Many proteins are enzymes (3.1.4.2)

Question 1

what are enzymes?

Answer 1

biological catalysts

Question 2

what are intracellular enzymes?

Answer 2

enzymes found inside cells

Question 3

what are extracellular enzymes?

Answer 3

enzymes found outside cells

Question 4

what type of protein are most enzymes and what features does this give them?

Answer 4

globular proteins - have hydrophilic amino acids on their surface which makes them soluble in water

Question 5

what do the properties of an enzyme relate to?

Answer 5

the tertiary structure of its active site and its ability to combine with complementary substrates to form an enzyme-substrate complex

Question 6

what do enzymes do to form products from a substrate?

Answer 6

1. the tertiary structure of the enzyme’s active site is complementary to the structure of the substrate (enzymes are specific for the substrate they bind to)
2. the substrate binds to the active site to form an enzyme-substrate complex
3. amino acids on the surface of the active site form temporary bonds with the substrate molecule
4. the enzyme catalyses the reaction to produce and release the products

Question 7

how do enzymes increase the rate of a reaction?

Answer 7

they provide a pathway for the reaction with a lower activation energy barrier - this means more substrate molecules have the energy to react

Question 8

what was scientists’ initial hypothesis for how enzymes worked?

Answer 8

lock and key model - tertiary structure of active site was fixed so didn’t change shape

Question 9

how does the current hypothesis of enzyme action (induced fit model) work?

Answer 9

1. the tertiary structure of the active site is specific to the shape of substrate but not complementary
2. as the substrate starts to form bonds with amino acids on the active site, the tertiary structure of the enzyme changes
3. this means the active site becomes complementary to the substrate
4. the bonds formed help catalyse the reaction

Question 10

why can’t molecules which aren’t the substrate bind to an active site in the induced fit model?

Answer 10

they are unable to form the correct bonds to the correct amino acids on the active site, meaning there is no change in tertiary structure and the active site is not complementary

Question 11

what does the frequency of an enzyme-controlled reaction depend on?

Answer 11

the frequency of successful collisions between the substrate and active site of the enzyme

Question 12

what is the effect of an initial increase in temperature on the rate of an enzyme-controlled reaction?

Answer 12

it increases the rate of reaction because the enzyme and substrate have increased kinetic energy - they move more rapidly so there is an increased frequency of a successful collision

Question 13

what does the optimum temperature mean?

Answer 13

the temperature where there are the maximum frequency of collisions between the substrate and active site

Question 14

what happens to the rate of an enzyme-controlled reaction after the optimum temperature is reached?

Answer 14

the rate decreases - enzyme molecules vibrate more rapidly causing bonds to break so the tertiary structure changes
eventually the enzyme denatures because the shape of the active site has changed so much that the substrate can’t bind

Question 15

why doesn’t an enzyme renature when cooled down after denaturing at high temperatures?

Answer 15

the tertiary structure has changed so much that it can’t be reversed

Question 16

why does pH affect the rate of an enzyme-controlled reaction?

Answer 16

1. pH depends on the concentration of H^+ ions present in the solution
2. the hydrogen ions bond with the R groups of amino acids in the protein, including in the active site
3. in the active site, this prevents the amino acids from forming temporary bonds with the substrate, which can’t bind as effectively
   in the rest of the enzyme it causes bonds which hold the tertiary structure in place to break

Question 17

how can pH cause an enzyme to denature?

Answer 17

the bonding of hydrogen ions may cause the shape of the active site to change so much that it is no longer complementary to the substrate

Question 18

how do you calculate the pH of a solution from the concentration of H^+ ions?

Answer 18

pH = -log10 [H^+]
(put the concentration of hydrogen ions in the brackets)

Question 19

what is the effect of an initial increase in substrate concentration on the rate of an enzyme-controlled reaction?

Answer 19

the rate of reaction is directly proportional to the substrate concentration (and substrate concentration is the limiting factor)

Question 20

what happens to the rate of reaction as you continue increasing substrate concentration?

Answer 20

the enzymes reaches its fastest rate (Vmax) and then the rate levels off
the enzymes become saturated so there are no free active sites for the extra substrate molecules to collide with (enzyme concentration is the limiting factor)

Question 21

what is the effect of an initial increase in enzyme concentration on the rate of an enzyme-controlled reaction?

Answer 21

the rate of reaction is directly proportional to the enzyme concentration (and enzyme concentration is the limiting factor)

Question 22

what happens to the rate of reaction as you continue increasing enzyme concentration?

Answer 22

the rate levels off because there aren’t enough substrate molecules to collide with all of the available active sites (substrate concentration is the limiting factor)

Question 23

how do competitive inhibitors work?

Answer 23

they are molecules with a similar but not identical shape to the active site of the enzyme - they bind to it for a short time preventing the actual substrate from binding (lower frequency of collisions between substrate and active site), which reduces the rate of reaction

Question 24

how do non-competitive inhibitors work?

Answer 24

they bind to a site in the enzyme called the allosteric site - this causes a change in the tertiary structure of the enzyme so the active site is no longer complementary and the substrate can’t bind (reduced rate of reaction)

Question 25

how can you reduce the effect of a competitive inhibitor?

Answer 25

increase the substrate concentration - if there are more substrates, it is less likely that the competitive inhibitor will bind to the active site

Question 26

when is it not possible to reduce the effect of a competitive inhibitor?

Answer 26

when the inhibitor binds irreversibly (permanently) to the active site

Question 27

is it possible to reduce the effects of a non-competitive inhibitor?

Answer 27

no, because even if the substrate concentration increases, the active site will still not be complementary, so they will be unable to bind

Question 28

what is end-product inhibition?

Answer 28

when a sequence of enzyme-catalysed reactions leads to the production of an end product, which is an inhibitor for the enzyme in an earlier reaction