Enzymes

Question 1

6.6.What are enzymes?

Answer 1

Tertiary structure, globular proteins that are catalysts

• not changed by reaction
• can be used repeatedly so - effective in small amounts
• hv a high turn-over (catalyse many reactions per sec)

Question 2

What is a catalyst?

Answer 2

a molecule that speeds up a chemical reaction but remains unchanged/isn’t used up at the end

Question 3

Why will each enzyme only catalyse one specific reaction?

Answer 3

• enzyme, so active site has a specific 3D tertiary structure/shape
• so - active site only complementary to & will bind to 1 substrate
• to form an e-s complex

Question 4

What are the 2 types of metabolism?

Answer 4

(all reactions in the body)
1. Anabolic reactions: building up molecules e.g. protein synthesis
2. Catabolic reactions: breaking molecules down e.g. digestion

Question 5

How are enzymes secreted from cells?

Answer 5

by exocytosis

Question 6

Induced fit?

Answer 6

• Before reaction, active site is not complementary to substrate
• as substrate binds, the active site changes shape to become complementary to substrate - forming e-s complex
• this stresses/distorts bonds in substrate (due to enzyme moulding around substrate) - lowering the activation energy
  *accepted model

Question 7

Induced fit&raquo_space;> lock and key model, why?

Answer 7

Lock and key suggest AS is rigid structure & substrate is exact fit to AS…
Induced fit matches current observations that AS changes shape slightly upon binding to become a more exact fit

Question 8

For an enzyme to catalyse a reaction it must…?

Answer 8

• come into physical contact w substrate
• substrate must be complementary to active site
• they must collide w enough energy & suitable orientation

Question 9

How do enzymes speed up the rate of reactions?

Answer 9

Lower the activation energy needed for reactions to take place
(the minimum amount of energy required to activate a reaction)

Question 10

Temperature?

Answer 10

• too low: not enough kinetic energy (to move fast enough) for successful collisions between enzyme + substrate so - fewer e-s complexes
• too high/beyond optimum: enzyme denatures, active site changes shape so - e-s complexes can’t form

Question 11

Vmax?

Answer 11

max velocity/rate at which enzyme catalyse reactions
- happens when all enzyme active sites are saturated w substrate

Question 12

Why do enzymes denature if there’s too much kinetic energy?

Answer 12

• bonds holding amino acids in fixed 3D tertiary structure in active site broken

Question 13

pH?

Answer 13

• too high - too many OH- ions, too low - too many H+
• interfere w charges in amino acids in the active site - can break bonds holding tertiary structure in place and so- active site changes shape (mainly ionic + h bonds)
• denatures - fewer e-s complexes

Question 14

Calculating pH?

Answer 14

pH = -log10[H+}

• [H+] = H+ ion conc.
• put - sign in front of log yourself

Question 15

Enzyme concentration?

Answer 15

low - too few enzyme molecules to allow all substrate molecules to find an active site at 1 time so - rate of reaction lower

intermediate - max rate of reaction - all active sites r filled

high - addition of further enzyme molecules: no effect - alr enough active sites to accommodate all available substrates (active sites become saturated w substrate - unable to work any faster aka point of saturation) - no increase in rate

Question 16

Substrate reaction?

Answer 16

low - too few substrate molecules to occupy all available active sites - rate of reaction lower (fewer collisions so few e-s complexes)

intermediate - max rate of reaction - all active sites r filled

high - addition of further substrate molecules: no effect - all active sites alr occupied at 1 time (active sites become saturated w substrate - unable to work any faster) - no increase in rate

Question 17

Limiting factor definition?

Answer 17

Factor is limiting when an increase in its value causes an increase in the rate of reaction

Question 18

Buffer definition?

Answer 18

an aq solution that has a highly stable pH
- if if add acid/base - pH not change significantly
- resists changes in pH

Question 19

Enzyme inhibitors?

Answer 19

substances that directly or indirectly affect the functioning of the active site of a specific enzyme & so prevent the substrate binding, preventing the formation of e-s complexes & so - decrease the rate of reaction

Question 20

Competitive inhibitors?

Answer 20

similar shape/structure to substrate so - can bind & occupy the active site of enzyme
- compete w substrate for active site
- binding of competitive inhibitor to active site is NOT permanent (aka is reversible) so - when leaves active site - another molecule can enter
- e-inhibitor complex prevents substrate from binding so - no e-s complexes so - lower rate
- if add very high conc. substrate - can knock inhibitor out of active site aka…
- increasing sub conc. overcome competitive inhibition because reversible

Question 21

Non-competitive inhibitors?

Answer 21

• binds to enzyme at a site that is NOT/away from the active site aka the allosteric site (that it’s complementary to)
• this causes a change in the tertiary structure of the enzyme so - changing shape of the active site - usually permanent
• active site no longer complementary to substrate so - substrate can no longer bind - no e-s complexes formed & no reaction

Question 22

Graphs of inhibitors?

Answer 22

• competitive: lower rate of reaction than w no inhibitor (less steep) the whole way but then meet same end point (at very high substrate conc. - so much substrate - outcompetes & knocks out inhibitor - bind - rate returns to normal)
• non-competitive: lower rate of reaction than w no inhibitor (less steep) initially but plateaus at MUCH lower rate (even if add more substrate - doesn’t knock out inhibitor, active site diff shape so - no catalysing reaction so - max rate at much lower