Enzymes

Question 1

Substrate + enzyme =

Answer 1

Enzyme/substrate complex -> enzyme and products

Question 2

Is the enzyme/substrate complex reversible?

Answer 2

Yes

Question 3

Explain the lock and key hypothesis

Answer 3

Within the globular protein structure of each enzyme is the active site with a very specific shape - only one substrate will fit.
They slot together to form a complex, which lowers the activation energy of the reaction, by affecting the bonds in the substrate, making it easier for them to break,
and the reacting substances are brought closer together, making it easier for bonds to form between them.
When the reaction is done the products aren’t the right shape to stay in the active site so the complex breaks up, releasing products + enzyme.

Question 4

What makes scientists think the lock and key hypothesis isn’t perfect?

Answer 4

Chemical analysis of active sites suggests that it’s not a rigid shape.

Question 5

Explain the induced-fit hypothesis

Answer 5

Active site has a distinctive shape and arrangement but it’s flexible. Once the substrate enters the active site, the shape of the site is modified to form complex. This lowers the activation energy of the reaction by affecting bonds in the substrate, making it easier for them to break,
and the reacting substances are brought closer together, making it easier for bonds to form between them.
Then the products leave the complex and the enzyme reverts to inactive relaxed form until another molecule binds.

Question 6

How do you investigate the way a factor affects the RoR?

Answer 6

Measure initial RoR each time the independent variable is changed - keep every other factor the same.

Question 7

How much enzyme do you provide in enzyme experiments?

Answer 7

Large excess so that the RoR isn’t limited by it, unless the effect of substrate concentration is under investigation.

Question 8

What is the initial RoR?

Answer 8

When the reaction proceeds at its fastest rate/maximum reaction rate

Question 9

How does end-product inhibition work?

Answer 9

The regulatory enzyme is inhibited by one of the end products of the chain, so if the product concentration increases, the product binds non-competitively and changes the shape of the active site. Then, as product levels fall, the molecules detach and the enzyme becomes active again.

Question 10

What happens in irreversible inhibition?

Answer 10

The inhibitor combines with the enzyme by permanent covalent bonding to one of the groups- changes shape and structure of molecule so it can’t be reversed.

Question 11

What happens in non-competitive inhibition?

Answer 11

Inhibitor forms a complex with the enzyme or enzyme/substrate complex, changing the shape of the active site so it can no longer catalyse the reaction.

Question 12

How can percentage of inhibition in competitive inhibition be reduced?
How about in non-competitive inhibition?

Answer 12

Increase substrate concentration, so it’s less likely for inhibitor molecules to bind to the active site.
You can’t because it joins elsewhere.

Question 13

What happens in competitive inhibition?

Answer 13

Inhibitor molecule is similar in shape to substrate, competing with it for binding at the active sites, forming a complex.

Question 14

What is molecular activity/turnover number?

Answer 14

Number of substrate molecules transformed per minute by a single enzyme molecule.

Question 15

How does pH affect rate of enzyme-catalysed reactions?

Answer 15

Change in pH affect interactions between R groups, e.g. H bonds and ionic bonds that hold the 3D structure of the protein together.

Question 16

How can thermophilic bacteria’s enzymes not denature?

Answer 16

They are made of temperature-resistant proteins that have a high density of H bonds and disulphide bonds, holding them together.

Question 17

How does temperature affect the rate of an enzyme-catalysed reaction?

Answer 17

Number of successful collisions leading to a reaction increase at higher temperatures. At temperatures over 40 degrees, the shape of the active site changes and the enzymes lose their territory and quaternary structures, meaning they can no longer catalyse.

Question 18

How does substrate concentration affect reactions?

Answer 18

There ate of the enzyme-catalysed reaction increases but only until all of the active sites are occupied by substrate molecules, and at that point, a further increase in substrate concentration will not increase the rate of reaction further.

Question 19

Why do we think that there is a physical site on the enzyme with a particular shape?

Answer 19

Some catalysts can catalyse many reactions, but some are so specific that they will only catalyse one or one type.

Question 20

Enzyme controlled reactions are affected by …

Answer 20

enzyme concentration

Question 21

Explain why there’s initially a positive gradient with substrate concentration vs rate (3)

Answer 21

Substrate concentration is limiting factor,
some active sites are free
so increase in substrate concentration increases rate

Question 22

Explain why substrate concentration vs rate plateaus (3)

Answer 22

all available active sites occupied with substrate
number of substrate molecules exceed number of active sites.
Therefore increase in substrate concentration can’t increase rate

Question 23

Why is pH vs rate a bell-shaped curve? Explain (4)

Answer 23

Enzymes have optimum pH,
pH changes from optimum cause changes in shape,
therefore reduces number of enzyme-substrate complexes,
thus reduces rate

Question 24

Define “competitive inhibitor” (2)

Answer 24

Substance which is structurally similar to normal substrate,

and competes with it for active site of enzyme

Question 25

Explain why rate of reaction isn’t greatly reduced by competitive inhibitors when there’s a high substrate concentration (2)

Answer 25

Substrate molecules occupy most of the active sites,

therefore little inhibition

Question 26

3 differences between competitive and non-competitive inhibitors

Answer 26

• Cs are structurally similar to substrate
• Cs bind to active site
• effect of C can be reversed by high substrate concentration

Question 27

Explain how the Benedict’s test on sucrase acting on sucrase would indicate activity of enzyme (2)

Answer 27

Forms brick-red precipitate with reducing sugars,
sucrose is a non-reducing sugar,
but if sucrose is digested by sucrase it’ll give a positive test

Question 28

Why does increasing enzyme concentration increase RoR? (2)

Answer 28

More enzyme means more active sites,

so more collisions between substrate and active site

Question 29

What is the normal substrate of glucose phosphorylase?

Answer 29

GLUCOSE

Question 30

How could someone show that enzyme hasn’t been used up in an experiment?

Answer 30

Add more substrate/repeat experiment

Question 31

How do you find mean rate of production in a time frame?

Answer 31

Take readings of graph for x-min and y-min, and take them away from each other, and divide by x - y

Question 32

Give three commercial uses for protease

Answer 32

Tenderising of meat, predigesting baby foods, biological washing powders

Question 33

What organs release amylase?

Answer 33

Salivary, pancreas

Question 34

Ligase uses energy from … to assemble a bond

Answer 34

ATP

Question 35

How does pectinase clarify fruit juices? (3)

Answer 35

Middle lamella contains pectin,
broken down by pectinate,
thus releasing cell contents,
fewer bits of cell wall left suspended.

Question 36

Suggest why the optimum pH of an intracellular enzyme is not necessarily identical with the pH of the surroundings.(4)

Answer 36

Many enzymes need to work together,
some need to work quicker/slower than others so products are formed at correct rate,
having different optimum pHs means they aren’t all working at peak rate,
so intracellular pH has a controlling influence on enzyme actions.