2.5 Enzymes

Question 1

What are enzymes?

Answer 1

Enzymes are biological catalysts. They are globular proteins that can speed up a biochemical reaction.

Question 2

What is the active site of an enzyme?

Answer 2

-A region of an enzyme molecule where the substrate molecule binds.

-Only one type of substrate fits into the active site. This is what is meant by enzyme-substrate specificity: one enzyme can only catalyze one type of reaction.

Question 3

What is the active site a result of?

Answer 3

-The folding of the polypeptide chain(s).

-The resulting (3D) shape that is formed by the polypeptide chain forms the active site, which is where the substrate interacts with the enzyme.

Question 4

Diagram showing a substrate entering the active site of an enzyme.

Answer 4

Question 5

What happens when the substrate binds to the enzyme’s active site?

Answer 5

-The enzyme changes shape slightly.

-This ‘induced fit’ results in tighter binding of the substrate to the active site.

Question 6

Structure of enzymes

Answer 6

Large globular polypeptides with a tertiary or quaternary structure.

Question 7

When describing an enzyme structure, don’t forget to mention an ___

Answer 7

Active site and specificity of enzymes as well as the fact that all enzymes are globular proteins.

Question 8

The reaction that converts the substrate into the products takes place in ___

Answer 8

The active site of the enzyme.

Question 9

When the substrate enters the active site, it triggers a change in ___

Answer 9

-The three-dimensional shape of the enzyme that allows a tighter fit.

-This is called an induced fit and is possible because of the flexibility of the protein molecules that make up the enzyme.

Question 10

When the enzyme and substrate(s) fit together tightly, the enzyme induces ___

Answer 10

The weakening of bonds within the molecules of the substrate(s), thus reducing the activation energy needed for the reaction.

Question 11

When the enzyme-catalysed reaction is completed, ___

Answer 11

The products are released from the enzyme.

Question 12

The induced fit theory has replaced the ___

Answer 12

Lock and key theory

Question 13

According to the lock and key hypothesis, ___

Answer 13

-The enzyme functions as a lock while the substrate functions as a key.

-There is a perfect matching between both components.

Question 14

The induced fit model suggests that ___

Answer 14

-A substrate is capable of inducing a change of the active site that will enable the enzyme to perform its catalytic function.

-The active site can slightly change its shape to fit a substrate.

Question 15

Diagram showin the induced fit model (how a substrate enters the active site of an enzyme, the enzymatic reaction occurs and the products are released from the enzyme.)

Answer 15

Question 16

Key points about enzymes

Answer 16

1) An enzyme is a globular protein that acts as a catalyst for a (bio)chemical reaction.

2) An active site is the region of an enzyme to which substrates bind and where reactions are catalysed.

3) Enzymes are specific: they catalyse only one type of reaction.

Question 17

What is an active site?

Answer 17

A region of an enzyme molecule where the substrate molecule binds.

Question 18

What does enzyme–substrate specificity mean?

Answer 18

Enzymes can only catalyse one type of reaction.

Question 19

How do enzymes speed up a reaction?

Answer 19

By lowering the activation energy of that reaction.

Question 20

Diagram showing how enzymes lower the activation energy of a reaction

Answer 20

Question 21

Define activation energy

Answer 21

The minimum energy that reacting particles should possess in order for a reaction to occur.

Question 22

What is a catalytic reaction?

Answer 22

When an enzyme converts the substance into products, such as when amylase hydrolyses starch to produce di- and monosaccharides.

Question 23

What kind of motion does enzyme catalysis involve?

Answer 23

Molecular motion and the collision of substrates with the active site.

Question 24

The motion of atoms and molecules in a liquid is very ___

Answer 24

Random and depends on the tempeture.

Question 25

In humans, where the body temperature is around 37   °C, the movement of atoms and molecules is ___

Answer 25

Rapid and many millions of collisions take place every second.

Question 26

When is catalysis of a reaction only possible?

Answer 26

If the substrate and active site happen to be correctly aligned when they collide to allow binding to take place.

Question 27

Exothermic or exorgonic reaction

Answer 27

A reaction where product formation is associated with release of energy, usually in the form of heat.

Question 28

Enrothermic or endergonic reaction

Answer 28

A reaction where product formation is associated with absorption of energy, usually in the form of heat.

Question 29

All enzymes are proteins. Therefore, they can be ___

Answer 29

-Denatured and lose their catalytic properties.

-The bonds between different amino acids break, causing structural changes to the active site of an enzyme.

Question 30

What factors can cause denaturation?

Answer 30

1) Temperature

2) pH

3) Substrate concentration

4) Enzyme concentration

Question 31

Effect of temperature on enzymes

Answer 31

-Enzymes are sensitive to temperature changes.

-When temperature is low, molecules tend to move slowly.

-The chance of collision between substrate and enzyme molecules is also low.

-When temperature rises, molecules move more rapidly and it is more likely that they will collide with each other.

-Each enzyme has an optimum temperature in which the rate of enzymatic reaction is the highest.

-The optimal temperature for human enzymes is around 37   °C.

-Interestingly, for enzymes in the bacterium Thermus thermophilus that lives in hot springs the optimal temperature is 65   °C.

-If the temperature is higher than optimal, an enzyme can be denatured.

-As a result, the rate of enzymatic reaction rapidly decreases.

Question 32

Graph showing effect of temperature on enzymes

Answer 32

Question 33

Where do most enzymes have their optimal temperature?

Answer 33

Around 40°C

Question 34

Effect of substrate concentration on enzyme

Answer 34

-When the substrate concentration is low, there are more enzyme molecules available than substrate.

-The rate of reaction is relatively low.

-Increasing the substrate concentration causes more chances of collision between substrate and enzyme molecules.

-Hence, the rate of enzymatic reaction rises gradually.

-However, this increase is halted when all active sites are occupied by substrate molecules.

-After this point, adding more substrate does not affect the rate of reaction. (V max is the maximum rate possible).

-Once V max is reached (the maximal rate of reaction), substrates have to wait for enzyme active sites to become available before they can bind.

Question 35

Graph showing the effect of substrate concentration on enzyme activity.

Answer 35

Question 36

Effect of pH on enzymes

Answer 36

-Enzymes work in different environments.

-Examples include the stomach, where the pH is very low (pH = 2), and the small intestine, which is more alkaline (pH = 7.5).

-Figure 3 shows two enzymes: pepsin and trypsin.

-Pepsin is an enzyme produced in the stomach, whereas trypsin is formed in the small intestine.

-Each enzyme has its own optimum pH at which its activity is the highest.

Question 37

Effect of pH on pepsin (red line) and trypsin (blue line)

Answer 37

Question 38

How can extreme pH values denature an enzyme?

Answer 38

-By altering the three-dimensional structure of its active site.

-If pH is lower or higher than the optimal, the rate of reaction gradually decreases.

Question 39

Drawing graphs to show the expected effects of temperature, pH and substrate concentration on the activity of enzymes.

Answer 39

-When sketching graphs you should remember the shape of each curve.

-For temperature, the rate of enzymatic reaction drastically drops after reaching the optimal temperature.

-When you sketch a rate of reaction for different substrate concentrations don’t forget to draw a plateau phase in which the rate remains constant.

Question 40

Denaturation is an ___ change

Answer 40

Irreversible

Question 41

Denaturation destroys the ___

Answer 41

-Tertiary or quaternary conformation of a protein.

-In some cases, when the temperature is high enough or the pH is extreme, the secondary structure of a protein can be altered.

Question 42

When there is only a minor temperature increase or change in pH, it is possible that ___

Answer 42

The denaturation is still reversible and the protein can fold back to its original and functional conformation.

Question 43

Diagram denaturation of a protein a protein (loses its conformation with denaturation. The beta sheets and alpha helices lose their form, and the protein reverts to a primary conformation. This means there is no longer a functional active site.)

Answer 43

Question 44

Examples of enzymes in every-day life.

Answer 44

1) Proteases are used to produce milk powders for babies and carbohydrase helps to manufacture corn syrup.

2) Enzymes are widely used in industries such as medicine, fuels, agriculture and even production of textiles.

3) Brewing, baking and cheese-making are the most common processes in which enzymes play a crucial role.

Question 45

What needs to happen for enzymes to be made mode stable and useful?

Answer 45

They need to be immobilised

Question 46

What is immobilisation?

Answer 46

The process of attaching an enzyme to a material so that its movements are restricted.

Question 47

Advantages of immobilisation

Answer 47

-For example, if enzymes are not immobilised, they are often present in the final product, which restricts the concentration that can be used to process food for human consumption to avoid adverse effects.

-The use of immobilised enzymes permits higher concentrations of enzymes to be used, allowing a faster rate of reaction.

-Moreover, immobilisation of enzymes allows immediate separation of the enzymes from the reaction mixture, which allows them to be recycled, reducing production costs.

Question 48

Use of enzymes in industry

Answer 48

-Protein hydrolysis

-Detergent

-Food processing

Question 49

Use of enzymes in the environment

Answer 49

-Degredation of residual waste

-Detoxification of toxic substances

Question 50

Use of enzymes in agriculture

Answer 50

-Feed processing

-Animal feed additive

-Agroprocessing

Question 51

What industries can enzymes be used in?

Answer 51

-Industry

-Environment

-Agriculture

Question 52

What is lactose?

Answer 52

-Lactose is a naturally occurring disaccharide present in mammalian milk and dairy products such as cheese and yogurt.

-It consists of two monosaccharides: glucose and galactose.

Question 53

What is lactase?

Answer 53

An enzyme that can break down lactose into monosaccharides.

Question 54

Diagram of lactase breaking down lactose into galactose and glucose

Answer 54

Question 55

What does lactase break down lactose into?

Answer 55

Galactose and glucose

Question 56

Lactose intolerance: Lack of lactase causes ___

Answer 56

Lactose to build up and remain in the digestive system, where it is fermented by bacteria.

Question 57

Lactose-intolerant people should consume lactose-free products to avoid ___

Answer 57

Gastric problems caused by the excess of undigested lactose in the alimentary gut.

Question 58

Advantages of the production of lactose-free products

Answer 58

-No ill effects after consumption

-Quicker fermentation, for example, in yogurt production as bacteria ferment glucose and galactose more readily than lactose

-Sweeter tasting milk (glucose and galactose are sweeter tasting than lactose).

Question 59

How are lactose-free products produced?

Answer 59

-By adding the enzyme lactase (usually made on industrial scale from the fungus Aspergillus niger) to milk.

-Lactase breaks down lactose into its constituent monomers, glucose and galactose.

Question 60

Explain how lactase can be immobilized

Answer 60

-Lactase can be immobilized in alginate beads, while the milk is allowed to flow past

-With this method, no lactase ends up in the final dairy product, making it better for consumption.

Question 61

Diagram showing the production of lactose-free milk by using immobilized enzymes.

Answer 61

Question 62

Yogurt is tolerated better than milk by lactose-intolerant people because ___

Answer 62

Lactase from the bacteria in yogurt helps digest the lactose.

Question 63

Immobilized enzymes are used in industry because ___

Answer 63

Enzymes can be recycled and used several times.

Question 64

2.5.6

Question 65

Enzyme and substrate: reaction steps - diagram

Answer 65

Question 66

Where is lactase produced?

Answer 66

By cells in the lining of the small intestine.