2.4 - Enzymes

Question 1

What is an Enzyme?

Answer 1

An enzyme is a globular protein which catalyses chemical reactions and speeds up metabolic reactions.

Question 2

What makes an Enzyme so special?

Answer 2

Chemical catalysts, which provide a similar role to enzymes, usually require very high temperatures and extreme values of pH in order for them to function efficiently. An enzyme can speed up reactions in life sustaining conditions , do not produce unwanted products and do not usually make any errors.

Question 3

What is an active site?

Answer 3

This is a cleft in the surface of the enzyme. The tertiary structure of the active site is complementary to the shape of the substrate molecule. This increases the specificity of the enzyme.

Question 4

What is an anabolic enzyme?

Answer 4

This is an enzyme which joins substrates together.

Question 5

What is a catabolic enzyme?

Answer 5

This is an enzyme which breaks down substrates.

Question 6

What is an intracellular enzyme?

Answer 6

These are enzymes which catalyse within the cell they are produced. i.e. ribosomes, DNA helicase and DNA polymerase.

Question 7

What is a metabolic pathway?

Answer 7

In a cell many metabolic reactions occur simultaneously. Some of these follow a pathway where the product of one enzyme-substrate complex is the substrate for another. An example of a metabolic pathway is respiration.

Question 8

What is an example of an intracellular enzyme?

Answer 8

CATALASE is an enzyme which is found in nearly in all living organisms which are exposed to oxygen. It protects a cell from damage by breaking down HYDROGEN PEROXIDE. 2H2O2 → O2 + 2H2O. Catalase is made up of four polypeptide chains and an iron haem group. In a eukaryotic cell it is found in small vesicles which are called PEROXISOMES.

Question 9

What is an extracellular enzyme?

Answer 9

These are enzymes which are secreted from the cell that they are produced in. This is particularly used in the digestive system where enzymes are used to break down food.

Question 10

What is an example of an extracellular enzyme?

Answer 10

AMYLASE is produced in the SALIVARY GLANDS and acts in the MOUTH in order to break down the polysaccharide STARCH into the disaccharide MALTOSE.It is also made in the pancreas where it catalyses the same reaction in the lumen of the small intestine.
TRYPSIN is made in the pancreas and acts in the lumen of the SMALL INTESTINE to break down PROTEINS into PEPTIDES.

Question 11

What is a cofactor?

Answer 11

Some enzymes, especially the on which catalyse redox reactions, only work if another SMALL NON- PROTEIN MOLECULE is attached to the enzyme. For example a CHLORINE ION (Cl-) is the cofactor for AMYLASE.

Question 12

What is a prosthetic group?

Answer 12

This is a cofactor which is PERMANENTLY bound to the enzyme by a covalent bond./ For example, CARBONIC ANHYDRASE contains a ZINC ION (ZN+) prosthetic group. It catalyses the interconversion of CO2 and H20 to H2CO3 (Carbonic acid) which breaks down to protons (H+) and Hydrogen Carbonate ions (HCO3-). This is important as it enables CO2 to be carried in the blood.

Question 13

What is a coenzyme?

Answer 13

These are SMALL ORGANIC NON - PROTEIN molecules, which are a type of cofactor, that bind temporarily to the active site either just before or at the same time as when the substrate binds to the enzyme. During this process the coenzyme is chemically changed and are then recycled.

Question 14

Can you name a source of coenzymes?

Answer 14

Many coenzymes can be found in vitamins. For example Nicotinamide provides the coenzymes NAD and NADP which may cause diarrhoea , dementia etc. if there is a deficiency.

Question 15

What is the ‘lock and key’ hypothesis?

Answer 15

The random movement of the enzyme and substrate brings the substrate into the active site. The substrate molecule fits the tertiary structure of the active site as it completely specific to that substrate - like a lock and key.
An enzyme substrate complex is temporarily formed by hydrogen bonds when the R groups in the amino acids of the active site interacts with the substrate. The substrate molecule is either broken down or built into a larger product molecule. The product molecule leaves the active site.

Question 16

What is the ‘induced fit’ hypothesis?

Answer 16

This was a modification suggested by Daniel Koshland in 1958, that the active site of an enzyme is not a fixed structure. Instead the exact shape of the active site is induced by the presence of a substrate molecule in order to ensure a perfect fit. The active site must still be complementary to the shape of the substrate and it returns to it’s original shape once the product is released from the enzyme.

Question 17

What factors affect the rate of an enzyme controlled reaction?

Answer 17

Temperature, pH, substrate concentration, enzyme concentration and inhibitor concentration.

Question 18

What is the effect of temperature on the rate of an enzyme controlled reaction?

Answer 18

If a substance is heated more kinetic is provided to the molecule. This increases the rate of collisions and the force at which they collide. This increases the rate at which E-S complexes are formed. As well as this thermal energy makes the molecules vibrate which could break the hydrogen and ionic bonds in the tertiary structure. This changes the shape of the active site and reduces the effectiveness of an E-S complex forming. This can also cause further irreversible changes to the shape of the active site which makes it no longer complementary to the substrate. However the heat does not break any peptide bonds so the primary structure is not affected.
goo.gl/64wcyI

Question 19

How do you find the rate of reaction?

Answer 19

1/(time to reach endpoint)

Question 20

What is the Temperature coefficient (Q10) and how do you find it?

Answer 20

This shows the change in the rate of reaction for an increase in temperature by 10 degrees. For metabolic reactions this should be around 2, when the temperature is above the optimum temperature the Q10 decreases.
Q10 = [rate of reaction at (T+10)] / [rate of reaction at T]

Question 21

Explain an experiment to test the effect of temperature on the rate of reaction.

Answer 21

The phosphatase enzyme breaks down organic phosphates in a cell. Use the chemical PPP (Phenolphthalein Phosphatase) and add in excess sodium carbonate. This produces phenolphthalein which produces a pink colour solution, which is proportional to concentration. The darker the pink, the more molecules have been hydrolysed. You can use a colorimeter to measure quantitively the amount produced and then repeat and compare at other temperatures

Question 22

What is the effect of pH on the activity of an enzyme?

Answer 22

Acids dissociate in solution to produce H+ ions. A proton has a positive charge which is attracted to negatively charged ions. This interferes with the hydrogen bonds and ionic forces in the tertiary structure of the active site. Excess H+ ions changes the shape and interferes in the binding of the substrate. As well this if there are too few H+ ions the shape of the active site is not at it’s optimum and therefore the rate of reaction would not be at it’s maximum.

Question 23

What is a buffer?

Answer 23

This is something that resists changes in pH. These can donate or accept H+ ions. pH is found by using the equation log (1/[H+]).

Question 24

Explain an experiment to test the effect of pH on the rate of reaction.

Answer 24

Use a pH buffer in order to make the pH level constant. Measure the time taken for amylase to break down starch into maltose. This can be checked by taking regular samples until they no longer give a blue- black colour when iodine is added. Use the time taken to reach endpoint to find the rate of reaction and repeat at other pH levels.

Question 25

What is the effect of substrate concentration on the activity of an enzyme?

Answer 25

At first if there is no substrate present, the reaction cannot occur. As the concentration of substrate increases, so does the rate of reaction as it increases the likelihood of an enzyme - substrate collision which then forms a complex. However the increase plateaus as another factor is the limiting factor. For example the concentration of enzymes as there may be no more enzyme active sites which are not occupied.
goo.gl/rDYgLO

Question 26

Explain an experiment to test the effect of substrate concentration on the rate of reaction.

Answer 26

Use Urease, a hydrolytic enzyme which breaks down C-N bonds in amides such as Urea. (NH2)CO + H20→ 2NH3 + CO2. Use a colorimeter and a cuvette to measure the amount and absorbance of ammonia produced.

Question 27

What is the effect of enzyme concentration on the activity of an enzyme?

Answer 27

At first if there is no enzyme present, the reaction cannot occur. As the concentration of enzyme increases, so does the rate of reaction as it increases the likelihood of an enzyme - substrate collision which then forms a complex. However the increase plateaus as another factor is the limiting factor. For example the concentration of substrate as there may not be any left for the reaction to occur.

Question 28

Explain an experiment to test the effect of enzyme concentration on the rate of reaction.

Answer 28

Use Urea at increasing concentrations. Measure the time taken to produce a set amount of ammonia or carbon dioxide. Use this to find the rate of reaction and then compare this to your other results. Draw a graph to demonstrate the trend shown.

Question 29

What is an inhibitor?

Answer 29

This is a substance which reduces or stops a reaction.

Question 30

What is a competitive inhibitor?

Answer 30

These are inhibitors which have a similar shape to the substrate and fit into the enzyme’s active site. How much inhibition occurs depends on the concentration of the inhibitors. If the concentration of the inhibitor is fixed, increases the substrate concentration dilutes the effect of the inhibitor as it is less likely to collide with an active site than a substrate. https://goo.gl/gkZEBq

Question 31

What is an inactivator?

Answer 31

This is a competitive inhibitor which binds IRREVERSIBLY to the active site of an enzyme.

Question 32

What is a non - competitive inhibitor?

Answer 32

These bind to the enzyme somewhere else other than at the active site. They bond to the allosteric site and change the shape of the active site. This prevents an active site from being complementary and an enzyme forming an E-Z complex.
https://goo.gl/gkZEBq

Question 33

What is end product inhibition?

Answer 33

This is after a reaction is catalysed and the product stays bound to the active site and prevents the formation of more products than needed.

Question 34

What is the control of metabolic sequences?

Answer 34

This is in processes such as photosynthesis and respiration which have a series of enzyme controlled reactions. One product from a reaction becomes the substrate in the next reaction. In order to prevent surplus the final product may reattach to the first enzyme and act as an irreversible non- competitive inhibitor to prevent excess products from forming.

Question 35

What is a metabolic poison?

Answer 35

Many toxins act by inhibiting or inactivating enzymes which prevent key life sustaining processes from completing

Question 36

What is the effect of cyanide?

Answer 36

Cyanide, KCN, inhibits aerobic respiration by inhibiting an enzyme found in mitochondria. This prevents aerobic respiration from completing.

Question 37

What is the effect of a medicine such as Aspirin?

Answer 37

Aspirin ( salicylic acid) binds to an enzymes called COX 1 and 2 which catalyse the formation of prostaglandins. The Prostaglandins are cell signalling molecules which are part of the metabolic process that causes pain, due to more sensitive nerve cells, during inflammation.