Enzymes

Question 1

what is an enzyme ?

Answer 1

a biological catalyst which increase the rate of a chemical reaction by lowering the activation energy of the reactions they catalyse

Question 2

structure of an enzyme

Answer 2

globular protein it has a tertiary structure
- helix folds back on itself, giving the molecule a unique 3D shape - hydrogen, disulphide + ionic bonds
- active site is formed

Question 3

what are the two types of reactions when they catalyse ?

Answer 3

anabolic reaction
catabolic reaction

Question 4

anabolic reaction

Answer 4

larger molecules are built from smaller molecules

Question 5

catabolic reaction

Answer 5

larger molecules are broken down

Question 6

what is the active site ?

Answer 6

the area of the enzyme where the substrate binds

Question 7

as the enzyme and substrate binds what forms ?

Answer 7

an unstable enzyme - substrate complex, which breaks down to form products

Question 8

Induced fit model

Answer 8

• enzyme actually changes shape as the substrate binds to its active site
• active site does not correspond exactly to the shape of the substrate
• the active site has a more flexible shape + able to mould around the shape of the substrate

Question 9

activation energy

Answer 9

the amount of energy needed to raise the substrate molecules to their transition state

Question 10

state the 4 factors which affect the rate of enzyme-controlled reactions

Answer 10

• temperature
• pH
• substrate concentration
• enzyme concentration

Question 11

Temperature

Answer 11

1. low temp = low rate
   - enzyme have too little kinetic energy and move slowly
   - few successful collisions between enzyme and substrate leads to low rate of reaction
2. optimum temp = high rate
   - enzymes have a lot of kinetic energy and move quickly
   - lots of successful collisions between enzyme and substrate leads to high rate of reaction
3. high temp = denatured = low rate
   - heat breaks the bonds maintaining the tertiary structure of the enzyme
   - the active site becomes denatured and substrate no longer fits
   - no successful collisions leads to a low rate of reaction

Question 12

pH

Answer 12

1. low optimum = low rate
   - pH has this effect because the structure of the protein is maintained by bonds in the tertiary structure
   - a change in pH alters the bonding and the shape of active site changes thus the substrate can no longer bond
2. optimum pH = high rate
   - active site has the appropriate shape to bind with the substrate so the reaction is catalysed the fastest
3. pH above optimum = low rate

Question 13

Substrate concentration

Answer 13

1. low substrate concentration = low rate
   - few substrate molecules limits chance of successful collisions between enzyme + substrate
2. high substrate concentration = high rate
   - more substrate molecules increase the chance of successful collisions between enzyme + substrate

Question 14

what are the two phases of substrate concentration ?

Answer 14

• lower substrate concentration = rate increases in proportion to the increase in substrate concentration
• higher substrate concentration = rate of reaction becomes constant, the rate will not increase even though the substrate concentration is still increasing

Question 15

Substrate concentration - low

Answer 15

there is an excess of enzyme molecules, lots of free enzyme molecules
substrate concentration = limiting factor

Question 16

Substrate concentration - high

Answer 16

there becomes a point where the concentration of substrate is higher than the concentration of enzyme molecules - no longer free enzymes molecules that can react
enzyme concentration = limiting factor

Question 17

what do the substrate molecules have to do to react? - substrate conc high

Answer 17

queue up

Question 18

what happens when adding more substrate molecules? - substrate conc high

Answer 18

only increases the number that are waiting - DOES NOT INCREASE RATE

Question 19

Enzyme concentration

Answer 19

1. low enzyme concentration = low rate
   - few enzyme molecules limits the chance of successful collisions between enzyme and substrate
   - excess of substrate with no free enzymes
2. high enzyme concentration = high rate
   - more substrate molecules increase the chance successful collisions between enzyme and substrate

Question 20

inhibitors

Answer 20

substances which stop the enzyme from binding to its substrate, therefore controlling the progress of a reaction

Question 21

what are two types of inhibition ?

Answer 21

• competitive
• non-competitive

Question 22

Competitive inhibition

Answer 22

an inhibitor molecules compete with the substrate for binding to the active site of the enzyme, therefore preventing the substrate from binding
- can be reversed by increasing the substrate concentration

Question 23

Non-competitive inhibition

Answer 23

an inhibitor doesn’t bind to the active site but binds to a different part of the enzyme and changes the shape of the enzyme
- decreases the reaction as the active site does not fit the substrate and the substrate cannot bind to the enzyme
- cannot be reversed by increasing substrate concentration

Question 24

what happens if the concentration of substrate molecules is high enough ? - competitive inhibitor

Answer 24

it will displace the inhibitor from the active site - less likely the inhibitor will bind

Question 25

when adding more substrate it does not dislodge why? - non competitive inhibitor

Answer 25

they are not competition for the enzymes active site - permanent

Question 26

what is end product inhibition ?

Answer 26

the final product often acts as a regulator of the pathway in a process

Question 27

process of end-product inhibition

Answer 27

• when the amount of end product is high, it binds non competitively to an enzyme in the pathway, blocking further production itself
• regulation of a metabolic pathway by negative feedback
• when the amount of end product falls, inhibition ends and the pathway restarts