Biological Molecules - Enzymes

Question 1

What do enzymes do?

Answer 1

Lower the activation energy of the reaction it catalyses so increases the rate of reaction

Question 2

What does it mean that enzymes are globular proteins?

Answer 2

It means their shape (as well as the shape of the active site of an enzyme) is determined by the complex tertiary structure of the protein that makes up the enzyme and is therefore highly specific

Question 3

What is the lock and key model?

Answer 3

In the 1890’s the first model of enzyme activity was described by Emil Fischer:
Suggested that both enzymes and substrates were rigid structures that locked into each other very precisely, much like a key going into a lock

Question 4

What is the induced fit model?

Answer 4

The enzyme and substrate interact with each other
The enzyme and its active site (and sometimes the substrate) can change shape slightly as the substrate molecule enters the enzyme
This ensures an ideal binding arrangement between the enzyme and substrate is achieved
This maximises the ability of the enzyme to catalyse the reaction

Question 5

What is it called when an enzyme and a specific substrate bind?

Answer 5

An enzyme-substrate complex
Only temporary before the enzyme catalyses the reaction and the product(s) are released

Question 6

What is denaturation?

Answer 6

A change in shape of the enzyme’s active site
Preventing substrate binding
Caused by extremes of heat or pH

Question 7

What must happen for a reaction to occur?

Answer 7

Substrates to collide with the enzymes active site and this must happen at the correct orientation and speed

Question 8

What is the specificity of an enzyme?

Answer 8

A result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)

Question 9

What is the specificity/ shape of the active site determined by?

Answer 9

the complex tertiary structure of the protein that makes up the enzyme:
Proteins are formed from chains of amino acids held together by peptide bonds
The order of amino acids determines the shape of an enzyme
If the order is altered, the resulting three-dimensional shape changes

Question 10

What factors effect the rate of enzyme-controlled reactions?

Answer 10

Enzyme concentration
Substrate concentration
Concentration of competitive and of non-competitive inhibitors
pH
Temperature

Question 11

How does a high temperature effect rate of reaction?

Answer 11

They speed up reactions:
Molecules move more quickly
Higher frequency successful collisions between substrate molecules and active site of enzyme
More frequent enzyme-substrate complex formation
Substrate and enzyme collide with more energy, making it more likely for bonds to be formed or broken (allowing the reaction to occur)

Question 12

How does a low temperature effect rate of reaction?

Answer 12

Prevent reactions from proceeding or slow them down:
Molecules move relatively slow
Lower frequency of successful collisions between substrate molecules and active site of enzyme
Less frequent enzyme-substrate complex formation
Substrate and enzyme collide with less energy, making it less likely for bonds to be formed or broken (stopping the reaction from occurring)

Question 13

How does a high temperature continuing to increase effect the rate of reaction?

Answer 13

The rate at which an enzyme catalyses a reaction drops sharply, as the enzyme begins to denature:
Bonds (eg. hydrogen bonds) holding the enzyme molecule in its precise shape start to break
This causes the tertiary structure of the protein to change
This permanently damages the active site, preventing the substrate from binding
Denaturation has occurred if the substrate can no longer bind
High temperatures causes the hydrogen bonds between amino acids to break, changing the conformation of the enzyme

Question 14

What happens to enzymes at extremes of pH?

Answer 14

Hydrogen and ionic bonds hold the tertiary structure of the protein together
Below and above the optimum pH of an enzyme, solutions with an excess of H+ ions (acidic solutions) and OH- ions (alkaline solutions) can cause these bonds to break
This alters the shape of the active site, which means enzyme-substrate complexes form less easily
Eventually, enzyme-substrate complexes can no longer form at all
At this point, complete denaturation of the enzyme has occurred

Question 15

Why use buffer solutions to measure the rate of reaction at different pH values?

Answer 15

Buffer solutions each have a specific pH
Buffer solutions maintain this specific pH, even if the reaction taking place would otherwise cause the pH of the reaction mixture to change

Question 16

How does enzyme concentration effect the rate of reaction?

Answer 16

The higher the enzyme concentration in a reaction mixture, the greater the number of active sites available and the greater the likelihood 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 17

How does substrate concentration effect rate of reaction?

Answer 17

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 18

What are competitive inhibitors?

Answer 18

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

Question 19

What are non-competitive inhibitors?

Answer 19

They bind to the enzyme at an alternative site (allosteric site), which alters the shape of the active site and therefore prevents the substrate from binding to it

Question 20

How does inhibitor concentration effect rate of reaction?

Answer 20

Both types of inhibitors slow down or stop enzyme activity
Increasing the concentration of an inhibitor, therefore, reduces the rate of reaction and eventually, if inhibitor concentration continues to be increased, the reaction will stop completely

Question 21

What happens if you increase substrate concentration when using competitive inhibitors?

Answer 21

It can increase the rate of reaction once more
More substrate molecules mean they are more likely to collide with enzymes and form enzyme-substrate complexes

Question 22

What happens if you increase substrate concentration when using non-competitive inhibitors?

Answer 22

It cannot increase the rate of reaction once more, as the shape of the active site of the enzyme remains changed and enzyme-substrate complexes are still unable to form