Module 1: Topic 2.4: Enzymes

Question 1

What is the role of an enzyme?

Answer 1

Enzymes speed up chemical reactions by acting as biological catalysts.

Question 2

What is a catalyst?

Answer 2

It is a substance that speeds up a chemical reaction without being used up in the reaction itself.

Biological catalysts are those found in living organisms. They catalyse metabolic reactions.

Question 3

Give one intracellular enzyme and its function.

Answer 3

Catalase is an enzyme that works inside cells to catalase the breakdown of hydrogen peroxide to harmless oxygen (O2) and water (H2O).

Hydrogen peroxide (H2O2) is the toxic by -product of several cellular reactions. if left to build up, it can kill cells.

Question 4

Give two Extracellular enzymes and its function.

Answer 4

Amylase is found in saliva and is produced in the pancreas. it is secreted into the mouth by cells in the salivary glands. It catalyses the breakdown of starch into maltose in the mouth.

Trypsin catalyses the hydrolysis of peptide bonds- turning big polypeptides bonds into smaller ones (which further break into amino acids by other enzymes). Trypsin is produced in the pancreas and secreted in the small intestine.

Question 5

What are Enzymes?

Answer 5

Enzymes are globular proteins that have an active site.

The active site of an enzyme has a specific shape and allows the substrate to bind. The actives site tertiary structure is complimentary to the substrate.

The substrate must fit perfectly into the active site for it to work. If not, the reaction will not be catalysed. When the substrate binds to an enzymes active site, an enzyme- substrate complex is formed.

Question 6

Define Activation energy.

Answer 6

The minimum amount of energy needed for a reaction to happen.

(often provided as heat)

Question 7

Explain the lock and key model.

Answer 7

This model suggests that the substrate fits into the enzyme’s active site in the same way in which a key fits into a lock. The shape of the substrate and the active site are perfectly complementary to each other. Catalysis happens in the following stages:

The substrate binds to the enzyme’s active site, forming an enzyme-substrate complex (ES complex).

The enzyme converts the substrate into product, forming an enzyme-product complex (EP complex).

The product is released from the enzyme’s active site.

Question 8

Explain the induced fit model.

Answer 8

The induced fit model suggests that the shapes of the enzyme’s active site and its substrate are not exactly complementary, but when the substrate enters the active site, a conformational change (change of shape) occurs which induces catalysis. The induced fit model can be broken down into the following stages:

The substrate enters the enzyme’s active site, forming an Enzyme Substrate complex.

The enzyme undergoes a conformational change which causes the conversion of substrate into product, forming an Enzyme Product complex.

The product is released from the enzymes active site.

Question 9

What is an advantage of the Lock and key model?

Answer 9

The advantage of the lock-and-key model is that it explains why most enzymes display such high specificity to their substrates.

Each enzyme will catalyse only a certain type of reaction and will only bind to a single specific substrate out of the millions of different molecules that are floating around our bodies.

However, not all enzymes catalyse a single chemical reaction.

Question 10

What is an advantage of the Induced fit model?

Answer 10

lipase exhibits broader specificity and can bind to a variety of lipids, which only the induced fit model is able to explain.

In addition, the induced fit model is better able to explain how catalysis actually occurs.

A conformational change, which would place stress on the bonds within the substrate, can explain how bonds would break in order for the products to form.

This makes the induced fit model the more widely accepted model of the

Question 11

How can you measure the rate of reaction?

Answer 11

Measure the rate of reaction by drawing a tangent.

Work out the gradient by drawing a triangle.

y1 - y2 / x1 - x2 = rate (mg/s)

Question 12

What are the factors that affect Enzyme Activity?

Answer 12

Temperature.

pH

Concentration.

Question 13

How does Temperature affect Enzyme activity?

Answer 13

At low temperatures, the rate of reaction will be slow because the enzyme and substrate have low amounts of kinetic energy.

This means that there wont be many collisions, so there will be reduced formation of enzyme substrate complexes.

As the temperature is increased, the number of collisions increases, this increases the formation of enzyme substrate complexes as well as increasing the rate of reaction.

If the temperature becomes really high, the hydrogen bonds will begin to break within the protein, causing it to unravel and become denatured.

If enzymes are denatured, they lose the shape of their active sites which means they cannot bind to their substrate, decreasing the rate of reaction.

Question 14

Formula for Temperature Coefficient.

Answer 14

R2 (rate at a higher temperature) / R1 (rate at a lower temperature)

(mostly have a rate of 2).

Question 15

How does pH affect enzyme activity?

Answer 15

Each enzyme has its own optimum pH at which it works best.

Pepsin, the enzyme which digests protein in the stomach, works best in acidic environments whereas the enzymes responsible for the digestion of carbohydrates work better at a more neutral pH.

Deviations from the optimum pH change the charge on the enzyme, which affects ionic bonding within its structure.

Deviations in pH also break hydrogen bonds. This causes it to change shape and become denatured, decreasing the rate of reaction as pH deviates from the enzyme’s optimum conditions.

Question 16

How does enzyme concentration affect enzyme activity?

Answer 16

As enzyme concentration increases, the rate of reaction increases since more active sites will be available to bind to substrate molecules.

This means that there will be more frequent collisions between the enzyme and substrate, so there will be more formation of enzyme-substrate complexes.

However, a point will be reached when increasing enzyme concentration does not result in further increases in reaction rate. At this point, something else has become a limiting factor, such as the availability of substrate.

Question 17

How does Substrate Concentration affect enzyme activity?

Answer 17

As substrate concentration increases, the rate of reaction increases since there are more substrate molecules to fill the enzyme’s active sites.

There will be more frequent collisions so more formation of Enzyme Substrate complexes.

At some point a ‘saturation’ point is reached where all of the enzyme’s active sites are occupied with substrate molecules, so the addition of more substrate molecules will have no effect on the rate of reaction.

At this point, the reaction is proceeding as fast as possible, which is referred to as Vmax. The only way the reaction can go any faster is by increasing enzyme concentration.

Question 18

What is a Cofactor?

Answer 18

Cofactors are non-protein molecules that need to be bound to an enzyme in order for it to work.

Some cofactors are inorganic and work by helping the enzyme bind to its substrate.

They aren’t involved in the reaction so they are chemically unchanged. For example, magnesium ions are cofactors for the enzyme

Question 19

What is a Coenzyme?

Answer 19

Other cofactors are organic molecules and are called coenzymes.

These work by directly participating in the reaction so are chemically altered by the end of the reaction.

They often carry chemical groups between different enzymes. For example, NAD+ is a coenzyme which transfers hydrogen (and electrons) between the enzymes involved in respiration. Many coenzymes are derived from vitamins.

Question 20

What is a Prosthetic group?

Answer 20

When a cofactor (non- protein) is tightly bound to its enzyme, it’s called a prosthetic group.

For example, the iron-containing heme group is a prosthetic group found in haemoglobin, forming a permanent part of its structure.

Question 21

What are Enzyme inhibitors.

Answer 21

Molecules that bind to the active site to prevent enzymes activity.

Question 22

What are the two types of Enzyme Inhibitors?

Answer 22

Competitive Inhibitors and Non- Competitive inhibitors.

Question 23

What is a Competitive Inhibitor.

Answer 23

A competitive inhibitor molecule can also bind to the active site. 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 molecule can fit in it.

By occupying the active site for a short time, competitive inhibitors prevent the actual molecule substrate from colliding with the active site. As a result the rate of reaction decreases because the frequency of collisions between the substrate and the active site is reduced.

Question 24

How can the effect of a competitive inhibitor be reduced?

Answer 24

The effect of a competitive inhibitor can be reduced by increasing substrate concentration.

If there is way more substrate compared to inhibitor then the substrate is much more likely to collide with the enzyme’s active site.

Question 25

What are examples of Competitive Inhibitors?

Answer 25

The drug Methotrexate is used to treat certain cancers. It is a reversible competitive inhibitor of an enzyme of an enzyme found in human cells.

Penicillin is also a competitive inhibitor of an enzyme involved in the synthesis of bacterial cell walls.

Penicillin binds irreversibly to the enzyme it inhibits. this weakens the cell well and prevents the bacterium from regulating its osmotic pressure. As a result the cell burst and the bacterium is killed.

Question 26

What are Non - Competitive inhibitors?

Answer 26

Non-competitive inhibitors are ones which bind to a site on the enzyme away from its active site. This region is known as its allosteric site.

This causes the active site to change shape so the substrate molecules can no longer bind to it.

The effect of a non-competitive inhibitor cannot be reduced by increasing substrate concentration.

Question 27

What does it mean by Reversible and Irreversible inhibitors?

Answer 27

Reversible inhibitors form weak bonds (e.g. hydrogen, ionic bonds) with the enzyme and are easily removed.

Irreversible inhibitors form strong covalent bonds with the enzyme and can’t be removed easily

Question 28

What are Metabolic poisons?

Answer 28

Metabolic poisons interfere with metabolic reactions (The reactions that occur in cells), causing damage , illness or death.

They are often enzyme inhibitors.

Question 29

What are 3 Examples of Metabolic poisons?

Answer 29

Cyanide = It is a non - competitive, irreversible inhibitors of cytochrome oxidase, an enzyme that catalyses the respiration reactions. Cells that cannot respire die.

Malonate = It is a Competitive inhibitor of Succinate Dehydrogenase. This also catalyses respiration reactions.

Arsenic = It is a non- competitive inhibitor of Pyruvate dehydrogenase. It is also an enzyme catalyses respiration reactions.

Question 30

What is a metabolic pathway?

Answer 30

It is a series of connected metabolic reactions, all catalysed by enzymes.

Question 31

What is End- Product Inhibition?

Answer 31

End-product inhibition is when the final product in a metabolic pathway inhibits an enzyme that acts earlier on in the pathway.

For example, glycolysis is the first stage of respiration and is made up of a series of reactions which converts glucose into energy (ATP).

Phosphofructokinase is an enzyme which catalyses one of these reactions and is inhibited by ATP, preventing further glycolysis if plenty of energy is already available.

Question 32

Why is End- product Inhibition useful?

Answer 32

End-product inhibition is a good way of regulating metabolic pathways and controlling the amount of product produced.

It is reversible, so when the product begins to drop, the enzymes will be able to function again to make more of the product.