

Enzymes are proteins but what do some of them have?

Answer 4

Non-protein components.

Question 5

What are substrates?

Answer 5

The molecules that enzymes act on to speed up reactions.

Question 6

What do all enzymes have?

Answer 6

An active site.

Question 7

What is the active site of an enzyme?

Answer 7

The part of the enzyme that the substrate fits into so that it can interact with the enzyme.

Question 8

Which part of the enzyme does the substrate fit into?

Answer 8

The active site.

Question 9

Describe the structure of the active site

Answer 9

It is 3D - it’s part of the tertiary structure of the enzyme protein.

Question 10

Which level of structure is the active site part of and why?

Answer 10

The tertiary structure of the enzyme protein because it is 3D.

Question 11

How do enzymes have high specificity?

Answer 11

They only work with specific substrates - usually only one.

Question 12

Why do enzymes usually work with only one substrate?

Answer 12

Because, for the enzyme to work, the substrate has to fit into the active site. Substrates are 3D, and if their shape doesn’t match the active site’s shape, then the reaction won’t be catalysed.

Question 13

What is the model that explains how a reaction will only be catalysed if the substrate is a specific shape to fit the active site called?

Answer 13

The ‘lock and key’ model.

Question 14

What does the ‘lock and key’ model explain?

Answer 14

How a reaction will only be catalysed if the substrate is a specific shape to fit the active site.

Because, for the enzyme to work, the substrate has to fit into the active site. Substrates are 3D, and if their shape doesn’t match the active site’s shape, then the reaction won’t be catalysed.

Question 15

How is the substrate held in the active site in the lock and key model?

Answer 15

By temporary bonds such as hydrogen bonds and instantaneous dipole-induced dipole forces.

Question 16

Where do the temporary bonds form between in the lock and key model when explaining how enzymes work?

Answer 16

Between the substrate and ‘R’ groups of the enzyme’s amino acids.

Question 17

What is formed when the substrate has fit into/bonded to the enzymes active site?

Answer 17

An enzyme-substrate complex.

Question 18

What conditions have to be right for enzymes to work?

Answer 18

pH and temperature.

Question 19

What has to be right about the temperature and pH for enzymes to work at their best?

Answer 19

There has to be optimal pH and optimal pH levels for enzymes to work at their best.

Question 20

Why is there an optimum temperature and pH?

Answer 20

Because in these conditions, the enzyme works best and the reaction rate is at a maximum.

Question 21

What happens to enzyme activity/reaction rate at low temperatures and why?

Answer 21

The reaction is slow because the reactant molecules have low kinetic energy.

Question 22

What happens to enzyme activity/reaction rate at higher temperatures, or at higher or lower pH values and why?

Answer 22

The reaction rate drops off dramatically because the enzyme becomes denatured. It stops working properly and can no longer effectively catalyse the reaction.

Question 23

What happens to enzymes at higher temperatures, or at higher or lower pH values?

Answer 23

The enzyme becomes denatured - it stops working properly and can no longer effectively catalyse the reaction.

The bonds that define the shape of the active site break, changing the tertiary structure of the enzyme molecule. The active site is no longer the correct shape for the substrate to fit into.

Question 24

Explain what happens to an enzyme when it becomes denatured.

Answer 24

The bonds that define the shape of the active site break, changing the tertiary structure of the enzyme molecule. The active site is no longer the correct shape for the substrate to fit into.

Question 25

When do enzymes denature?

Answer 25

When they become too hot, or are exposed to too high a concentration of acid or alkali (the pH is too low or too high).

Question 26

What level of protein structure is affected when an enzyme becomes denatured?

Answer 26

The tertiary structure as the shape of the active site changes which is part of the overall 3D structure of the enzyme protein.

Question 27

What do competitive inhibitors do?

Answer 27

Slow down the rate of reaction.

They compete with the substrate to bond to the active site, but no reaction follows. Instead they block the active site, so no substrate can fit in it.

Question 28

What are competitive inhibitors?

Answer 28

Molecules with a similar shape to the substrate that fits the enzymes active site.

Question 29

How much competitive inhibition happens depends on what?

Answer 29

The relative concentrations of competitive inhibitor and substrate.

How strongly the inhibitor bonds to the active site.

Question 30

How much inhibition will there be if there’s a lot more of the competitive inhibitor than the substrate?

Answer 30

It’ll take up most of the active sites and very little substrate will be able to get to the enzyme.

Question 31

In a very simple uncatalysed, first order reaction, what might happen to a substrate?

Answer 31

It might become a molecule of product.

S –> P

Question 32

In an uncatalysed, first order reaction, what happens to the rate of reaction as the concentration of the substrate increases?

Answer 32

It speeds up - in fact, because it is a first order reaction, doubling the concentration of S should double the rate of reaction as they are directly proportional.

Question 33

In an enzyme catalysed, first order reaction, briefly what happens to the rate of reaction as the concentration of the substrate increases?

Answer 33

At first it will increase proportionately to the increasingly level of substrate but will reach a stage where it can’t go any faster so will plateau.

Question 34

What happens to the order of an enzyme catalysed reaction as substrate is added?

Answer 34

It changes - it is first order when the substrate concentration is low but it changes to zero order when there is no longer an increase in the rate of reaction as the substrate concentration becomes greater than the enzyme concentration.

Question 35

Why does an enzyme catalysed reaction go from first order to zero order when substrate is added?

Answer 35

It is first order when the substrate level is low but towards the end the substrate concentration becomes greater than the enzyme concentration. This is because all of the enzyme active sites are involved in catalysis at any moment, so the rate has reached its maximum. The rate is no longer affected by the concentration of S so it has become a zero order reaction.

Question 36

What experiments/techniques (or equipment, etc) can be used to study rates of enzyme-catalysed reactions and how they are affected by changes in conditions such as pH, substrate concentration or temperature?

Answer 36

Gas syringe: If the reaction produces a gas, you can measure the volume evolved at regular intervals using a gas syringe.

Mass balance: If the reaction produces a gas, you can measure the mass of gas lost at regular time intervals using a mass balance.

Titrations: You can take samples at regular intervals and use titrations to find the concentration of a substrate or product in the solution.

• You would repeat all the techniques in multiple experiments with varying conditions to see the change of rate between them
• You can also use any other techniques that are used for other rate experiments to monitor the rates of enzyme-catalysed reactions.