Enzymes

Question 1

What is an enzyme?

Answer 1

A biological catalyst that increases the rate of reaction and stays unchanged and reusable.

Question 2

What are advantages of using enzymes?

Answer 2

They are more specific than chemical catalysts.
Conditions that damage cells not needed
Not used up.

Question 3

What is the turnover number?

Answer 3

The number of reactions which an enzyme can catalyse per second.

Question 4

What is the structure of an enzyme?

Answer 4

There is a complementary active site to the substrate.
The 3D shape comes from the primary structure.

Question 5

How do enzymes work?

Answer 5

They increase the rate by providing an alternative reaction pathway with a lower activation energy - more molecules have sufficient energy to react at lower temperatures.

Question 6

How does the active site lower the activation energy?

Answer 6

Holds molecules closer so there is less repulsion and bond easier
Physical pressure on substrate - break easier.

Question 7

What is the lock and key hypothesis?

Answer 7

The substrate fits the active site, temporary hydrogen bonds hold them together to form a complex. The substrate is broken into smaller products.
Enzymes can also catalyse the formation of a bigger product.

The active site is rigid - not explain how complex is stabilised.

Question 8

What is the induced fit model?

Answer 8

The active site is relaxed and not fully complementary. When the substrate binds, the AS changes shape of the of R groups - more precise. Complex is more effectively bound by non covalent forces. As the substrate is converted, the product is still in the active site until it is a different shape as the products.

Question 9

What is a cofactor?

Answer 9

A substance which must be present to enable an enzyme to catalyse a reaction at the right rate.

Question 10

How do cofactors work?

Answer 10

As a prosthetic group - permanently bound by covalent bonds.
Coenzyme - temporarily bonded to enzyme.

Question 11

What is an example of prosthetic groups?

Answer 11

Carbonic anhydrase contains Zn ion - CO2 + H2O to carbonic acid.

Question 12

What is a coenzyme?

Answer 12

Non protein molecules which are changed by the reaction they help catalyse and must be regenerated afterwards.

Question 13

What are cosubstrates?

Answer 13

Cofactors which pair with a substrate to make it the right shape. Some affect the charges to make the temporary bonds easier to form.

Question 14

What is an intracellular enzyme?

Answer 14

They can affect the structure and function of biological molecules. They normally take part in metabolic pathways.

Question 15

What are metabolic pathways?

Answer 15

A series of reactions to produce a specific product. The reactants and intermediates act as substrates for specific enzymes = metabolites.

Question 16

What is an example of an enzyme that catalyses intracellular reactions?

Answer 16

Catalase - poisonous hydrogen peroxide into water and oxygen.
Fastest acting enzyme.
Made of 4 polypeptide chains and 1 haem group.

Question 17

What are extracellular enzymes?

Answer 17

Formed within cells and secreted to work outside the cell.
Used for digestion.

Question 18

What are examples of extracellular enzymes?

Answer 18

Amylase - secreted from salivary gland and pancreas. Breaks down starch into maltose - body can’t absorb starch. Not function without Cl-.

Trypsin - protease secreted from pancreas. Hydrolyses peptide bonds to break down proteins into amino acids.

Question 19

What are optimal conditions?

Answer 19

Conditions that maximise an enzyme’s rate of reaction.

Question 20

How can enzyme concentration be controlled by cells?

Answer 20

Cells can turn on/off translation of genes coding for enzymes.
Enzyme degradation up/down. Cell can regulate own metabolism and eliminate abnormal/unnecessary enzymes which could cause cell damage.

Question 21

How can a reaction occur?

Answer 21

If the enzyme and substrate collide to form a complex. If there is excess substrate, the more enzymes, the greater the chance of collision. Increasing rate.

But substrate concentration can be a limiting factor.

Question 22

What is the effect of substrate concentration?

Answer 22

The higher the concentration, the more collisions there are, and the higher the rate.
After a point, all AS are occupied so increasing concentration will have no effect on the rate.
= Saturation point.

Question 23

What is the effect of temperature?

Answer 23

Increases kinetic energy, so collide more often and with greater energy. Increasing rate at optimum temperature.

If temperature increased more, bonds in the enzyme break and the AS shape is disrupted. Enzyme denatures due to permanent changes in the tertiary structure.

Question 24

What is the effect of pH?

Answer 24

Deviations from the optimum can change the tertiary structure by breaking bonds, changing AS shape, decreasing rate.
Excess H+ ions can also alter charges on AS as more protons closer around negative groups. Affects the binding.

Question 25

What are different pHs of enzymes?

Answer 25

Blood pH is between 7.35 and 7.45.
Intracellular enzymes have optimum pH close to 7.
Extracellular enzymes pH vary. eg. pepsin has very low optimum because it is in the stomach.

Question 26

What is an enzyme inhibitor?

Answer 26

Substances that interfere with an enzyme, so the reaction slows or stops.

Question 27

What is a competitive inhibitor?

Answer 27

Similar molecular shape to substrate - bind to AS, forming a barrier and preventing complexes forming.

Question 28

What is a non competitive inhibitor?

Answer 28

Bind to the allosteric site, changing the shape of the AS so it is not complementary anymore.

Question 29

How do competitive inhibitors bind?

Answer 29

Reversibly. Depends one their concentration vs substrate. If there is a high concentration, it will occupy most AS.

Question 30

What is end product inhibition?

Answer 30

Prevents the cell overproducing products.
eg. product remains bound to the enzyme, so no more substrate can enter the AS.

Question 31

How does end product inhibition work?

Answer 31

The product becomes the substrate for the next reaction in the pathway. One of the products may act as a non-competitive inhibitor to another enzyme and change the AS shape (reversible).

Greater inhibition decreases the amount of end product. Negative feedback loop occurs.

Question 32

What are examples of poisons?

Answer 32

KCN - forms hydrogen cyanide gas which dissociates into H+ and CN-. CN- bind to an enzyme in mitochondria, inhibiting it, stopping aerobic respiration –> death.

Snake venom - ACh is the neurotransmitter between nerves and muscles. It is broken down by AChE. Venom contains this enzyme’s inhibitor so the muscles continue contracting –> paralysis and death (suffocation).

Question 33

Which drugs work with enzymes?

Answer 33

Aspirin - reduces pain in infections as contains salicylic acid - inhibits enzymes that catalyse production of prostoglandins. Prostaglandins are cell signalling made by infected tissue.

Protease inhibitors - treat some viral infections by stopping viral replication. Competitive as inhibits protease - viral protein coats not made.

Question 34

How are enzymes secreted as inactive precursors?

Answer 34

Need to be activated before they can function. Normally some amino acids have to be removed from AS - so is exposed or becomes right shape.
eg. digestive enzymes - stops damage while inside cells.