2.1.4: Enzymes

Question 1

Turnover number

Answer 1

The number of substrate molecule transformed per minute by 1 enzyme molecule.

Question 2

Vmax

Answer 2

The maximum initial velocity or rate of an enzyme catalysed reaction.

Question 3

Temperature coefficient (Q10)

Answer 3

(of a reaction or process) a measure of how much the rate of the reaction increases with a 10°C rise in temperature; usually taken as 2 for enzyme controlled reactions.

Question 4

Cofactor

Answer 4

A non-protein component necessary for the effective functioning of an enzyme.

Question 5

Factors that affect enzyme activity

Answer 5

• Temperature
• pH
• Substrate concentration

Question 6

What happens to a reaction as [substrate] increases?

Answer 6

• Collisions between enzyme’s active site ad substrate happen more often.
• More enzyme-substrate complexes form
• More products are formed
• Saturation means reaction rate will reach max value

Question 7

What happens to a reaction as temperature increases? (0-40℃)

Answer 7

0-40℃
• Below 0℃, enzyme is inactive
∵ Not enough kinetic energy for enzyme and substrate to collide and form ES complex
• As temperature increases, more thermal energy ∴ more kinetic energy ∴ more collisions
• Nearing 40℃ some enzyme are beginning to denature

Question 8

What happens to a reaction as temperature increases? (40-70℃)

Answer 8

40-70℃
• Some enzymes start to denature
• Tertiary structure starts to break down
• H bonds affected
• Active site loses shape, cannot form ES complexes
• Not reversible

Question 9

What happens to a reaction as pH changes?

Answer 9

• Small deviation from pH that is quickly resolved = some enzymes may be usable
• Only large deviations from optimum pH causes denaturation

Question 10

Why are enzymes in low concentrations in cells?

Answer 10

• Can catalyse reactions fast –> high turnover number
• They are reusable
• Only low concentrations needed
• Low concentrations means the enzyme’s effects are easier to control

Question 11

Types of cofactors

Answer 11

• Coenzymes
• Prosthetic groups
• Inorganic ion cofactors

Question 12

How do coenzymes work?

Answer 12

• Donate/accept chemical groups
• Coenzymes often participate in reaction -> are chemically modified
• Recycled to participate in another reaction

–> Vitamins can be a source of coenzymes

Question 13

Features of prosthetic groups

Answer 13

• Permanent, integral part of enzyme
• Not necessarily covalently bonded to enzyme
• Do not dissociate from enzyme
• Essential for enzyme’s function
• Can be organic or inorganic
• Non-protein: wide variety of molecular structures can act as prosthetic groups

Question 14

How do prosthetic groups work?

Answer 14

• Contribute to overall 3D shape of enzyme
• Can contribute to shape/charge of active site ∴ substrate binding
• e.g. carbonic anhydrase, Zn²⁺ ion helps weaken H–O bond

Question 15

Inorganic ion cofactors

Answer 15

• Not a permanent part of the enzyme
• Can combine with enzyme or substrate
• Can combine with ES complex by affecting shape/charge of active site
• Amylase requires chloride ions to function

Question 16

How does a competitive inhibitor work?

Answer 16

1) Inhibitor enters environment
2) Shape of inhibitor is similar to shape of substrate; both have a shape complementary to the enzyme’s active site
3) Due to kinetic energy, all molecules moving
4) Inhibitor reaches active site before substrate –> binds to AS ∴ AS blocked and substrate prevented from binding
5) Other AS available so reaction continues at a slower rate

Question 17

How does a non-competitive inhibitor work?

Answer 17

1) Inhibitor enters enzyme’s environment
2) Shape of inhibitor is different to shape of substrate
3) Shape of inhibitor is complementary to enzyme’s allosteric site
4) Inhibitor binds to allosteric site –> causes reaction which changes tertiary structure of enzyme
5) Substrate can no longer fit into active site
6) If inhibitor reversible, it can dissociate with the enzyme. The enzyme can then regain its tertiary structure.

Question 18

Example of an enzyme that catalyses an intracellular reaction

Answer 18

Catalase (reduction of hydrogen peroxide, think liver)

Question 19

Example of an enzyme that catalyses an extracellular reaction

Answer 19

Amylase (starch and glycogen breakdown)

Trypsin (protein digestion)

Question 20

Example of a competitive inhibitor (poison)

Answer 20

• Fasciculin (snake venom)
• Competitive inhibitor to enzyme that degrades acetylcholine neurotransmitter
• Stops muscle contraction ⟶ flaccid paralysis

Question 21

Example of a non-competitive inhibitor (poison)

Answer 21

• Potassium cyanide
• Cytochrome oxidase combines hydrogen and oxygen to form water and allow ATP creation; potassium cyanide non-competitively inhibits cytochrome oxidase
• Reactions requiring ATP are no longer supplied ⟶ body eventually has no energy causing total cell failure

Question 22

Example of a competitive inhibitor (medicine for viral infection)

Answer 22

• HIV Protease inhibitor
• Competitively inhibits HIV virus’ protease enzyme ⟶ virus cannot cut RNA into pieces to inject into host cell’s DNA
• Host cell can be infected but not hijacked by the HIV virus

Question 23

Example of a competitive inhibitor (medicine for poison)

Answer 23

• Ethanol as a treatment for antifreeze poisoning
• Ethylene glycol = found in antifreeze; if ingested is digested into oxalic acid (toxic)
• Ethanol competitively inhibits alcohol dehydrogenase (i.e. less ethylene glycol gets broken down)
⟶ Less oxalic acid produced (drunk rather than dead)

Question 24

End-product inhibition

Answer 24

the product of a reaction inhibits the enzyme required for the reaction.