2.1.4 Enzymes

Question 1

What is an enzyme?

Answer 1

• A globular protein with complex tertiary structure (some have a quaternary structure)
• Biological catalyst
• Speeds up a reaction without being used up

Question 2

How do enzymes affect the activation energy?

Answer 2

They lower it

Question 3

What happens when an enzyme is catalysing a breakdown?

Answer 3

• Fitting into the active site puts a strain on the bonds of the substrate
• They can be broken more easily

Question 4

How are two substrate molecules joined?

Answer 4

• Enzyme holds them together
• They can bond more easily

Question 5

What is intracellular enzyme action?

Answer 5

When enzymes are made and retained inside the cell
e.g. catalase, polymerase, lysosomal enzymes, ATPase, ATP synthase

Question 6

What is catalase?

Answer 6

• An intracellular enzyme
• Breaks down hydrogen peroxide into water and oxygen
• 2H2O2 –> O2 + 2H2O
• Found in peroxisomes (vesicles in liver cells)

Question 7

What is extracellular enzyme action?

Answer 7

• Secreted by cells and work outside of the cells
  E.g. amylase

Question 8

What is amylase?

Answer 8

• An extracellular enzyme
• Works outside of cells in saliva to catalyse the hydrolysis of starch into maltose

Question 9

What is trypsin?

Answer 9

• An extracellular enzyme
• Hydrolyses peptide bonds of large polypeptides into smaller ones
• Smaller ones are eventually broken down to amino acids by other enzymes

Question 10

What enzymes are in fungi?

Answer 10

• Extracellular
• Cells in fungal hyphae synthesise enzymes that are secreted outside the cell to digest organic matter
• Absorbed and used for growth

Question 11

What is the structure of an enzyme?

Answer 11

Globular protein:
- 3D shape
- Almost spherical
- Soluble in water

Active site:
- Specific shape
- Where substrate binds to

Question 12

What is the lock and key model?

Answer 12

• The old model
• Enzymes will only catalyse certain reactions because the active site is a specific shape to fit the substrate
• There is no change as active site is exactly complimentary

Question 13

What is the induced fit model?

Answer 13

• New model
• Enzyme isn’t complimentary to the substrate
• Enzyme changes shape slightly as the substrate binds

Question 14

How does temperature affect enzyme activity?

Answer 14

• At low temperature, enzyme is deactivated (temporary)
• As temperature increases so does kinetic energy
• The number of successful collisions with the active site increases the rate of reaction
• Optimum temperature is where the enzyme activity is at its highest as there will be the maximum amount of enzyme-substrate complexes
• When temperature is too high, vibrations break bonds that hold enzyme together so enzyme denatures and activity decreases

Question 15

What is denaturation?

Answer 15

• Irreversible change to the tertiary structure of the active site
• No longer complementary to substrate
• No E-S complezes

Question 16

How are enzymes in thermophiles adapted?

Answer 16

• Thermophiles live in very hot environments
• Enzymes have more hydrogen bonds and disulfide bridges in the enzyme protein molecules
• More resistant to temperature rises
• Cyanobacteria live between 40 and 85 degrees
• Spirochetes live between 40 and 73 degrees

Question 17

What is Q10?

Answer 17

A factor by which the rate of enzyme activity increases when the temperature is raised by 10 degrees

Question 18

What is the formula for Q10?

Answer 18

Q10 = rate at higher temperature/rate at lower temperature

Question 19

How does pH affect enzyme activity?

Answer 19

• As you move away from the optimum temperature, enzymes start to denature and activity decreases
• H+ and OH- ions break the ionic and hydrogen bonds that hold the enzymes tertiary structure in place
• No longer able to form E-S complexes
• Either side of the optimum is permanent denaturation
• Optimum pH is the pH the enzyme works best at

Question 20

What are the optimum pHs of enzymes?

Answer 20

• Pepsin is in the stomach with HCl so optimum is low (1-2)
• Amylase is in the mouth so needs to be neutral (6-7)

Question 21

How do you control the pH in an investigation?

Answer 21

Buffer solutions

Question 22

How does enzyme concentration affect enzyme activity?

Answer 22

• As enzyme concentration increases, rate of reaction increases as there are more active sites
• Substrate concentration is a limiting factor so eventually increasing enzyme concentration doesn’t increase ROR

Question 23

How does substrate concentration affect enzyme activity?

Answer 23

• As substrate concentration increases, the rate of reaction increases
• More active sites are engaged
• More successful collisions with active sites
• Point of saturation (Vmax) is when all the active sites are filled
• Past this, enzymes concentration is a limiting factor so ROR doesn’t increase (graph plateaus)

Question 24

What is a cofactor?

Answer 24

A non-protein which binds to enzymes to make them work

Question 25

What is an inorganic cofactor?

Answer 25

• An inorganic ion or molecule
• Helps the enzyme and substrate to bind together but doesn’t directly participate in the reaction
• Not used up or changed
• Not inhibitory and enables the reaction

E.g. Chloride ions (Cl-) are cofactors for amylase

Question 26

What are organic cofactors/coenzymes?

Answer 26

• Participate in the reaction and are changed
• They often act as carriers, moving chemical groups between enzymes
• Continually recycled during the process

E.g. NAD

Question 27

What is a prosthetic group?

Answer 27

• A cofactor that is tightly bound to an enzyme

E.g. Zinc ions (Zn2+) are a prosthetic group for carbonic anhydrase so are a permanent part of the enzyme’s active site

Question 28

What are competitive inhibitors?

Answer 28

• Similar shape to active site and substrates
• Bind to the active site but no reaction takes place
• This blocks the active site and leads to a slower reaction

E.g. Malonate is a competitive inhibitor for succinate dehydrogenase

Question 29

What are non-competitive inhibitors?

Answer 29

• These bind to the allosteric site
• Causes a change in shape of the active site
• The substrate molecule can no longer fit so enzyme-substrate complexes are no longer able to be formed

E.g. Copper sulfate is a non-competitive inhibitor for catalase

Question 30

How do the graphs of inhibitors compare?

Answer 30

• A competitive inhibitor will have a slower rate of reaction but still reach the same end point
• A non-competitive inhibitor has a slower rate of reaction and lower end point as the enzyme’s active site is permanently damaged
• All 3 lines start from the same point

Question 31

What is a reversible inhibition?

Answer 31

• Inhibitor can be removed
• Due to weaker hydrogen bonds or weak ionic bonds

Question 32

What is a non-reversible inhibition?

Answer 32

• Inhibitor cannot be removed easily
• Due to strong covalent bonds

Question 33

What do metabolic poisons do?

Answer 33

They interfere with metabolic reactions in cells

Question 34

What is cyanide?

Answer 34

• Non-competitive inhibitor of cytochrome C oxidase
• Involved with respiration

Question 35

What is arsenic?

Answer 35

• Non-competitive inhibitor of pyruvate dehydrogenase
• Involved with glycolysis

Question 36

What is malonate?

Answer 36

• Competitive inhibitor of succinate dehydrogenase
• Involved with the kreb cycle