Enzymes

Question 1

What do enzymes do?

Answer 1

Catalyse almost every cellular reaction

Increase the rate of biochemical reactions to more than a billion times their normal rate

Remain unchanged throughout the reaction; are not used up; can be re-used

Cannot make réactions that do not naturally occur happen

Do not alter amount of product produced, nor the enthalpy of the reaction

Make reactions more likely to occur

Question 2

Describe enzyme specificity

Answer 2

Enzymes catalyse reactions involving one substrate

Tertiary structure is complementary to the substrate

Question 3

Describe an enzyme active site

Answer 3

A globular structure with a small pocket/cleft

Question 4

Why are enzymes necessary?

Answer 4

Covalent molecules are very stable

Question 5

Describe the induced-fit theory

Answer 5

• active site can change its shape to enfold a substrate
• enzyme takes up its most effective catalytic shape after binding with the substrate; shape of the enzyme is affected by the shape of the substrate
• the distorted enzyme distorts the substrate - breaks, strains and twists the bonds (reducing substrate stability and therefore potential energy; reduces activation energy)
• products that are formed no longer bind to the active site
• enzyme returns to its original shape

Question 6

Why is the induced-fit theory more widely accepted than the lock-and-key theory?

Answer 6

It does not depend upon angle of collision

Question 7

What are the factors affecting enzymes?

Answer 7

1. Temperature
2. pH
3. Concentration of enzyme
4. Concentration of substrate
5. Incubation Time
6. Inhibitors (competitive + non-competitive)
7. Cofactors

Question 8

Define optimum temperature (OT)

Answer 8

The temperature at which the rate of enzyme reaction is greatest

Question 9

Describe how temperature affects enzymes

Answer 9

• increasing temperature increases kinetic energy (because heat energy = kinetic energy)
• enzymes move faster; more likely to collide with substrates; collisions are more frequent
• more ESCs are formed
• more products are formed
• rate of reaction increases until OT is reached
• after OT, rate falls dramatically because enzymes denature (increased energy causes hydrogen bonds in the tertiary structure to break, so ESCs could not form as the active site changed shape; no longer complementary)

Question 10

Describe how increasing temperature past OT causes enzymes to denature

Answer 10

after OT, rate falls dramatically because enzymes denature (increased energy causes hydrogen bonds in the tertiary structure to break, so ESCs could not form as the active site changed shape; no longer complementary)

Question 11

Describe the relationship between temperature and kinetic energy

Answer 11

increasing temperature increases kinetic energy (because heat energy = kinetic energy)

Question 12

Describe Q10

Answer 12

• the temperature coefficient
• the change in rate of reaction for each 10°C rise in temperature
• below OT, Q10 is usually approximately 2

Question 13

Give the equation for Q10

Answer 13

rate of reaction at x+10°C / rate of reaction at x

Question 14

Describe optimum pH

Answer 14

• the pH at which enzyme rate of reaction is greatest
• most enzymes are only effective in a narrow range
• usually matches an enzyme’s usual pH environment

Question 15

Describe denaturation caused by changes in pH

Answer 15

• as H+ concentration changes, the charges interact with attraction bonds (hydrogen bonds) and charged bonds (ionic bonds), causing some to break and some to form in different places - changes R group ionisation
• changes tertiary structure, changes shape, no longer complementary, no ESCs
• negatively charged amino acids form part of the active site, causing H+ ions to cluster around it and interfere with the binding of the substrate and the active site
• slight shifts are reversible
• extreme shifts are permanent

Question 16

Describe the relationship between rate of enzyme reaction and concentration of the enzyme

Answer 16

• when substrate concentration is not the limiting factor (when pH, pressure, temperature and substrate are in excess)
• rate of enzyme reaction is directly proportional to concentration of enzyme present
• the more active sites there are, the more substrate can be converted into product

Question 17

Name the marking points for questions regarding the effect of changes of concentration of an enzyme on rate of enzyme reaction

Answer 17

1. Higher enzyme concentration
2. More active sites available
3. More collisions
4. More ESCs formed
5. At very high concentrations of enzyme, concentration of substrate becomes the limit g factor; all of the substrate molecules are being used up
6. Further increased in enzyme concentration has no effect

Question 18

Describe the relationship between concentration fo substrate and rate of enzyme reaction

Answer 18

• rate of enzyme reaction increases in direct proportion to substrate concentration
• until concentration of enzymes becomes the limiting factor
• all of the active sites are full, so adding more substrate has no effect

Question 19

What are inhibitors?

Answer 19

Substances which interfere with enzymes

Question 20

Describe competitive inhibitors b

Answer 20

• shape resembles an enzyme’s substrate (complementary)
• competes with substrate to bind with active site
• binding prevents substrate from binding with it (forms an EIC)
• efficacy depends on inhibitor concentration relative to substrate concentration + how tightly the inhibitor binds with the enzyme

Question 21

Describe non-competitive inhibitors

Answer 21

• binds to allosteric site
• causes active site to change shape (because tertiary structure of enzyme changes)
• fewer ESCs can form

Question 22

Describe reversible inhibitors

Answer 22

• binds to an enzyme with weak bonds (e.g. hydrogen bonds)

* affects enzyme only when attached to it

Question 23

Describe irreversible inhibitors

Answer 23

• attach to an enzyme with strong covalent bonds; difficult to break without damage to the enzyme
• effect is permanent

Question 24

What is a cofactor?

Answer 24

• a non-protein substance
• can be inorganic or organic
• ensure enzyme controlled reactions take place at a certain rate

Question 25

What is a prosthetic group?

Answer 25

A cofactor which is tightly bound to its enzyme

Question 26

Describe activators

Answer 26

• inorganic cofactors
• attach to the active site of an enzyme to make its shape more efficient
• chloride -> neeeed by amylase
• iron -> needed by catalase
• zinc and magnesium -> needed by DNA Polymerase

Question 27

Describe coenzymes

Answer 27

• organic cofactors
• binds to active site (usually changed in some way, usually recycled)
• some transfer chemical groups, atoms or electrons from one enzyme to another
• e.g. nicotinamide adenine dinucleotide -> derives from vitamin B3 -> transfers hydrogen during cellular respiration by helping pyruvate dehydrogenase function