Unit 2: Section 3 - Enzymes

Question 1

1. What are enzymes? What type of protein are they?
2. What is a catalyst?
3. What can enzyme action be?

Answer 1

1. Enzymes are biological catalysts that catalyse (speed up) metabolic reactions in your body e.g. digestion and respiration. Enzymes are globular proteins.
2. A catalyst is a substance that speeds up a chemical reaction without being used up in the reaction itself.
3. Enzyme action can be intracellular - within cells, or extracellular - outside cells (e.g. in places like the blood and digestive sytem)

Question 2

1. Describe an enzyme’s structure in relation to its function.
2. Briefly explain how enzymes work and why they are so specific.

Answer 2

1. Enzymes have an active site, which has a specific shape. The active site is the part of the enyzme where the substrate molecules (the substance that the enzyme interacts with) bind to. The specific shape of the active site is determined by the enzyme’s tertiary structure.
2. For the enzyme to work, the substrate has to fit into the active site (its shape has to be complementary). If the substrate shape doesn’t match the active site, the reaction won’t be catalysed. This means that enzymes work with very few substrates - usually only one.

Question 3

1. Briefly explain what activation energy is.
2. What do enzymes do to the activation energy and what is the result of this?

Answer 3

1. In a chemical reaction, a certain amount of energy needs to be supplied to the chemicals before the reaction will start. This is called the activation energy - its often provided as heat.
2. Enzymes reduce the amount of activation energy that’s needed, often making reactions happen at a lower temperature than they could without an enzyme. This speeds up the rate of reaction.

Question 4

When a substrate binds to an enzyme’s active site, an enzyme-substrate complex is formed. It’s the formation of the enzyme-substrate complex that lowers the activation energy. Explain 2 reasons why.

Answer 4

• If the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate. This strain means the substrate molecule breaks up more easily.
• If two substrate molecules need to be joined, attatching to the enzyme holds them closer together, reducing any repulsion between the molecules so they can bond more easily.

Question 5

Explain the Lock and Key Model.

Answer 5

Early scientists studying the action of enzymes came up with the lock and key model. This is where the substrate fits into the enzyme in the same way that a key fits into a lock. The model states that the enzyme and the substrate have to fit together at the active site of the enzyme. This creates an enzyme-substrate complex. The active site then causes changes in the substrate (e.g. bringing molecules closer together or putting a strain on bonds). This change results in an enzyme-product complex and eventually the substrate being broken down/ joined together (to another substrate) to form products.

Question 6

1. What problem was discovered regarding the Lock and Key Model?
2. What did scientists modify the Lock and Key Model into?

Answer 6

1. Scientists soon realised that the lock and key model didn’t give the full story. The enzyme and substrate do have to fit together in the first place, but new evidence showed that the enyzme-substrate complex changed shape slightly to complete the fit. This locks the substrate even more tightly to the enzyme.
2. The Induced Fit Model/Theory.

Question 7

1. What does the Induced Fit Model help to explain?
2. Explain the Induced Fit Model and it’s difference to the Lock and Key Model.

Answer 7

1. The Induced Fit Model helps to explain why enzymes are so specific and usually bond to only one particular substrate.
2. The induced fit model has the same basic mechanism as the lock and key model:

The model states that the enzyme and the substrate have to fit together at the active site of the enzyme. This creates an enzyme-substrate complex. The active site then causes changes in the substrate (e.g. bringing molecules closer together or putting a strain on bonds). This change results in an enzyme-product complex and eventually the substrate being broken down/ joined together (to another substrate) to form products.

The difference with the lock and key model is that the substrate is thought to cause a change in the enzyme’s active site shape (and so slightly change the shape of the enzyme-substrate complex), which enables a better fit (locks the substrate even more tightly to the enzyme).

Question 8

Explain the effect of temperature on enzyme activity.

Answer 8

Like any chemical reaction, the rate of an enzyme-controlled reaction increases when the temperature’s increased. More heat means more kinetic energy, so the molecules move faster. This creates more random collisions and so makes the enzymes more likely to collide with the substrate molecules. Because of this, more enzyme-substrate complexes are formed and the rate of reaction increases. The energy of these collisions also increases, which means each collision is more likely to result in a reaction.

But, if the temperature gets too high, the reaction stops. The rise in temperature makes the enzyme’s molecules vibrate more. If the temperature goes above a certain level (above the optimum temperature), this vibration breaks some of the bonds (e.g. hydrogen and ionic bonds) that hold the enzyme in shape. This alters the tertiary structure and so changes the active site shape making it no longer complementary (so that the enzyme and substrate no longer fit together). There is now a massive decrease in enzyme-substrate complexes. At this point, the enzyme is denatured (no longer functions as a catalyst) and the rate of reaction stops.

Question 9

Every enzyme has an optimum temperature. What is this temperature for most human enzymes and those enzymes used in biological washing powders?

Answer 9

For most human enzymes it’s around 37^oc but some enzymes, like those used in biological washing powders, can work well at 60^oc.

Question 10

1. Define pH.

• State the pH values in relation to H⁺ions.
  1. State the 2 types of bonds that are affected by changes in pH.

Answer 10

1. pH is a measure of the concentration of H⁺ions.

• High concentration of H⁺ ions = Acidic (low pH of 1-6)
• Low concentration of H⁺ ions = Alkaline (high pH of 8-14)

1. Changes in pH affect Hydrogen bonds and ionic bonds.

Question 11

1. Give 2 examples of optimum pH values in relation to enzymes.
2. Explain what happens to enzyme activity when the optimum pH is reached.

Answer 11

1. All enyzmes have an optimum pH value. Most human enzymes work best at pH 7 (neutral), but there are exceptions. Pepsin, for example, works best at acidic pH 2, which is useful because it’s found in the stomach.
2. At the optimum pH, the concentration of H+ ions is such that there is minimum interference with hydrogen and ionic bonds of the enzyme. This means that the tertiary structure is unaltered as is the active site. This allows the maxium amount of enzyme-substrate complexes to be formed which means that the rate of reaction is at a maxium.

Question 12

Explain what happens to enzyme activity when pH is above or below the optimum pH.

Answer 12

Above and below the optimum pH, the concentration of H⁺ions and OH^- ions found in acids and alkalis begins to affect the hydrogen and ionic bonds that hold the enzyme’s tertiary structure in place. This alters the tertiary structure and the enzyme’s active site. As a result, the rate of reaction is reduced as less enzyme-substrate complexes are formed due to the active site being no longer complementary. At this point, the enzyme is denatured.

Question 13

Explain the effect of increasing enzyme concentration on enzyme activity.

Answer 13

The more enzyme molecules there are in a solution, the more likely a substrate molecule is to collide with one (so more active sites will be used) and form an enzyme-substrate complex. So increasing the concentration of the enzyme increases the rate of reaction.

But, if the amount of substrate is limited, this effect is only true up until a ‘saturation’ point when there’s more than enough enzyme molecules to deal with all the available substrate, so adding more enzyme has no further effect.

- here the limiting factor is the substrate concentration

Question 14

Explain the effect of increasing substrate concentration on enzyme activity.

Answer 14

The more substrate molecules there are in a solution, the more likely an ennzyme molecule is to collide with one (so more active sites will be used) and form an enzyme-substrate complex. So increasing the concentration of the substrate increases the rate of reaction.

But, if the amount of enzyme is limited, this effect is only true up until a ‘saturation’ point when there’s more than enough substrate molecules to deal with all the available enzyme, so adding more substrate has no further effect.

- here the limiting factor is the enzyme concentration

Question 15

What are Cofactors and explain how they carry out their role?

Answer 15

Cofactors are usually non-protein, inorganic molecules that bind to either the enzyme or the substrate. They work by helping the enzyme and substrate to bind together and allowing enzyme-substrate complexes to form easier.

They don’t directly participate in the reaction so aren’t used up or changed in any way.

e.g. manganese ions are cofactors found in hydrolase (enzymes that catalyse the hydrolysis of chemical bonds).

Question 16

What are Coenzymes and explain how they carry out their role?

Answer 16

Some cofactors are non-protein, organic molecules called coenzymes, that bind to the active site at a similar time as the substrate. They often act as carriers, moving chemical groups between different enzymes. They’re continually recycled during this process.

They participate in the reaction and are changed by it (they act like a ‘second substrate’).

Question 17

1. Enzyme activity can be inhibited - prevented by enzyme inhibitors. Define enzyme inhibitors.
2. State the 2 types of inhibitor molecules.
3. What are the 2 types of inhibition?

Answer 17

1. Enzyme inhibitors are molecules that bind to the enzyme that they inhibit.
2. There are competitive inhibitor molecules and non-competitive inhibitor moleucles.
3. Inhibition can be competitive or non-competitive.

Question 18

Explain the process of Competitive Inhibition.

Answer 18

Competitive inhibitor molecules have a similar shape to that of the substrate molecules and so are complementary to the enzyme’s 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 and form an enzyme-inhibitor complex, so that no substrate molecules can fit in it.

How much the enzyme is inhibited depends on the relative concentrations of the inhibitor and substrate. If there’s a high concentration of the inhibitor, it’ll take up nearly all the active sites and hardly any of the substrate will get to the enzyme.

Question 19

Explain the process of Non-Competitive Inhibition.

Answer 19

Non-competitive inhibitor molecules bind to the enzyme away from its active site. When attatched they affect the tertiary structure of the enzyme and so cause the active site to change shape meaning that the substrate molecules can no longer bind to it. Because the inhibitors have a different shape to the substrate they don’t compete with the substrate molecules to bind to the active site.

Question 20

Describe the effect of increasing the substrate concentration during competitive inhibition.

Answer 20

Even when a competitive inhibitor is present, it’s effect can be reduced/overcome by increasing the substrate concentration - this increases the chance of a successful enzyme-substrate collision occuring.

Question 21

Describe the effect of increasing the substrate concentration during non-competitive inhibition.

Answer 21

Increasing the concentration of substrate won’t make any difference during non-competitive inhibition - enzyme activity will still be inhibited as the non-competitve inhibitors bind to the enzyme away from its active site.

Question 22

Inhibitors can be reversible or irreversible. How is each one determined?

Answer 22

Most of the inhibiton that occurs is reversible (or temporary), other types of inhibiton are irreversible (or permanent). Which one they are depends on the strength of the bonds between the enzyme and inhibitor.

• If they’re strong, covalent bonds, the inhibitor can’t be removed easily and the inhibiton is irreversible.
• If they’re weaker hydrogen bonds or weak ionic bonds, the inhibitor can be removed and the inhibition is reversible.

Question 23

1. Some Metabolic Poisons are enzyme inhibtors. Briefly describe what they do.
2. Describe 3 examples of inhibtors that inhibit enzymes in which catalyse respiration reactions.

Answer 23

1. Metabolic poisons interfere with metabolic reactions (the reactions that occur in cells), causing damage, illness or death.
2. 3 examples:

• Cyanide is an irreversible inhibitor of cytochrome c oxidase, an enzyme that catalyses respiration reactions. Cells that can’t respire die.
• Malonate inhibits succinate dehydrogenase, an enzyme that catalyses respiration reactions. Cells that can’t respire die.
• Arsenic inhibits the action of pyruvate dehydrogenase, an enzyme that catalyses respiration reactions. Cells that can’t respire die.

Question 24

Some medicinal drugs are enzyme inhibitors and work by inhibiting enzymes. Describe and explain 2 examples.

Answer 24

• Some antiviral drugs (drugs that stop viruses like HIV) - e.g. reverse transcriptase inhibitors inhibit the enzyme reverse transcriptase, which catalyses the replication of viral DNA. This inhibition prevents the virus from replicating.
• Some anitbiotics - e.g. penicillin inhibits the enzyme transpeptidase, which catalyses the formation of proteins in bacterial cell walls. This inhibition weakens the cell wall and prevents the bacterium from regulating its osmotic pressure. As a result the cell bursts and the bacterium is killed.