Enzymes

Question 1

Define the term ‘Enzyme’

Answer 1

Enzymes are biological catalysts that interact with substrate molecules to facilitate chemical reactions. They are usually globular proteins.

Question 2

Define the term ‘substrate’

Answer 2

A substance which is used or acted on for or by another process or substance. For example, a reactant in an enzyme catalysed reaction.

Question 3

Define the term ‘product’

Answer 3

The result of a chemical reaction.

Question 4

Explain why enzymes are necessary to life.

Answer 4

Because most processes necessary to life involve chemical reactions which need to happen very fast so they are catalysed by an enzyme.

Question 5

Define the term ‘anabolic reactions’.

Answer 5

Anabolism refers to chemical reactions in which simpler substances are combined to form more complex molecules.
Anabolic reactions build new molecules and/or store energy, and they normally require energy.

Question 6

Define the term ‘catabolic reactions’.

Answer 6

Catabolism refers to chemical reactions that result in the breakdown of more complex organic molecules into simpler substances.
Catabolic reactions usually release energy that is used to drive chemical reactions i.e anabolic reactions.

Question 7

Explain how enzymes can affect both the structure and function of cells and whole organisms.

Answer 7

Enzymes can affect the structures in an organism because they are involved in the production of important proteins i.e. collagen. Enzymes are specific so they carry out a specific function on behalf of the cell/ organism. (bit dodge)

Question 8

Define the terms ‘digestion’.

Answer 8

The breakdown of large nutrient molecules into smaller molecules.

Question 9

Define the term “metabolism”.

Answer 9

Metabolism is a term that is used to describe all chemical reactions involved in maintaining the living state of the cells and the organism.

Question 10

Define the term “intracellular enzyme”. State where they can be found in the cell and give an example.

Answer 10

An enzyme which performs its function inside the cell that produces it. They can be found in the cytoplasm, chloroplasts or mitochondria. Catalase is an intracellular enzyme.

Question 11

Define the term extracelluar enzyme. give two examppes.

Answer 11

Enzymes that perform their function outside of the cell. Amylase and trypsin (digestive enzymes).

Question 12

State the substrates and products for the enzyme catalase.

Answer 12

Substrate: Hydrogen peroxide (H2O2)
Products: Water and oxygen
(Hydrogen peroxide is very toxic and it is the by-product of several celluar reactions and can kill cells if it builds up.)

Question 13

State the substrates and products for the enzyme amylase.

Answer 13

Catalyses the hydrolysis of starch into maltose in the mouth.
It is found in salvia and secreted into the mouth by salivary glands.

Question 14

State the substrates and products for the enzyme trypsin.

Answer 14

Trypsin catalyses the hydrolysis of peptide bonds, turning big polypeptides into smaller polypeptides. It is produced in the pancreas and secreted into the small intestine.

Question 15

Explain the roles of extracellular enzymes in general.

Answer 15

Extracellular enzymes are released from cells to breakdown large nutrient molecules into smaller molecules. This is so the smaller molecules can enter the cell to meet its demand for nutrients.

Question 16

Summarise the digestion of starch.

Answer 16

Starch polymers are partially broken down by amylase into maltose (a disaccharide).
Maltose is then broken down into glucose by maltase (a monosaccharide).

Question 17

Define the term active site.

Answer 17

The area of an an enzyme with a shape complementary to a specific substrate, allowing the enzyme to bind to a substrate with specificity.

Question 18

Define the term complementary shape.

Answer 18

It means that the shape of the Active Sites of Enzymes are exactly Complementary to the shape of the Substrate - they fit together perfectly. This is called lock and key hypothesis.

Question 19

Define the term specific.

Answer 19

It means that the active site is only complementary to one type of substrate.

Question 20

Explain why an enzyme only catalyses one type of reaction.

Answer 20

Because for an enzyme to work the substrate has to fit into the active site so its shape has to be complementary. If it the substrate shape doesn’t match the active site the reaction won’t be catalysed.

Question 21

State the sequence of events in an enzyme-controlled reaction.

Answer 21

• A substrate binds to the active site forming an enzyme-substrate complex which lowers activation energy.
• Products are then produced

Question 22

Describe the “lock and key” hypothesis of enzyme action.

Answer 22

The substrate fits into the enzyme in the same a key fits into a lock. The active site and substrate have a complementary shape, they are specific to eachother.

Question 23

Describe the “induced-fit” hypothesis of enzyme action.

Answer 23

This helps to explain why a substrate might bind to an enzyme, but no reaction is catalysed.
The shape of active sites are not exactly complementary, but change shape in the presence of a specific substrate to become complementary. This happens because interactions between the substrate and enzyme induce changes in the enzymes tertiary structure.
However if the substrate is not the correct one for the enzyme no enzyme-substrate complex will form.

Question 24

Suggest how the R-groups of amino acids are involved in catalysing reactions.

Answer 24

The R-groups contain the features which are responsible for the tertiary structure in proteins. The tertiary structure determines the active site’s shape.

Question 25

Define the term activation energy.

Answer 25

The energy required to initiate a reaction.

Question 26

Define the term rate of reaction.

Answer 26

The speed of a reaction proceeds.

Question 27

Draw an energy-level graph to show how a reaction progresses with and without an enzyme present (the transition state model).

Answer 27

With an enzyme the peak is lower and it declines faster because the reaction is completed faster.
Without an enzyme the peak is higher because the activation energy is higher.

Question 28

State what the presence of an enzyme does to the activation energy for the reaction and explain why this increases the rate of reaction.

Answer 28

It lowers the activation energy because an enzyme substrate complex is formed. This lowers the activation energy because:
- If two substrate molecules need to be joined, attaching to the enzyme holds them close together reducing any repulsion between the molecules so they can bond more easily.
-If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate. This strain means the substrate molecule breaks up more easily.
A lower activation energy increases the rate of reaction because the reaction can happen at a lower temp than it could without the enzyme.

Question 29

State 5 factors that affect the rate of an enzyme controlled reaction.

Answer 29

• Temperature
• Surface area of substrate
• Concentration of substrate
• Concentration of enzyme
• PH

Question 30

Draw a graph showing how the total amount of product produced from an enzyme-controlled reaction changes over time following the start of an experiment. Explain the shape of the graph and explain the significance of the gradient of the line at any one point.

Answer 30

It has a steep incline initially due to a high concentration of substrate but starts to plateau as the substrate concentration decreases.

Question 31

Draw a graph showing how temperature affects the initial rate of an enzyme-controlled reaction.

Answer 31

The graph has a steep incline until it reaches the optimum temperature, then it sharply declines until it stops.

Question 32

Explain how temperature affects the rate of reactions.

Answer 32

• A higher temperature means the molecules have more kinetic energy so they move much faster
• This means substrate molecules are more likely to sucessfully collide with enzymes to form more enzyme-substrate complexes which result in a reaction.
• The rate increases until the enzyme reaches its optimum temperature where the rate is fastest.
• Past this temperature the enzyme’s molecules vibrate more and these vibrations break some of the bonds that hold the enzyme in shape.
• The active site changes shape and the enzyme and substrate no longer fit together. The enzyme is denatured.
• The rate stops completely.

Question 33

Define the term “temperature coefficient, Q10” and state its usual value for enzyme controlled reactions.

Answer 33

The value for a reaction showing how much the rate of the reaction changes when the temperature is raised by 10 celcius.
Before the optimum temp a Q10 value of 2 means the rate has doubled, a value of 3 means has trebled.
Most enzyme-controlled reactions have a Q10 of 2.

Question 34

Draw a graph showing how pH affects the initial rate of an enzyme-controlled reaction.

Answer 34

It’s a symetrical graph with the peak at the optimum PH.

Question 35

Explain why a pH change away from the optimum decreases the rate of reaction.

Answer 35

Above and below the optimum PH the H+ ions and the OH- ions can break the ionic and hydrogen bonds that hold the enzyme’s tertiary structure in place. This changes the shape of enzyme’s active site so the enzyme is denatured.

Question 36

Draw a graph showing how substrate concentration affects the initial rate of an enzyme-controlled reaction.

Answer 36

The graph increases in a straight line and then curves and plateaus.

Question 37

Define the term “Vmax”

Answer 37

Maximum initial velocity or rate of an enzyme catalysed reaction.

Question 38

Explain how increasing the substrate concentration affects the initial rate of an enzyme-controlled reaction.

Answer 38

The higher the substrate concentration the higher the rate of reaction. More substrate molecules means a collision between substrate and enzyme is more likely so more active sites will be occupied and more enzyme-substrate complexes will be formed.
This is only true up to a certain point, then after that there are so many enzyme-substrate molecules that the enzymes have as much as they can cope with - all the active sites are full.
This means adding more makes no difference and so enzyme concentration becomes the limiting factor.

Question 39

Explain how increasing the enzyme concentration affects the initial rate of an enzyme-controlled reaction.

Answer 39

The more enzyme molecules there are the more likely a substrate molecule is to collide and form an enzyme-subsrate complex.
Increasing the concentration of enzyme increases the rate of reaction. But if the substrate supply is limited there comes a point when there’s more than enough enzyme molecules to deal with all the available substrate so adding more enzymes will not make a difference. The substrate concentration becomes the limiting factor.

Question 40

Draw a graph showing how enzyme concentration affects the initial rate of an enzyme-controlled reaction.

Answer 40

It is a straight line if the supply substrate is unlimited, but it starts to curve if the substrate amount is limited.

Question 41

Describe and explain how to investigate any of the factors that affect the rate of enzyme-controlled reactions.

Answer 41

1) Set up boiling tubes containing the same volume and concentration of H202
2)To keep PH constant add equal volumes of suitable buffer solution to each boiling tube.
3) Set up a trough of water with an upside down measuring cylinder in it and a delivery tube leading to a boiling tube with a bung
4) Put each tube in separate water baths at different temperatures along with another tube of a catalase - leave for 5 mins
5) Use a pipette to add the same volume of catalase to each boiling tube then add the bung
6) Record how much oxygen is produced in the first minute.
7) Calculate the mean rate of reaction at each temperature by dividing oxygen produced by time taken.
Change different factors accordingly.

Question 42

Explain how to calculate the rate of reaction from a curved graph.

Answer 42

1) Draw a tangent across from the point needed.
2) Calculate the gradient of the tangent (chnage in y / change in x)
3) Work out the units by doing units of y/ units of x e.g. cm3/1 = cm3s-1

Question 43

Define the term “cofactor”.

Answer 43

a non-protein substance which binds to an enzyme and activates it, it can be inorganic or organic. Inorganic cofactors cannot be changed in a reaction.

Question 44

Define the term “coenzyme”.

Answer 44

Coenzymes are organic non-protein molecules which move different chemical groups between enzymes. They are not a permanent feature of the enzyme they bind to and they can be changed in a chemical reaction. They are necessary for the functioning of an enzyme.

Question 45

Define the term prosthetic group.

Answer 45

A cofactor that is tightly bound to an enzyme, it is a permanent feature of the protein.

Question 46

Explain why the chloride ion necessary for the correct formation of the active site in amylase is called a cofactor not a coenzyme or prosthetic group.

Answer 46

It is inorganic so it cannot be a coenzyme, also it is not a permanent feature of the protein so it cannot be a prosthetic group.

Question 47

Explain why the zinc ion that forms an important part of the structure of carbonic anhydrase (an enzyme necessary of the metabolism of carbon dioxide) is called a prosthetic group not a cofactor or coenzyme.

Answer 47

The zinc ions are a permament part of the enzyme’s active site.

Question 48

Give two examples of coenzymes that are synthesised from vitamins in our diet.

Answer 48

The coenzyme NAD is derived from vitamin B3 and it is responsible for the transfer of Hydrogen atoms in respiration. NADP is also derived from vitamin B3.

Question 49

Define the term enzyme inhibitor

Answer 49

Inhibitors are molecules that prevent enzymes from carrying out their normal function of catalysis (or slow them down).

Question 50

Define the term competitive inhibitor

Answer 50

A molecule with a similar shape to that of a substrate so it competes with the substrate to bind with the enzymes active site.

Question 51

Define the term non-competitive inhibitor

Answer 51

A molecule that binds to an enzyme away from its active site. This changes the shape of the active site so the substrate can no longer bind.

Question 52

Define the terms “reversible inhibitor”.

Answer 52

Reversible means it doesn’t bind permanently to an enzyme.

Question 53

Define the term “irreversible inhibitor”.

Answer 53

It binds permanently to an enzyme.

Question 54

Define the term “allosteric site”.

Answer 54

The place on an enzyme where a molecule that is not a substrate may bind, thus changing the shape of the enzyme and influencing its ability to be active.

Question 55

Explain how a competitive inhibitor affects the rate of an enzyme-controlled reaction.

Answer 55

They have a similar shape to that of substrate molecules, they compete with the substrate molecules to bind to the active site, but no reaction takes place. Instead they block the active site so no substrate molecules can fit in.
If there is a high concentration of the inhibitor then it’ll take up nearly all the active sites and drastically slow the rate.

Question 56

Explain how a non-competitive inhibitor affects the rate of an enzyme-controlled reaction.

Answer 56

The inhibitor binds to the enzyme at the alloesteric site. This binding causes the tertiary structure of the enzyme to change, meaning the active site changes shape.
The active site is no longer complementary to the substrate so an enzyme-substrate complex cannot form. Increasing the substrate concentration will not make a difference because the enzyme has changed.

Question 57

Draw graph showing how substrate concentration affects the rate of an enzyme-controlled reaction if a competitive or non-competitive inhibitor is present. Explain the effect of competitive and non-competitive inhibitors on the Vmax of an enzyme-controlled reaction.

Answer 57

Competitive - the line is under the original and is straighter but still reaches the same maximum.
non-competitive - the line is much lower and plateaus at a much lower point and has a smaller maxiumum.

Question 58

Define the term “end-product inhibition” and describe its usefulness in controlling metabolic pathways.

Answer 58

Metabolic pathways are regulated by end product inhibition. A metabolic pathway is a series of metabolic reactions connected. The product of the first reaction takes part in the second. Each reaction is catalysed by a different enzyme. Many enzymes are inhibited by the product of the reaction they catalyse - product inhibition.
End product inhibition is when the final product in a pathway inhibits an enzyme that acts earlier on in the pathway.

Question 59

Describe how ATP is involved in end-product inhibition of the enzyme phosphofructokinase.

Answer 59

phosphofructokinase is an enzyme in the metabolic pathway that breaks down glucose to make ATP. ATP inhibits the action of phosphofructokinase - so a high level stops more being made.

Question 60

Define the term “inactive precursor enzyme” and explain why enzymes may be produced in this form.

Answer 60

Enzymes are sometimes synthesized as inactive precursors in metabolic pathways to prevent them causing damage to cells. Part of the precursor molecule inhibits its action as an enzyme. Once this part is removed the enzyme becomes active.

Question 61

Describe three ways in which inactive precursor may be activated.

Answer 61

For a precursor to be active it needs to undergo a change in tertiary structure. This can be done by:

• the addition of a cofactor
• the action of another enzyme e.g. protease
• a change in conditions such as PH or temperature.

Question 62

Define the term “apoenzyme”

Answer 62

An apoenzyme is an inactive enzyme, activation of the enzyme occurs upon binding of an organic or inorganic cofactor

Question 63

Define the terms “holoenzyme’’.

Answer 63

An active enzyme formed by the combination of an inactive enzyme with a cofactor.

Question 64

Define the term zymogens

Answer 64

An inactive substance which is converted into an enzyme when activated by another enzyme.

Question 65

Define the term proenzymes.

Answer 65

An inactive precursor enzyme which requires a chemical change to function. This could be a change in conditions or the action of another enzyme.

Question 66

What is the active site made up of?

Answer 66

Catalytic amino acids.

Question 67

What is the importance of chloride ions?

Answer 67

They are a cofactor that aids the formation of the active site in amylase.

Question 68

What is the importance of zinc ions?

Answer 68

They are a prosthetic group in the structure of carbonic hydrase.