Enzymes

Question 1

What are enzymes?

Answer 1

Enzymes are biological catalysts. They speed up chemical reactions without being used up. They catalyze metabolic reactions, both at a cellular level and for the organism as a whole.
They can affect both structures and functions in an organism.

Question 2

What is an example of an intracellular enzyme?

Answer 2

Catalase is an intracellular enzyme.
- hydrogen peroxide is the toxic by-product of several cellular reactions. If left to build up, it can kill cells.
- Catalase is an enzyme that works inside cells to catalyze the breakdown of hydrogen peroxide into harmless oxygen and water

Question 3

What are examples of extracellular enzymes?

Answer 3

Amylase and Trypsin are Extracellular enzymes
- They both work outside of cells in the human digestive system
- Amylase is found in saliva. It is secreted into the mouth by cells in the salivary glands. It catalyzes the hydrolysis of starch into maltose.
- Trypsin catalyzes the hydrolysis of peptide bonds (turning big polypeptides into smaller polypeptides). Trypsin is produced by cells in the pancreas and secreted into the small intestine.

Question 4

What kind of proteins are enzymes?

Answer 4

They are globular proteins

Question 5

What is an enzyme’s active site?

Answer 5

Enzymes have an active site that has a specific shape, complementary to the substrate. The specific shape of the active site is determined by the enzyme’s tertiary structure. For the enzyme to work, the substrate has to fit into the active site. If the substrate shape doesn’t match the active site, the reaction won’t be catalyzed.

Question 6

How do enzymes speed up the rate of reaction?

Answer 6

The activation energy is the energy required to initiate a reaction. (often provided as heat). Enzymes reduce the amount of energy needed to start a reaction (e.g. a lower temperature) which consequently increases the rate of reaction.

Question 7

What is the enzyme-substrate complex?

Answer 7

It’s what’s formed when an enzyme and substrate bind together. The formation of it is what lowers the activation energy.

Question 8

How does the formation of the enzyme-substrate complex lower activation energy?

Answer 8

• If two substrate molecules need to be joined, attaching to the enzyme holds them closer together, reducing any repulsion between the molecules so they can bond more easily.
• If the substrate needs to be separated, attaching to the enzyme puts a strain on the bond between them. This strain means that it is easier to separate the molecule.

Question 9

What is the enzyme “lock and key” model?

Answer 9

This is the diagram where the substrate fits into the enzyme in the same way that a key fits into a lock. They are completely complementary and the enzyme doesn’t change shape- it is rigid.

Question 10

What is the enzyme “induced fit” model?

Answer 10

This is the diagram where the substrate doesn’t only have to be the right shape to fit the active site, it has to make the active site change shape in the right way as well.

Question 11

How does temperature affect enzyme Activity?

Answer 11

Increased temperature increases the kinetic energy of particles and therefore they move faster. This makes it more likely for enzymes to collide with the substrate molecules. The energy of the collisions also increases so it is more likely for the collision to be successful and have the correct activation energy.

Question 12

How does temperature denature enzymes?

Answer 12

• the rise in temperature makes the enzyme’s molecules vibrate more
• as temperature goes above a certain level this vibration breaks some of the bonds that hold the enzyme in shape
• the active site changes shape and the enzyme and substrate no longer fit together
• At this point, the enzyme is denatured and it no longer functions as a catalyst

Question 13

What is the equation for the temperature coefficient?

Answer 13

Q10= rate at highest temperature/rate at lowest temperature

Question 14

What is the temperature coefficient of a reaction?

Answer 14

It shows how much the rate of a reaction changes when the temperature is raised by 10 degrees C

Question 15

What Q10 values do most enzyme-controlled reactions have?

Answer 15

2

Question 16

How does pH affect enzyme activity?

Answer 16

All enzymes have an optimum pH value (most human ones are around 7 except pepsin)
Above and below the optimum pH, the H+ and OH- ions found in acids and alkalis can mess up the ionic bonds and hydrogen bonds that hold the enzyme’s tertiary structure in place. This makes the active site change shape, so the enzyme is denatured.

Question 17

How does enzyme concentration affect the rate of reaction?

Answer 17

• the more enzyme molecules there are in a solution, the more likely a substrate molecule is to collide with one and form an enzyme-substrate complex. So increasing the concentration increases the rate of reaction
• however, if the amount of substrate is limited, there comes a point where there are more than enough enzyme molecules to deal with all the available substrate, so adding more enzyme has no further effect.

Question 18

How does substrate concentration affect the rate of reaction?

Answer 18

• the higher the substrate concentration the faster the reaction as more substrate molecules mean more collisions between enzyme and substrate so more active sites will be used.
• this is only true up to the ‘saturation point’ as there are so many substrate molecules and only a fixed amount of active sites so they overwhelm the enzymes and thus adding more substrate has no further effect.
• substrate concentration decreases over time so the rate of reaction will decrease over time as well.

Question 19

What factors affect the rate of enzyme activity?

Answer 19

• pH
• temperature
• enzyme concentration
• substrate concentration

Question 20

How do you measure the rate of an enzyme-controlled reaction?

Answer 20

1) you can measure how fast the product of the reaction appears. for example, measuring the volume of gas produced per minute
2) you can measure the disappearance of the substrate.

Question 21

How would you investigate the effect of temperature on catalase activity?

Answer 21

1) Set up boiling tubes containing the same volume and concentration of hydrogen peroxide and add equal volumes of buffer to each tube
2) put each boiling tube in a water bath set to a different temperature along with another tube containing catalase
3) use a pipette to add the same volume and concentration of catalase to each boiling tube
4) record how much oxygen is produced in the first 60 seconds (using a gas syringe)
5) repeat the experiment at each temperature x3 times and use the results to find the mean vol of O2 produced
6) calculate the mean rate of reaction

Question 22

How do you calculate the rate of reaction?

Answer 22

volume produced/time taken

Question 23

What are enzyme cofactors?

Answer 23

They are non-protein substances bound to enzymes which allow them to work.

Question 24

How do inorganic molecules as cofactors work?

Answer 24

They work by helping the enzyme and substrate to bind together. They don’t participate in reaction so aren’t used up or changed in any way. For example, chloride ions are cofactors in the enzyme amylase.

Question 25

How do organic molecules as cofactors work?

Answer 25

They are called coenzymes They participate in the reaction and are changed by it. They often act as carriers, moving chemical groups between different enzymes. They’re continually recycled during this process. Vitamins are often sources of coenzymes.

Question 26

How are cofactors related to prosthetic groups?

Answer 26

If a cofactor is tightly bound to the enzyme, it’s known as a prosthetic group. For example, zinc ions are a prosthetic group for carbonic anhydrase. The zinc ions are a permanent part of the enzyme’s active site.

Question 27

What are enzyme inhibitors?

Answer 27

They are molecules that bind to the enzyme that they inhibit. Inhibition can be competitive and noncompetitive.

Question 28

What is competitive inhibition?

Answer 28

• competitive inhibitor molecules have a similar shape to that of the substrate molecule
• 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 it.
• how much the enzyme is inhibited depends on the relative concentrations of the inhibitor (if there is a large concentration more active sites will be used) (if there is a large substrate concentration they are more likely to get to the active sites first)

Question 29

What is non-competitive inhibition?

Answer 29

-Non-competitive inhibitor molecules bind to the enzyme away from its active site. The site that they bond to is known as the allosteric site
- This causes the active site to change shape so the substrate molecules can no longer bind to it.
- they don’t compete with the substrate as they have a different shape
- Increasing the concentration of the substrate won’t make any difference to the reaction rate- enzyme activity will still be inhibited

Question 30

When are enzyme inhibitors reversible?

Answer 30

If there are weaker hydrogen bonds or weak ionic bonds the inhibitor can be removed and the inhibition is reversible.

Question 31

When are enzyme inhibitors irreversible?

Answer 31

If there are strong covalent bonds, the inhibitor can’t be removed easily and the inhibition is irreversible

Question 32

How are some antiviral drugs enzyme inhibitors?

Answer 32

Some drugs that stop viruses- e.g. reverse transcriptase inhibitors inhibit the enzyme reverse transcriptase, which catalyzes the replication of viral DNA. Thus preventing the virus from replicating.

Question 33

How are some antibiotics enzyme inhibitors?

Answer 33

Some antibiotics- e.g. penicillin inhibits the enzyme transpeptidase, which catalyzes the formation of proteins in bacterial cell walls. This weakens the cell wall and prevents the bacterium from regulating its osmotic pressure. As a result, the cell bursts, and the bacterium is killed.

Question 34

What is cyanide in relation to enzyme inhibition?

Answer 34

Cyanide is an irreversible inhibitor of cytochrome c oxidase, an enzyme that catalyzes respiration reactions. Cells that can’t respire die.

Question 35

What is Malonate in relation to enzyme inhibition?

Answer 35

Malonate inhibits succinate dehydrogenase which catalyzes respiration reactions. Cells that can’t respire die.

Question 36

What is Arsenic in relation to enzyme inhibition?

Answer 36

Arsenic inhibits the action of pyruvate dehydrogenase, another enzyme that catalyzes respiration reactions.

Question 37

What is a metabolic pathway?

Answer 37

A metabolic pathway is a series of connected metabolic reactions. The product of the first reaction takes part in the second reaction. Each reaction is catalyzed by a different enzyme.

Question 38

What is product inhibition?

Answer 38

Product inhibition is when enzymes are inhibited by the product of the reaction they catalyze.

Question 39

What is end-product inhibition?

Answer 39

End-product inhibition is when the final product in a metabolic pathway inhibits an enzyme that acts earlier on in the pathway.
It is a good way to regulate the pathway and control the amount of end-product that gets made.

Question 40

Why are both product and end-product inhibition reversible?

Answer 40

When the level of product starts to drop the level of inhibition will start to fall and the enzyme can start to function again. This means more products can be made.

Question 41

How can enzyme inhibition help to protect cells?

Answer 41

• enzymes are sometimes synthesized as inactive precursors in metabolic pathways to prevent them from causing damage to cells. For example, some proteases are synthesized as inactive precursors to stop them from damaging proteins in the cell in which they’re made.
• part of the precursor molecule inhibits its action as an enzyme. Once this part is removed the enzyme becomes active.