Chapter 4: Enzymes :>

Question 1

What is a catalyst?

Answer 1

• A catalyst is a substance that speeds up the rate of a chemical reaction without altering itself at the end of the reaction.

Question 2

What are biological catalysts?

Answer 2

• Biological catalysts are substances that are found in living things and are made out of protein.
• They can also alter the rate of chemical reactions without changing themselves at the end of the reactions.
  (E.g.) One example of a biological catalyst is an enzyme, like a biological molecule.

Question 3

How can enzymes change or speed up chemical reactions?

Answer 3

• Enzymes can change or speed up chemical reactions by lowering the activation energy of a chemical reaction.
• Activation energy is the energy needed to start a chemical reaction.
• When an enzyme is present, it speeds up the reaction by lowering the activation energy.
• Thus, the rate at which the products are formed is increased.

Question 4

What is an activation energy?

Answer 4

• Activation energy is the energy needed to start a chemical reaction.

Question 5

How many groups can enzyme-catalysed reactions be classified in?

Answer 5

• Enzyme-catalysed reactions can be classified into 2 groups:
  1. Anabolic reactions in which complex substances are synthesised from simpler ones.
  (simpler –> complex)
  2. Catabolic reactions in which complex substances are broken into simpler ones.
  (complex –> simpler)

Question 6

What are the examples of enzyme-catalysed reactions?

Answer 6

1. Synthesis of complex substances:
   - Special enzymes in the cytoplasm build up proteins from amino acids.
   - Enzymes are involved in the synthesis of glucose from carbon dioxide and water during photosynthesis. (Photosynthesis equation: Carbon dioxide + Water — (sunlight +chlorophyll) –> Oxygen + Glucose)
2. Breaking down of complex substances:
   - Enzymes such as digestive enzymes break down large molecules in food into smaller ones which are soluble in water and can pass through the cell membrane.
   - Certain enzymes break down large molecules that are toxic, into smaller and safer ones.
   (E.g.) Hydrogen peroxide is produced during some chemical reactions in plant and animal cells, but this is toxic to the cells.
   (E.g.) Catalyse breaks down hydrogen peroxide from water and oxygen.
   (E.g.) Catalyse is abundant in the blood and liver of mammals.
   - Various enzymes break down glucose to release energy, carbon dioxide, and water in cellular respiration.

Question 7

What are the enzymes that are involved in the breaking down of glucose to release energy, carbon dioxide, and water during cellular respiration?

Answer 7

• During glycolysis (the first stage of cellular respiration that takes place in the cytoplasm of cells), glucose is broken down into two molecules of PYRUVATE.
  (E.g.) Dehydrogenase:
• It oxidises glucose by stripping 2 electrons from it and transferring them to a coenzyme called NAD+.

(E.g.) Pyruvate dehydrogenase:
- It helps in the conversion of pyruvate, which is transported into the mitochondria and is then converted into acetyl-CoA.

(E.g.) Isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, and succinate dehydrogenase:
- Enzymes that are found in the citric acid cycle which takes place in the mitochondrial matrix.
- During this stage, acetyl-CoA is broken down into carbon dioxide and water, thus releasing energy during this process.

(E.g.) ATP synthase:
- During oxidative phosphorylation, it takes place in the inner mitochondrial membrane.
- During this stage, the energy that is released from previous stages is used to generate ATP.

Question 8

What are the examples of digestive enzymes?

Answer 8

(Digestive enzymes aid in the digestion and breaking down of substances)
1. Carbohydrase:
Function: To digest carbohydrates.
(E.g.) (A) Amylase digests starch into maltose.
(B) Maltase digests maltose into glucose.
(C) Sucrase digests sucrose into glucose and fructose.
(D) Lactase digests lactose into glucose and galactose.

2. Protease:
   Function: To digest proteins.
   (E.g.) (A) Pepsin digests proteins into polypeptides.
   (B) Trypsin digests proteins into polypeptides.
   (C) Peptidases digest polypeptides into amino acids.
3. Lipase:
   Function: To digest fats (lipids)
   (E.g.) (A) Steapsin digests lipids into glycerol and fatty acids.

Question 9

What are substrates?

Answer 9

• Substrates are substances that enzymes act on.

Question 10

What is an active site and what are the substances that can fit into it?

Answer 10

• An active site is where a substrate fits into, which are also depressions on the surface of an enzyme.

Question 11

How are enzymes specific in their action?

Answer 11

• Enzymes are highly specific in their action.
• Each chemical reaction is catalysed by a unique enzyme, and this specifically is due to the enzyme’s 3D shape.
• Thus, this means that only a substrate with a shape complementary to that of the enzyme’s active site can fit into the active site.

Question 12

What is the lock-and-key hypothesis?

Answer 12

• According to the lock-and-key hypothesis, the enzyme is like a lock and the substrate is like a key.

Question 13

What is an enzyme-substrate complex and how is = it formed?

Answer 13

• An enzyme-substrate complex is created when a substrate binds to the active site of an enzyme.

Question 14

Why do chemical reactions occur at the enzyme’s active site?

Answer 14

• Chemical reactions occur at the enzyme’s active site to convert the substrate into products.

Question 15

What is the mode of action of an enzyme?

Answer 15

(Enzyme changes shape light as the substrate binds themselves together)

1. The substrate enters the active site, but it is not fully connected to the enzyme.
2. Enzyme-substrate complex.
3. Enzyme-product complex.
4. Products leave the active site.

Question 16

What happens after the products leave the active site?

Answer 16

• Once the reactions are completed, the products leave the active site.

Question 17

Why must only a small amount of an enzyme is needed to catalyse the reaction for a large amount of substrate 9substances that enzymes work on)?

Answer 17

• The enzyme remains unchanged, and it is free to catalyse another reaction.
• The same enzyme molecule can be used repeatedly so only a small amount of an enzyme is needed to catalyse the reaction for a large amount of substrate.

Question 18

What are the 2 factors that directly affect the rate of enzyme reactions?

Answer 18

1. Temperature/temperate
2. pH value/levels.

Question 19

What is the best condition the enzyme work best at to function at its best?

Answer 19

• Enzymes work best at their optimum (most suitable) temperature and pH value/level.

Question 20

What does “temperature sensitive” mean and how do we use it?

Answer 20

• enzymes are temperature sensitive which means that they are sensitive to temperature (changes?)

Question 21

Do enzymes and substrates have a higher/lower kinetic energy at low temperature?

Answer 21

• At low temperatures, the enzyme and substrate molecules have low kinetic energy.
• During the reactions of enzymes at low temperatures, the enzymes, and substrate molecules move slowly and there is a lower chance of the molecules colliding with one another.
• Thus, the rate of enzyme activity is low.

Question 22

What happens to the rate of enzyme reactions when the temperature increases from a low to an optimum temperature?

Answer 22

• When the temperature increases, the molecule has more kinetic energy.
• As the temperature increases, the molecules have more kinetic energy.
• This increases the chances of the substrate molecules colliding with the enzymes and fitting into the active site.
• For every 10℃ increase in temperature, the enzyme reaction rate doubles until the optimum temperature is reached.
• Enzymes in the human body have an optimum temperature of 37℃.

Question 23

What happens to the rate of enzyme reactions when the temperature of the surroundings is increased beyond the optimum temperature?

Answer 23

• When the temperature is increased beyond the optimum temperature, the rate of enzyme reactions rapidly increases.
• High temperatures will break the bonds of the enzyme which keeps it in shape (maintaining its structure).
• Thus, the enzyme’s active site loses its original shape, and the substrate molecule can no longer fit into the active site.
• The enzyme is then said to be DENATURED when the active site of the enzyme loses its original shape, and the substrate molecule can no longer fit into the active site, so it can no longer act as a catalyst.
• SIDE NOTE: most enzymes are irreversibly denatured above 60℃.

Question 24

What does it mean for an enzyme to b e denatured?

Answer 24

• An enzyme is denatured when the active site of an enzyme loses its original shape, and the substrate molecule can no longer fit into the active site.

Question 25

What is denaturation?

Answer 25

• Denaturation is the change in the 3D structure of an enzyme or any other soluble protein.
• It may be caused by heat or chemicals such as acids and alkalis.

Question 26

What does it mean when enzymes are pH sensitive?

Answer 26

• Enzymes are pH sensitive so as the pH deviates from the optimum pH of the enzyme, the rate of enzyme reaction rapidly decreases.
• The extreme changes in terms of the acidity or alkalinity of the solution in which the enzyme acts.

Question 27

Do all enzymes have the same optimum pH values at which they work best?

Answer 27

• No, different enzymes have different optimum pH values at which they work best in.