Enzymes

Question 1

Define metabolic reactions.

Answer 1

‘The complex of physical and chemical processes
occurring within a living cell or organism that are
necessary for the maintenance of life’.

Question 2

Describe the process of catabolism.

Answer 2

The process were complex molecules are broken down into simpler ones is known as catabolism – i.e. proteins
to amino acids.

Question 3

Describe the process of anabolism.

Answer 3

The process were complex molecules are synthesised
from simpler ones is known as anabolism – i.e. amino
acids to proteins – these can then be utilised in the body.

Question 4

describe the function of digestion

Answer 4

This is the function of digestion. To break down complex insoluble molecules (catabolism) into
simpler soluble molecules so they can be absorbed and assimilated (synthesised – anabolism)

Question 5

What are reactions called when they release energy and give an example?

Answer 5

Some of these reactions release energy and are exergonic – i.e. cellular respiration. .

Question 6

What are reactions called that require energy?

Answer 6

Some reactions require energy and are endergonic.

Question 7

What are enzymes?

Answer 7

Enzymes are globular proteins that act as biological catalysts.

Question 8

What is a catalyst and what do catalysts do?

Answer 8

A catalyst increases the rate at which chemical reactions occur but remain unchanged or are unaffected by the reaction.

Question 9

State the equation for a chemical reaction.

Answer 9

Reactants————————- Products

Question 10

Define the activation energy?

Answer 10

Activation energy is the minimum energy required

for a successful chemical reaction.

Question 11

why do enzymes speed up the rate of reactions?

Answer 11

Enzymes speed up the rate of reaction by lowering the activation energy needed for a
chemical reaction.

Question 12

How do enzymes speed up the rate of reactions and decrease the amount of activation energy needed?

Answer 12

Enzymes do this by helping to align the reactants through the formation of enzyme substrate
complexes and stress the bonds (bending bonds) within the substrate, so they require less
energy to break them. Collisions will occur between specific parts of the molecules involved
in the reaction, so bonds can break and reform more efficiently, therefore less activation
energy is required.

Question 13

Describe the structure of enzymes.

Answer 13

Enzymes are proteins which have a specific 3D/tertiary shape. They have a specifically
complementary active site to their substrate/s (reactant/s). This then allows the substrate
to bind to the enzyme active site forming an enzyme–substrate complex (ES). It is this that
aligns the reactant and lowers the activation energy.

Question 14

what does the lock and key theory state?

Answer 14

The lock and key theory states that the active site is specifically complementary to its substrate, fitting
it like a key in a lock. This model is useful as it helps to describe the specificity demonstrated by enzymes.

Question 15

describe the stages of the lock and key theory?

Answer 15

1. The substrate enters the enzymes active site
2. The substrate fits exactly into the active site – they are complementary
3. Products are formed and no longer fit into the active site – not complementary
4. The enzyme is free to take part in another reaction

Question 16

what does the lock and key model not take into consideration?

Answer 16

The lock and key model does not take into account that enzyme’s active sites are not rigid
structures and can be changed when substrates bind.

Question 17

Describe what the induced fit model states.

Answer 17

The induced fit theory states that the active site is not fully fixed and the substrate ‘induces’ a change in the shape of the active site making it
complimentary.

Question 18

Describe the stages of the induced fit model

Answer 18

1. The substrate enters the enzymes active site
2. The binding of the substrate molecule/s induces the change in the shape of the active
   site.
3. When the substrate leaves the active site it then returns to its previous shape.

Question 19

why does only one enzyme only catalyse one reaction?

Answer 19

This is due to the lock and key theory.

Question 20

As enzymes are proteins they are affected by temperature and pH. what are enzymes also affected by?

Answer 20

• Inhibitors

* Substrate concentration

Question 21

How does temperature affect enzymes?

Answer 21

Every enzyme has a temperature at which it works best (optimum). As the temperature increases up to its optimum the enzyme and substrate molecules have more kinetic energy and are more likely to collide and form more enzyme substrate complexes per unit time (the collisions are more successful). The rate of the reaction increases.

Question 22

Define the term ‘optimum temperature’

Answer 22

Optimum Temperature – The maximum heat energy that can be supplied to the enzyme
molecules before hydrogen and ionic bonds start to break

Question 23

Describe what would happen when temperature is above the optimum temperature.

Answer 23

Above the optimum temperature:
• further increase in kinetic energy
• causes amino acids in polypeptide chain to
vibrate
• breaks hydrogen and ionic bonds between the R
groups
• the tertiary structure is lost
• Active site shape is no longer specific to
substrate
• No enzyme substrate complexes can be formed.
• Enzyme is denatured and can no longer catalyse
any chemical reactions.

Question 24

Define the term pH

Answer 24

pH is a measure of hydrogen ion concentration.

Question 25

What occurs when the pH alters from the optimum level?

Answer 25

If the pH alters from the optimum then the charge on the R groups of the amino acids are altered and ionic bonds in the tertiary structure are broken. Therefore the active site changes shape and the substrate can no longer fit (not specifically complementary). No enzyme substrate complexes can be formed and the rate of the reaction decreases either side of the optimum. The enzyme is denatured.

Question 26

Describe the effect of substrate concentration

Answer 26

As the substrate concentration increases the rate of reaction increases and then levels off. The rate of reaction is low at low substrate concentrations as not all of the active sites of the enzymes are saturated (filled). The substrate is a limiting factor.

Question 27

What happens when the substrate concentration increases?

Answer 27

As the substrate concentration increases, the active sites are filled and the rate of reaction increases, due to more successful collisions between enzyme active sites and substrate. The substrate is no longer a limiting factor. HOWEVER At high levels of substrate, there is no further rise in rate of reaction since the enzymes cannot formed any more enzyme substrate complexes as all active sites are filled. The number of enzymes is now the limiting factor.

Question 28

describe competitive inhibitors

Answer 28

Competitive inhibitors have a similar structure to substrates. They fit into the active site but do not react. They prevent the normal substrate from binding and hence slow the rate of reaction as fewer enzyme substrates can be formed.

Question 29

Describe non competitive inhibitors

Answer 29

These bind to a site on the enzyme which is distant from the active site. They alter the shape of the active site so that substrate/s cannot fit. Fewer enzyme substrates can be formed and the rate of reaction is decreased.