2.4 - Enzymes

Question 1

Why enzymes are known as specific

Answer 1

• Shape of active site
• complementary
• to correct substrate
• will form ESC
• any other substrate will not

Question 2

Enzyme action – induced fit hypothesis

Answer 2

• As the substrate binds to the active site, the enzyme changes shape slightly
• The active site is tighter around the substrate molecules
• Oppositely charged groups on the substrate and active site interact and hold the substrate
  molecule in place. This is the enzyme substrate complex (ESC)
• The enzyme’s shape change puts strain on the bonds in the substrate which destabilises it,
  causing the reaction to occur more easily
• The product(s) is formed (enzyme product complex) and because it is a different shape to
  the reactant it is released from the active site.

Question 3

Explaining the effect of temperature on enzyme activity

Answer 3

Up to and at optimum:

• As molecules are heated they gain kinetic energy and move around faster - this results in more frequent collisions
• This results in more enzyme substrate complexes and therefore a higher (max) rate of reaction and more product formed

Above optimum:

• Molecules have more kinetic energy
• Enzymes vibrate too much and weaker bonds are broken (ionic and H)
• The tertiary structure of the enzymes are changed
• This means the active site loses complementary shape
• No ESCs can form as substrate doesn’t fit into active site
• The enzymes have denatured
• This is irreversible so reaction stops

Question 4

Explaining the effect of pH on enzyme activity

Answer 4

Not at optimum

• Change in pH or H+ ions alters distribution of charge on the enzyme molecule
• This causes the hydrogen and ionic bonds to break
• This means the enzyme loses its tertiary structure
• This changes the shape of the active site of the enzyme
• Substrates are no longer attracted to the active site beause the H+ ions have altered its charge
• Substrates can’t bind to the active site as it is no longer complementary
• No ESCs can form = no product = no reaction
• Enzymes are denatured at extremes of pH (for that enzyme)

At optimum
- At their optimum, the conc of H+ ions gives the tertiary structure the best shape = most complementary active site.

Question 5

Explaining the effect of increasing enzyme concentration on enzyme activity (fixed concentration of substrate)

Answer 5

As long as the substrate is in excess:

• as enzyme conc increase, reaction rate increases
• more enzymes = more active sites =more ESCs form so more product = higher rate of reaction

As the substrate is used up (not in excess):

• When substrate is used up the rate will decrease as less product is formed
• Substrate is limiting

Question 6

Explaining the effect of substrate concentration on enzyme activity (fixed concentration of enzyme)

Answer 6

As long as enzymes in excess:

• as substrate conc increase, rate of reaction increases
• more substrate = more frequent collisions between active sites and substrate = more ESCs = more product = higher rate

When all active sites occupied:

• it is not possible for more ESCs to form at any one time = increasing the substrate conc further has no further effect on the rate - the rate plateaus
• the enzyme concentration is a limiting factor

Question 7

Competitive inhibitors

Answer 7

• Competitive inhibitors have similar shapes to an enzymes’ substrate
• Their shape is complementary to the active site so they can bind with it and block it
• This prevents ESCs from forming and slows rate of reaction - no products can be formed
• do not bind permanently to the active site - action is reversible

Question 8

Non-competitive inhibitors

Answer 8

• Non-competitive inhibitors fit into the allosteric site on an enzyme
• This alters the tertiary structure of the enzyme and changes the shape of the active site
• This means the substrate can no longer fit into the active site, so no ESCs can form - rate of
  reaction decreases
• bind permanently to the enzymes - their effect is irreversible - the enzymes become
  useless

Question 9

Explaining the effect of competitive inhibitor concentration on enzyme activity

Answer 9

• rate determined by relative concentrations of substrate/inhibitor
• more inhibition if substrate conc. is lower than inhibitor conc
• there is a higher chance of inhibitor entering active site than substrate = less ESCs = less
  product
• effects can be reversed by increasing substrate conc.

Question 10

Explaining the effect of non-competitive inhibitor concentration on enzyme activity

Answer 10

• Increasing substrate conc has no effect on the rate because they bind irreversibly - if all
  enzymes have a non-competitive inhibitor bound, the reaction will stop
• Changing the conc of inhibitors will reduce the rate further = fewer ESCs = less product