2.4 - B - Enzymes

Question 1

What is an enzyme?

Answer 1

Enzymes are biological catalysts because they speed up metabolic reactions in living organisms.
They are globular proteins with specific tertiary structures.

Question 2

What is the difference between a catabolic enzyme and an anabolic enzyme?

Answer 2

Catabolic enzyme - break down substances

Anabolic enzyme - build (bond together) substances

Question 3

What is an active site?

Answer 3

An area made of a few (usually 6-10) amino acids (created by the tertiary structure) where the reaction takes place. Every active site is complementary to a specific substance.

Question 4

What are metabolic reactions?

Answer 4

The chemical reactions that take place inside living cells or organisms.

Question 5

Define intracellular enzymes and extracellular enzymes

Answer 5

Intracellular enzymes - catalyse reactions inside cells/within the cytoplasm or on membranes.
Extracellular enzymes - catalyse reactions outside cells. These are most of digestive enzymes (inside the body, outside cells).

Question 6

What are endotherms and how do they allow their enzymes to work in extreme temperatures?

Answer 6

Endotherms (maintain own internal body temp) don’t need to have enzymes adapted for extremes as they maintain the optimum temperatures for the enzymes with thermoregulation.

Question 7

What is the difference between saprophytic feeders and heterotrophs? Give examples of each.

Answer 7

Saprophytic feeders release enzymes onto their food for it to be digested outside the body and then absorb the monomers.
e.g. fungi
Heterotrophs consume other organisms and digest them inside the body.
e.g. animals

Question 8

Give an example of an enzyme used in digesting pathogens

Answer 8

Lysosomal enzymes - used by phagocytes to engulf and digest pathogens

Question 9

What is a cofactor?

Answer 9

A substance that has to be present to ensure that an enzyme-catalysed reaction takes places at the appropriate rate. Some cofactors (prosthetic groups) are part of the enzyme structure, and others (mineral ion cofactors and organic coenzymes) form temporary associations with the enzyme.

Question 10

What is a coenzyme?

Answer 10

A small, organic, non-protein molecule that is able to temporarily bind to an active site. The coenzyme is chemically changed
during the reaction, and they need to be recycled to their original state,
sometimes by a different enzyme.

Question 11

What is an enzyme-substrate complex?

Answer 11

A complex formed by temporary binding of enzyme and substrate molecules during an enzyme-catalyse reaction.

Question 12

What is a prosthetic group?

Answer 12

A cofactor that is permanently bound by covalent bonds to an enzyme molecule.

Question 13

What do enzymes do?

Answer 13

They reduce the amount of activation energy required by providing a different route for the reaction.

Question 14

Define activation energy

Answer 14

Activation energy is the amount of energy needed for a reaction to take place.

Question 15

Explain the lock and key hypothesis

Answer 15

Enzyme’s active site is complementary in shape to the substrate molecule.
Substrate (key) can fit into the enzyme’s active site (lock).

Question 16

Explain the induced fit hypothesis

Answer 16

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 17

When is an enzyme-product complex and an enzyme-substrate complex formed?

Answer 17

Lock and key hypothesis.
E.S.C. - when breaking apart 2 molecules - in the active site.
E.P.C - when bonding together 2 molecules - in the active site.

Question 18

What is a buffer?

Answer 18

Something that resists changes in pH

Question 19

What is pH?

Answer 19

A measure of H+ ions. More H+ ions = more acidic.

Question 20

Give the model answer for enzymes regarding pH

Answer 20

Change in pH or H+ ions alters distribution of charge on the enzyme molecule
This causes the hydrogen and ionic bonds to be disrupted
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)

Question 21

Define concentration

Answer 21

The number of molecules per unit of volume

Question 22

Give the model answer for enzymes regarding concentration

The substrate concentration remains the same

Answer 22

No enzyme = no enzyme-substrate-complexes (ESCs)= no reaction.
More enzymes = more active sites.
More ESCs form so more product = higher rate of reaction.
As long as the substrate is in excess, the rate will continue to rise with and increase in enzyme concentration.
After a point all substrate molecules are occupying active sites and a maximum rate.
will be reached increasing the enzyme conc further will have no more effect on rate.
At the point where increasing enzyme conc has no more effect on the rate, the substrate concentration is a limiting factor.
When substrate conc becomes limiting, the rate will decrease as the substrate is used up.

Question 23

Give the model answer for enzymes regarding concentration

The enzyme concentration remains the same

Answer 23

No substrate = no ESCs = no reaction.
More substrate = more frequent collisions between enzyme and substrate = more ESCs = more product = higher rate.
After a point, all active sites will be occupied - it is not possible for more ESCs to form at any one time = no more product = increasing the substrate conc further has no further effect on the rate.
At the point where increasing substrate conc has no more effect on the rate, the enzyme concentration is a limiting factor.

Question 24

What is an inhibitor?

What are the 2 types of inhibitors?

Answer 24

Inhibitors are molecules that slow down the rate of an enzyme controlled reaction by affecting the enzyme molecule.
Competitive and non-competitive.

Question 25

Explain what a competitive inhibitor is and does

Answer 25

They have similar shapes to an enzymes’ substrate.
Their shape is complementary to the active site so they can bind with it and block it, forming an enzyme‐inhibitor‐complex.
This prevents enzyme‐substrate complexes from forming and slows
down the rate of reaction ‐ no products can be formed.
Most competitive inhibitors do not bind permanently to the active site
‐ they bind for a while and then leave. Their action is reversible.

Question 26

Explain what happens when substrate concentration is increased with an enzyme and a competitive inhibitor.

Answer 26

More substrate = less inhibition = higher rate of reaction because the substrate molecules are more likely to collide with an active site than an inhibitor. Eventually there will be so much substrate that the impact of a competitive inhibitor will be minuscule.

Question 27

Explain what a non-competitive inhibitor is and does

Answer 27

Non‐competitive inhibitors fit into theallosteric 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
enzyme substrate complexes can form ‐ rate of reaction decreases.
Non‐competitive inhibitorsbind permanently to the enzymes ‐ their
effect is irreversible ‐ the enzymes become useless.

Question 28

Explain what happens when substrate concentration is increased with an enzyme and a non-competitive inhibitor.

Answer 28

Increasing substrate conc has no effect on the rate with non‐
competitive inhibitors because they bind irreversibly ‐ if all enzymes have a non competitive inhibitor bound, the reaction will stop.
Changing the conc of inhibitors will have an effect.

Question 29

Explain how a metabolic sequence works

Answer 29

Wave of substrate to enzyme. The product of the first is the substrate of the next. The last one in the loop attaches like a non-competitive inhibitor, impacting the enzyme. But because it is reversible, when the concentration of that inhibitor falls, it detaches and the enzyme reverts back to its normal shape, and the sequence repeats.

Question 30

Give an example of a non-competitive inhibitor that is a poison and explain its impact on a person

Answer 30

Potassium cyanide is a non‐competitive inhibitor of the enzyme
cytochrome oxidase needed for respiration.
Inhibiting this enzyme decreases use of oxygen so ATP can’t be made.
The organism has to respire anaerobically ‐ this causes a build up of
lactic acid in the blood.
Tiny amounts of cyanide will make an adult lose consciousness in 10s.
In 45 minutes they can fall into a coma and die in 2 hours.

Question 31

How are antibiotics inhibitors?

Answer 31

Antibiotics treat bacteria infections by killing or stopping their
growth. Penicillin is an inhibitor of the enzyme that helps to build cell walls in bacteria. Stopping the production of their cell walls, prevents
the bacteria from reproducing.
Some bacteria have an enzyme which breaks down penicillin ‐ this
makes them resistant to the antibiotic.

Question 32

Give an example of a viral infection that is an inhibitor

Answer 32

HIV are treated using protease inhibitors.
The viruses need protease enzymes to build their viral coat and the inhibitors prevent this from happening. They are competitive
inhibitors.

Question 33

Discuss inhibitors as alcohol

Answer 33

Alcohol is poisonous and damages the liver over a long period of time.
Ethylene glycol (in antifreeze) is broken down in the liver by alcohol 
dehydrogenase into oxalic acid which is highly toxic ‐ this can lead to 
death.
The treatment is a huge dose of ethanol which acts as an inhibitor of alcohol dehydrogenase ‐ this slows the production of oxalic acid so the ethylene glycol can be removed.

Question 34

What are coenzymes and prosthetic groups examples of?

Answer 34

Cofactors

Question 35

What is Q10?

Answer 35

The increase in the rate of a process when the temperature is increased by 10°C