module 2.1.4 - enzymes

Question 1

what type of protein is an enzyme

Answer 1

globular protein

Question 2

what is the role of enzymes

Answer 2

act as catalysts to metabolic reactions in living organisms, which means they usually speed up metabolic reactions
- so that they occur at a reasonably fast pace even at body temperature

are required to build all the structures of the body (e.g. the cytoskeleton of a cell can be built up and reduced by enzyme activity), as well as to control the activity of the body

Question 3

write an example of an intracellular and what it does

Answer 3

catalase
- converts hydrogen peroxide to oxygen and water

Question 4

what is an intracellular protein

Answer 4

working inside cell

Question 5

what is an example of extracellular proteins and what it does

Answer 5

amylase and trypsin
- released into the digestive system

Question 6

what is an extracellular protein

Answer 6

working outside cell

Question 7

what is the properties of enzymes

Answer 7

• the molecule has a three-dimensional shape — its tertiary structure
• part of the molecule is an active site that is complementary to the shape of the substrate molecule
• each enzyme is specific to the substrate
• there is a high turnover number
• they have the ability to reduce the energy required for a reaction to occur
• their activity is affected by temperature, pH, enzyme concentration and substrate concentration
• the enzyme is left unchanged at the end of the reaction

Question 8

comment on the specificity of enzymes

Answer 8

refers to its ability to catalyse just one reaction or type of reaction. only one particular substrate molecule will fit into the active site of the enzyme molecule

Question 9

comment on the lock and key hypothesis

Answer 9

the shape of the active site is caused by the specific sequence of amino acids. this produces a specific tertiary structure — the three-dimensional shape of the molecule

Question 10

explain how enzymes catalyse reactions (comment of activation energy)

Answer 10

enzymes can speed up the rate of a reaction at body temperature
- they lower the activation energy required for the reaction to occur

Question 11

what is activation energy

Answer 11

the amount of energy required to set off the reaction and break the bonds in the substrate molecule

Question 12

explain the induced fit hypothesis

Answer 12

1. the active site of an enzyme molecule does not have a perfectly complementary fit to the shape of the substrate.
2. when the substrate moves into the active site, it interacts with the active site and interferes with the bonds that hold the shape of the active site
3. as a result, the shape of the active site is altered to give a perfect fit to the shape of the substrate
   
   • this changes the shape of the active site, which also affects the bonds in the substrate, making them easier to make or break

Question 13

explain the course of an enzyme controlled reaction

Answer 13

substrate is complementary in shape to the enzyme’s active site, therefore the substrate enters the enzyme’s active site, combining to form the enzyme–substrate complex (ESC)
- destabilises and strains the bonds in the substrate, forming the enzyme product complex. The product is finally leaves the active site and the enzyme is then free to take up another substrate molecule

Question 14

describe and explain the effect of pH on enzymes

Answer 14

All enzymes have an optimum pH — the pH at which they work best. Therefore, they will not work as quickly at a pH outside their optimum range. This is because the hydrogen ions that cause acidity affect the interactions between R groups in the tertiary structure of the enzyme, so hydrogen and/or ionic bonds may break.

This affects the tertiary structure of the molecule and so alters the shape of the active site. The shape will no longer be complementary to the shape of the substrate molecule, so the enzyme substrate complex will no longer form, meaning that the enzyme is denatured. The higher the percentage of enzymes that are denatured, the slower the rate of reaction will be

Question 15

describe and explain the effects of temperature on enzymes

Answer 15

At low temperatures (0–45°C), the activity of most enzymes increases as temperature rises. At low temperatures, the molecules have little kinetic energy. They collide infrequently with the substrate molecules and activity is reduced. As temperature rises, the molecules gain more kinetic energy. They collide more frequently with the substrate molecules and are more likely to have sufficient energy to overcome the required activation energy. Therefore, activity increases.

At higher temperatures, the increased kinetic energy causes the enzyme to vibrate, causing vibration within the protein molecule, so hydrophilic and hydrophobic interactions, as well as hydrogen bonds and ionic bonds in the tertiary structure break. This changes the 3D structure/conformation of the protein, so the enzyme loses its shape, becoming denatured. The active site no longer fits the shape of the substrate and activity reduces quickly to zero

Question 16

describe and explain the effects on enzyme conc on enzymes

Answer 16

if there are more enzyme molecules in a particular volume of reaction medium, there are more active sites available
- there is a greater likelihood of collisions between the enzyme and the substrate molecules
- more interactions per second mean a higher rate of reaction. as the enzyme concentration increases, so does the rate of reaction

Question 17

describe and explain the effects on substrate conc on enzymes

Answer 17

if the substrate concentration is high, there is a greater chance of successful collisions between the enzyme active sites and substrate molecules, forming enzyme substrate complexes and thus resulting in higher product formation
- as the substrate concentration increases, so does the rate of reaction. if the number of enzyme molecules is limited, the rate of reaction plateaus once all the enzyme active sites are fully occupied as enzymes are working at maximum rate
- therefore further increases in substrate concentration will have no effect as enzyme concentration will then be the limiting factor

Question 18

Practical investigations
Effects of pH, temperature, enzyme concentration and substrate concentration

Answer 18

1 Volume and concentration of enzyme solution.
2 Volume and concentration of substrate solution.
3 Control of temperature. A thermostatic water bath is often the best way.
4 Control of pH. A buffer solution controls pH.
5 How can you make the test more reliable? Repeat the test a number of times and calculate the mean.
6 Testing reliability. Comparing raw data to the mean is a good indication. There should be little variation around the mean — this can be shown using range bars. Calculating the standard deviation is even better.
7 Whether the volume of the solution needs to be fixed or not
8 Distribution of the molecules in the solution. Even distribution can be attained by stirring regularly
9 What is the control? This is a test that omits one factor in the experiment to show that it is essential for the reaction to occur. It is usual to omit the enzyme from the reaction mixture to show that no reaction occurs without the enzyme.
10 What level of precision is appropriate in the measurements?
11 How valid is the experiment? Is it actually measuring what you think it should? Have you taken account of all possible conditions that may affect the reaction rate?

Question 19

what is a coenzyme

Answer 19

larger organic substances that take part in the reaction. they usually transfer other reactants between enzymes

Question 20

state some examples of coenzymes

Answer 20

coenzyme A, which takes part in aerobic respiration
NAD, which is involved in transporting hydrogen atoms to the inner mitochondrial membrane
- coenzyme A and NAD are both made from the B vitamins

Question 21

what is a cofactor and what does it do

Answer 21

inorganic substances, usually metal ions
- they increase rate of catalysis by binding to the active site of the enzyme so that the enzyme-substrate complex forms more quickly and easily

Question 22

examples of cofactors

Answer 22

Cl– in amylase
Zn2+ as a prosthetic group in carbonic anhydrase

Question 23

what is a competitive inhibitor and what do they do

Answer 23

competitive inhibitors have a shape similar to the shape of the substrate and complementary to the shape of the active site
- they fit into the active site, stopping the substrate molecules fitting in
- reduces the number of available active sites
- the amount of inhibition depends on the relative concentrations of inhibitor and substrate molecules

Question 24

what is a non competitive inhibitor and what do they do

Answer 24

Non-competitive inhibitors fit into a different site (the allosteric site) on the enzyme molecule
- cause a change in the shape of the enzyme molecule
- affects the active site, so the substrate molecule is no longer complementary to the active site and so can no longer fit in

Question 25

what is the difference between reversible and non reversible inhibitors

Answer 25

reversible inhibitors occupy the enzyme site only briefly whereas non-reversible inhibitors bind permanently to the enzyme

Question 26

what is an inhibitor

Answer 26

substances that reduce the rate of reaction and fit into a site on the enzyme

Question 27

what are examples of poisons inhibiting enzymes

Answer 27

many metabolic poisons act by inhibiting enzymes
- cyanide inhibits the action of the enzyme cytochrome oxidase in aerobic respiration. cytochrome oxidase contains iron ions and the cyanide binds to them
- alpha amanitin causes death by inhibiting the production of mRNA, preventing transcription and translation and thus protein synthesis meaning vital proteins in the body that are needed for survival like haemoglobin are not produced

Question 28

what are examples of medicinal drugs inhibiting enzymes

Answer 28

aspirin binds to enzymes, preventing the formation of cell-signalling molecules that normally stimulate pain sensitivity
- this is why taking the correct dose of medicinal drugs is important as overdosing can be lethal, especially if the inhibitor is non-reversible

Question 29

explain what product inhibition is

Answer 29

when the product of an enzyme-controlled reaction inhibits the enzyme
- can act to prevent too much product being formed