2.1.4 - Enzymes

Question 1

Enzymes

Answer 1

• Enzymes are biological catalysts
• Enzymes are globular proteins
• Enzymes increase the rate of reaction by lowering the activation energy of the reaction

Question 2

Metabolism

Answer 2

• Metabolism is the sum of all the different reactions and reaction pathways happening in a cell or organism
• Anabolic reactions are reactions of metabolism that construct molecules from smaller units (building up)
• Catabolic reactions are reactions of metabolism that break molecules down into smaller units. (breaking down)

Question 3

Intracellular reactions

Answer 3

• Enzyme works within the cell that it was made in
  • Catalase catalyses intracellular reactions(reactions within cells)
  o Catalyses the breakdown of hydrogen peroxide
  o Hydrogen peroxide is a toxic product of many metabolic pathways
  o Catalase breaks it down into oxygen and water

Question 4

Extracellular reactions

Answer 4

• Enzyme works outside the cell that it was made in
  • Amylase and trypsin catalyse extracellular reactions(work outside the cell that made them)
  o Amylase catalyses the breakdown of starch
  o Trypsin catalyses the breakdown of proteins

Question 5

Digestion of starch

Answer 5

• Begins in mouth and continues in the small intestine
• Amylase produced in salivary glands and pancreas
• Amylase works in the saliva in the mouth and small intestine
• Amylase: Starch to maltose
• Maltase : Maltose to glucose

Question 6

Digestion of proteins

Answer 6

• Trypsin produced in the pancreas
• Trypsin works in the small intestine
• Trypsin breaks down proteins into smaller peptides, then other professes will break it down into amino acids

Question 7

Tertiary structure

Answer 7

• The interactions between R-groups that leads to the further folding of the secondary structure
• It determines the shape of the active site

Question 8

Specificity

Answer 8

o The active site is specific. It is only complementary to the shape of a specific substrate molecule.
o Supported by lock and key hypothesis

Question 9

Active site

Answer 9

o Area of an enzyme with a shape complementary to a specific substrate, allowing the enzyme to bind a substrate with specificity

Question 10

Lock and key hypothesis

Answer 10

o Only the right key will fit into the lock, only a specific substrate will fit in to the active site. The active site is specific.

Question 11

Induced fit hypothesis

Answer 11

o The active site of the enzyme is modified in shape by binding to the substrate
o The initial interactions are weak, but they induce changes in the tertiary structure
o This strengthens binding and puts strain on the substrate molecule
o This can weaken particular bonds in the substrate lowering the activation energy

Question 12

Enzyme-substrate complex

Answer 12

o Complex formed when a substrate is bound to the active site of an enzyme

Question 13

Enzyme-product complex

Answer 13

o Complex formed as a result of an enzyme catalysed reaction, when a substrate is converted to a product or products while bound to the active site of an enzyme

Question 14

Lowering of activation energy

Answer 14

o The energy needed for a reaction to start is known as activation energy
o Enzymes lower the activation energy increasing rate of reaction

Question 15

Effect of temperature on enzyme activity

Answer 15

• Increasing the temperature, increases the kinetic energy of the particles
• The particles move faster and collide more frequently.
• As a result there are more frequent successful collisions between the substrate and enzyme.
• This leads to an increase in the rate of reaction
• At high temperatures the binds holding the protein together vibrate more
• The vibrations increase until the bonds strain and then break
• The breaking of these bonds result in a change in the precise tertiary structure of the protein.
• The active site changes shape and the enzyme is denatured, so the active site is no longer complementary to the substrate.

Question 16

Optimum temperature

Answer 16

The optimum temperature is the temperature at which the enzyme has the highest rate of activity.
The temperature coefficient, Q10 of a reaction is a measure of how much the rate of reaction increases with a 10 degrees Celsius rise in temperature.

Question 17

Effect of pH on enzyme activity

Answer 17

• Hydrogen bonds and ionic bonds between amino acid R-groups hold proteins in their precise 3D shape.
• A change in pH changes the hydrogen ion concentration.
• This affects the hydrogen and ionic bonds and the interactions between R-groups.
• The tertiary structure of the active site is altered, the enzyme is denatured.
• The substrate no longer binds to the active site.
• This reduces the rate of reaction.

Question 18

Effect of enzyme concentration on enzyme activity

Answer 18

• If the enzyme concentration is increased there are more active sites available
• More enzyme-substrate complexes are formed
• The concentration of substrate then become the limiting factor reducing the rate of reaction

Question 19

Effect of substrate concentration on enzyme activity

Answer 19

• Higher substrate concentration results in a higher collision rate with the active site
• More enzyme-substrate complexes are formed
• Once all enzymes are bound to a substrate the rate of reaction plateaus

Question 20

Cofactors, coenzymes

Answer 20

• Cofactors : Non-protein components necessary for the effective functioning of an enzyme
• They may transfer atoms or groups from one reaction to another in a multi-step pathway or they may form part of the active site of an enzyme.
• If the cofactor is an organic molecule it’s called coenzyme
• Cofactors are obtained via the diet as minerals, including iron, calcium, chloride and zinc ions
• The enzyme amylase contains a chloride ion that is necessary for the formation of a correctly shaped active site, so it can digest starch.
• Coenzymes are derived from vitamins in the diet

Question 21

Prosthetic groups

Answer 21

• Prosthetic groups are non-protein groups
• Prosthetic groups are tightly bound and form a permanent feature of the protein
• Zn2+ is a prosthetic group of carbonic anhydrase, which is necessary for the metabolism of carbon dioxide

Question 22

Precursor activation

Answer 22

• Many enzymes are produced in an inactive form
• Precursor enzymes need to undergo a change in shape of active site or change in conditions to be activated
• The precursor protein is called an apoenzyme. When the cofactor is added and the enzyme is activated it is called a holoenzyme.
• A change in pH or temperature can result in a change in the tertiary structure and activates the precursor enzyme. These types of precursor enzymes are called zymogens or proenzymes.

Question 23

Competitive inhibition

Answer 23

• A molecule or part of a molecule that has a similar shape to the substrate of an enzyme can fit into the active site of an enzyme
• This blocks the substrate from entering the active site
• The enzyme cannot carry out its function
• Substrate and inhibitor compete with each other to bind to the substrate
• Most competitive inhibitors bind temporarily, so their effect is reversible

Question 24

Non-competitive inhibition

Answer 24

• The inhibitor binds to the enzyme at a location other than the active site. This alternative site is called an allosteric site
• The binding of the inhibitor causes the tertiary structure of the enzyme to change, the active site changes shape
• The active site is no longer complementary to the substrate
• The substrate can’t bind to the enzyme

Question 25

Competitive inhibition : Effect on rate of reaction

Answer 25

• As the concentration of the competitive inhibitor is increased, the rate of reaction decreases
• If substrate concentration increased enough Vmax can be reached

Question 26

Non-competitive inhibition : Effect on rate of reaction

Answer 26

• As the concentration of the non-competitive inhibitor is increased, the rate of reaction decreases
• As the concentration of substrate is increased there is no effect on rate of reaction

Question 27

End-product inhibition

Answer 27

• The product of the reaction acts as an inhibitor
• Negative feedback
• Non-competitive reversible inhibition

Question 28

Explain the effect of substrate concentration on rate of reaction

Answer 28

Linear part:

• More successful collisions with active site
• More enzyme-substrate complex formed
• More product formed in a given time

Plateaus:

• All active sites are occupied
• Further increase in substrate concentration has no effect on the rate
• Enzyme concentration becomes limiting factor

Question 29

Suggest why the lock-and-key and induced-fit explanations are termed models.

Answer 29

idea of simple representation of the , process / structure or
idea of showing people how it works ;

Question 30

Suggest why most scientists now accept the induced-fit model rather than the lock and key model

Answer 30

supported by , more evidence / new research / more work ;
1 max
ACCEPT example, e.g. X-ray crystallography
idea of fitting evidence more closely (than lock & key) ;

Question 31

Explain the term biological catalyst

Answer 31

• Enzymes are proteins used in metabolism
• They alter rate of reaction by lowering activation energy and providing alternative route for reaction
• They are not changed or used up

Question 32

Why do Cl- ions increase rate of reaction

Answer 32

• Act as cofactor
• Cl- binds to enzyme
• ESC forms more easily