chapter 4 part 2

Question 1

Increase in concentration of substrate:

Answer 1

leads to a higher collision rate with the active sites of enzymes and the formation of more enzyme-substrate complexes.
The rate of reaction therefore increases.

Question 2

Increase in the concentration of the enzyme:

Answer 2

The same as increase in substrate as this will increase the number of available active sites in a particular area or volume, leading to the formation of enzyme-substrate complexes at a faster rate.
The rate of reaction increases up to its maximum (Vmax).
At this point all of the active sites are occupied by substrate particles and no more enzyme-substrate complexes can be formed until products are released from active sites.

Question 3

The only way to increase the rate of reaction:

Answer 3

add more enzyme or increase the temperature.
If the concentration of the enzyme is increased more active sites are available so the reaction rate can rise towards a higher Vmax.
The concentration of substrate becomes the limiting factor again and increasing this will once again allow the reaction rate to rise until the new Vmax is reached.

Question 4

effect of substrate concentration or rate of reaction graph

Answer 4

Question 5

Investigations into the effects of different factors on enzyme activity:

Answer 5

Investigating the effects of different factors on enzyme activity provides an insight into how enzymes work.
Catalase is an enzyme present in plant tissue and animal tissue, making it a good choice for use in investigations because it is readily available.
Catalase catalyses the breakdown of hydrogen peroxide into water and oxygen.
The volume of oxygen gas collected in a set length of time can be used as a measure of the rate of reaction.

Question 6

The experiment procedure:

Answer 6

A student conducted a series of experiments to determine the effect of temperature on enzymes.
In the first experiment, using apparatus similar to that shown in the diagram above, liver tissue was put into hydrogen peroxide solution and the volume of oxygen released every five seconds was measured.
In the second experiment the liver was boiled for five minutes before being placed in the hydrogen peroxide solution.
The graph below shows the results from the experiment.
The student then investigated the effect of substrate concentration on enzyme activity.

Question 7

apparatus used to collect released oxygen when catalase reacts with hydrogen peroxide

Answer 7

Question 8

What is a serial dilution:

Answer 8

a repeated, stepwise dilution of a stock solution of known concentration.

usually done by factors of ten, to produce a range of concentrations.

useful even if the concentration of the initial solution is unknown as they give us relative concentrations.

used in many different ways - for example to investigate the effect of changing the concentration of an enzyme or a substrate in a reaction, and in determining the numbers of microorganisms in a culture.

Question 9

diagram of serial dilution

Answer 9

Question 10

how a serial dilution might be set up.

Answer 10

Adding 1 ml of stock solution to 9 ml of distilled water gives 10 ml of dilute solution in which there is 1 ml stock/10 ml hence a 1/10 or 10 fold dilution.
This step is repeated a number of times to give a serial dilution.
Catalase is an enzyme that catalyses the breakdown of hydrogen peroxide.
To investigate the effect of different concentrations of catalase on the rate of breakdown of hydrogen peroxide, catalase-rich tissues such as liver or potato can be ground down to make a solution.
The solution will contain catalase released from cells.
Serial dilution of this solution will produce a range of solutions with different relative concentrations of catalase.
The effect of the different concentrations on the rate of reaction can be investigated by adding equal volumes of a given concentration of hydrogen peroxide to each solution.

Question 11

it is important that cellular conditions such as

Answer 11

pH and temperature are kept within narrow limits so that enzyme activity is not delayed.
This ensures that reactions can happen at a rate fast enough to sustain living processes, for example, respiration.

Question 12

Importance of Regulation in Complex Reaction Pathways:

Answer 12

It is also important that reactions do not happen too fast as this could lead to the build-up of excess products.
Living processes rarely involve just one reaction but are complex, multi-step reaction pathways.
These pathways need to be closely regulated to meet the needs of living organisms without wasting resources.

Question 13

Control of metabolic activity within cells:

Answer 13

The different steps in reaction pathways are controlled by different enzymes.
Controlling the activity of enzymes at crucial points in these pathways regulates the rate and quantity of product formation.
Enzymes can be activated with cofactors, or inactivated with inhibitors.
Inhibitors are molecules that prevent enzymes from carrying out their normal function of catalysis (or slow them down).
There are two types of enzyme inhibition - competitive and non-competitive.

Question 14

Competitive inhibition works in the following way:

Answer 14

A molecule or part of a molecule that has a similar shape to the substrate of an enzyme can fit into the active site of the enzyme.
This blocks the substrate from entering the active site, preventing the enzyme from catalysing the reaction.
The enzyme cannot carry out its function and is said to be inhibited.
The non-substrate molecule that binds to the active site is a type of inhibitor.
Substrate and inhibitor molecules present in a solution will compete with each other to bind to the active sites of the enzymes catalysing the reaction.
This will reduce the number of substrate molecules binding to active sites in a given time and slows down the rate of reaction.
For this reason such inhibitors are called competitive inhibitors and the degree of inhibition will depend on the relative concentrations of substrate, inhibitor, and enzyme.

• Most competitive inhibitors only bind temporarily to the active site of the enzyme, so their effect is reversible.
• However there are some exceptions such as aspirin.

Question 15

Competitive inhibition effect on rates of reaction:

Answer 15

A competitive inhibitor reduces the rate of reaction for a given concentration of substrate, but it does not change the Vmax of the enzyme it inhibits.
If the substrate concentration is increased enough there will be so much more substrate than inhibitor that the original Vmax can still be reached.

Question 16

Examples of competitive inhibition:

Answer 16

Statins are competitive inhibitors of an enzyme used in the synthesis of cholesterol.
Statins are regularly prescribed to help people reduce blood cholesterol concentration.
High blood cholesterol levels can result in heart disease.

Aspirin irreversibly inhibits the active site of COX enzymes, preventing the synthesis of prostaglandins and thromboxane, the chemicals responsible for producing pain and fever.

Question 17

Non-competitive inhibition works in the following way:

Answer 17

The inhibitor binds to the enzyme at a location other than the active site. This alternative site is called an allosteric site.
The binding of the inhibitor causes the tertiary structure of the enzyme to change, meaning the active site changes shape.
This results in the active site no longer having a complementary shape to the substrate so it is unable to bind to the enzyme.
The enzyme cannot carry out its function and it is said to be inhibited.
- As the inhibitor does not compete with the substrate for the active site it is called a non-competitive inhibitor.

Question 18

Non-competitive inhibition effect on rates of reaction:

Answer 18

Increasing the concentration of enzyme or substrate will not overcome the effect of a non-competitive inhibitor.
Increasing the concentration of inhibitor, however, will decrease the rate of reaction further as more active sites become unavailable.
the binding of the inhibitor may be reversible or non-reversible.
Irreversible inhibitors cannot be removed from the part of the enzyme they are attached to.
They are often very toxic, but not always.

Question 19

comparison of competitive and non competitive graph

Answer 19

Question 20

Examples of irreversible non-competitive inhibitors:

Answer 20

Organophosphates used as insecticides and herbicides irreversibly inhibit the enzyme acetyl cholinesterase, an enzyme necessary for nerve impulse transmission.
This can lead to muscle cramps, paralysis, and even death if accidentally ingested.

Proton pump inhibitors (PPIs) are used to treat long-term indigestion.
They irreversibly block an enzyme system responsible for secreting hydrogen ions into the stomach.
This makes PPIs very effective in reducing the production of excess acid which, if left untreated, can lead to formation of stomach ulcers.

Question 21

End-product inhibition:

Answer 21

End-product inhibition is the term used for enzyme inhibition that occurs when the product of a reaction acts as an inhibitor to the enzyme that produces it.
This serves as a negative-feedback control mechanism for the reaction.
Excess products are not made and resources are not wasted.
It is an example of non-competitive reversible inhibition.

Question 22

Example of End-product inhibition:

Answer 22

Respiration is a metabolic pathway resulting in the production of ATP.
Glucose is broken down in a number of steps.
The first step involves the addition of two phosphate groups to the glucose molecule.
The addition of the second phosphate group, which results in the initial breakdown of the glucose molecule, is catalysed by the enzyme phosphofructokinase (PFK).
This enzyme is competitively inhibited by ATP.
ATP therefore regulates its own production.

Question 23

Steps of ATP in end product inhibition:

Answer 23

When the levels of ATP are high, more ATP binds to the allosteric site on PFK, preventing the addition of the second phosphate group to glucose.
Glucose is not broken down and ATP is not produced at the same rate.

As ATP is used up, less binds to PFK and the enzyme is able to catalyse the addition of a second phosphate group to glucose.
Respiration resumes, leading to the production of more ATP.

Question 24

cofactors:

Answer 24

a non-protein ‘helper’ component needed by enzymes in order to carry out their function as biological catalysts.
They may transfer atoms or groups from one reaction to another in a multi-step pathway or they may actually form part of the active site of an enzyme.

Question 25

coenzyme.

Answer 25

if the cofactor is an organic molecule

Question 26

How are Inorganic cofactors obtained:

Answer 26

via the diet as minerals, including iron, calcium, chloride, and zinc ions.
For example, the enzyme amylase, which catalyses the breakdown of starch, contains a chloride ion that is necessary for the formation of a correctly shaped active site.
Many coenzymes are derived from vitamins, a class of organic molecule found in the diet.
For example, vitamin B3 is used to synthesise NAD (nicotinamide adenine dinucleotide), a coenzyme responsible for the transfer of hydrogen atoms between molecules involved in respiration.
NADP, which plays a similar role in photosynthesis, is also derived from vitamin B3.
Another example is vitamin B5, which is used to make coenzyme A.
Coenzyme A is essential in the breakdown of fatty acids and carbohydrates in respiration.

Question 27

Prosthetic groups:

Answer 27

Prosthetic groups are cofactors - they are required by certain enzymes to carry out their catalytic function.
Seen on the globular protein, haemoglobin, in which the prosthetic group is an iron (Fe) ion.
While some cofactors are loosely or temporarily bound to the enzyme protein in order to activate them, prosthetic groups are tightly bound and form a permanent feature of the protein.

Question 28

Example of Prosthetic group:

Answer 28

zinc ions (Zn2+) form an important part of the structure of carbonic anhydrase, an enzyme necessary for the metabolism of carbon dioxide.

Question 29

Precursor activation:

Answer 29

Many enzymes are produced in an inactive form, known as inactive precursor enzymes, particularly enzymes that can cause damage within the cells producing them or to tissues where they are released, or enzymes whose action needs to be controlled and only activated under certain conditions.
Precursor enzymes often need to undergo a change in shape (tertiary structure), particularly to the active site, to be activated.

Question 30

How does Precursor activation work

Answer 30

by the addition of a cofactor.
Before the cofactor is added, the precursor protein is called an apoenzyme.
When the cofactor is added and the enzyme is activated, it is called a holoenzyme.
Sometimes the change in tertiary structure is brought about by the action of another enzyme, such as a protease, which cleaves certain bonds in the molecule.
In some cases a change in conditions, such as pH or temperature, results in a change in tertiary structure and activates a precursor enzyme.
These types of precursor enzymes are called zymogens or proenzymes.
When inactive pepsinogen is released into the stomach to digest proteins, the acid pH brings about the transformation into the active enzyme pepsin.
This adaptation protects the body tissues against the digestive action of pepsin.

Question 31

Enzyme activation and the blood-clotting mechanism:

Answer 31

Blood clotting, or coagulation, is an important biological response to tissue damage.
The blood-clotting process only begins when platelets aggregate at the site of tissue damage.
The aggregated platelets release clotting factors, including factor X.
Factor X is an important component in the blood-clotting mechanism.
It is an enzyme that is dependent on the cofactor vitamin K for activation.
Activated factor X catalyses the conversion of prothrombin into the enzyme thrombin by cleaving certain bonds in the molecule, thus altering its tertiary structure.
Thrombin is a protease and catalyses the conversion of soluble fibrinogen to insoluble fibrin fibres.
Fibrin molecules, together with platelets, form a blood clot.
This series of successive enzyme activations in blood clotting is called the coagulation cascade.