2.4 Enzymes

Question 1

What are enzymes?

Answer 1

Biological catalysts
They are found in living organisms, and speed up the rate of reaction without being changed or used up

Question 2

What is the structure of enzymes?

Answer 2

Globular proteins

Question 3

Where do enzymes act?

Answer 3

Intracellular
Extracellular
both within and outside of cells

Question 4

What are intracellular enzymes?

Answer 4

produced and function inside the cell

Question 5

What are extracellular enzymes?

Answer 5

secreted by cells and catalyse reactions outside cells (eg. digestive enzymes in the gut)

Question 6

What is catalase an example of? What is its function?

Answer 6

Intracellular enzyme
Converts hydrogen peroxide to carbon dioxide and water
H2O2 is a harmful metabolic product of many reactions so needs to be disposed of

Question 7

What is amylase an example of? What is its function?

Answer 7

Extracellular enzyme
Digests carbohydrates - hydrolyses starches into simple sugars (glucose)
Secreted in salivary glands + small intestine

Question 8

What is an active site?

Answer 8

The active site of an enzyme has a specific shape to fit a specific substrate

Question 9

What is meant by tertiary structure?

Question 10

What is enzyme specificity?

Answer 10

The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate

Question 11

Explain the lock-and-key hypothesis:

Answer 11

1. Both enzyme and substrate have rigid structures
2. The substrate binds to the specific active site of the enzyme forming the enzyme-substrate complex
3. The bond in the substrate breaks - an enzyme-product complex is formed
4. Products of the substrate are released and the enzyme is free to bind to more substrates

Question 12

Explain the induced-fit theory:

Answer 12

1. The substrate collides with the specific active site of the enzyme molecule, binds to it and causes the enzyme molecule to change shape
2. Oppositely charges groups hold the enzyme and substrate together and the enzyme fits more closely around the substrate
3. This places greater strain on the substrate and destabilizes it - allows the reaction to occur more easily

Question 13

How doe enzymes effect activation energy?

Answer 13

They lower activation energy
Enzymes speed up chemical reactions because they reduce the stability of bonds in the reactants
The destabilisation of bonds in the substrate makes it more reactive
They provide an alternative reaction pathway

Question 14

What factors affect enzyme activity?

Answer 14

Temperature
Concentration
pH

Question 15

How does temperature affect enzyme activity?

Answer 15

Enzymes have a specific optimum temperature
—> the temp. at which they catalyse a reaction at the maximum rate
Higher temperatures cause reactions to speed up because:
Molecules move more quickly as they have more kinetic energy
Increased kinetic energy results in a higher frequency of successful collisions between substrate molecules and the active sites of the enzymes which leads to more frequent enzyme-substrate complex formation
Substrates and enzymes also collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)

Question 16

What is the temperature coefficient and what does it tell us?

Answer 16

Q10 = rate of reaction at T +10 / rate of reaction at T

This is the ratio between the rates of that reaction at two different temperatures

For most enzyme-catalysed reactions the rate of the reaction doubles for every 10 °C increase in temperature

Question 17

How does pH affect enzyme activity?

Answer 17

Enzymes have an optimum pH which they work at and are denatured when the pH is too high or too low
The extremes of pH causes the bonds in the tertiary structure of the enzyme to break - changing the shape of the active site, meaning the enzyme-substrate complex does not form as easily

Question 18

How does enzyme concentration affect enzyme activity?

Answer 18

A higher enzyme conc:
The greater the number of active sites available –> the greater the chance of enzyme -substrate complex formation
As long as there is sufficient substrate available, the initial rate of reaction increases linearly with enzyme concentration
If the amount of substrate is limited, at a certain point any further increase in enzyme concentration will not increase the reaction rate as the amount of substrate becomes a limiting factor

Question 19

How does substrate concentration affect enzyme activity?

Answer 19

The greater the substrate concentration, the higher the rate of reaction:
As the number of substrate molecules increases, the likelihood of enzyme-substrate complex formation increases
If the enzyme concentration remains fixed but the amount of substrate is increased past a certain point, however, all available active sites eventually become saturated and any further increase in substrate concentration will not increase the reaction rate
When the active sites of the enzymes are all full, any substrate molecules that are added have nowhere to bind in order to form an enzyme-substrate complex

Question 20

What are reversible inhibitors?

Answer 20

An enzyme’s activity can be reduced or stopped, temporarily, by a reversible inhibitor

Question 21

What are competitive inhibitors?

Answer 21

have a similar shape to that of the substrate molecules and therefore compete with the substrate for the active site

Question 22

Are competitive inhibitors reversible?

Answer 22

Yes - Effects are reversible because they only bind temporarily

Question 23

What are non-competitive inhibitors?

Answer 23

bind to the enzyme at an alternative site, which alters the shape of the active site and therefore prevents the substrate from binding to it
- can be reversible or permanent

Question 24

What is end-product inhibition?

Answer 24

Metabolic reactions can be controlled by using the end-product of a particular sequence of metabolic reactions as a non-competitive, reversible inhibitor:
As the enzyme converts the substrate into product, the process is itself slowed down as the end-product of the reaction chain binds to an alternative site on the original enzyme, changing the shape of the active site and preventing the formation of further enzyme-substrate complexes
The end-product can then detach from the enzyme and be used elsewhere, allowing the active site to reform and the enzyme to return to an active state
This means that as product levels fall, the enzyme begins catalysing the reaction once again, in a continuous feedback loop
This process is known as end-product inhibition

Question 25

What is a co-enzyme?

Answer 25

An organic molecule which binds to the active site of the enzyme, for a short period of time
Carries substances between enzymes in a reaction sequence.

Question 26

What is a co-factor?

Answer 26

Inorganic metal ions
Affects charge distribution and the shape of the enzyme and shape of the active site
Speeds up formation of enzyme-substrate complex
Prosthetic groups

Question 27

Give an example of a co-factor:

Answer 27

Cl- ions required for amylase
Iron ions for catalase

Question 28

How do you carry out a serial dilution?

Answer 28

1. Put 10cm cubed of stock solution in a test-tube
2. Pipette 1cm3 out and into the next tube, fill up with 9cm3 of de-ionised water
3. Take 1cm3 of this solution and pipette this into the next test-tube, fill up with 9cm3 of water.
4. Repeat until you have the required number of samples
   –> the concentration of each test-tube divides by 10 each time
   e.g. 1% –> 0.1% –> 0.01% –> 0.001%