Chapter 4 Enzymes

Question 1

What is an enzyme?

Answer 1

An enzyme is a biological catalyst that speed up chemical/metabolic reactions by lowering the activation energy.

Question 2

What is activation energy?

Answer 2

Activation energy is the amount of energy required to start a chemical reaction.

Question 3

What is the Lock and Key Hypothesis?

Answer 3

The lock and key hypothesis states that an enzyme’s function is specific as it can only react with the substrate, whose shape is complementary to the active site of the enzyme. When the substrate enters the active site, an enzyme-substrate complex is formed. After the reaction reaches completion, it becomes an enzyme-product complex. The product or products is released, leaving the enzyme (unchanged) to catalyse more reactions.

Question 4

What is the induced- fit hypothesis?

Answer 4

The induced-fit hypothesis states that the active site of an enzyme changes shape slightly to fit the substrate. The initial interaction between enzyme and substrate is weak, but these interactions induces changes on an enzyme’s tertiary structure, putting strain on the substrate molecule.

Question 5

What is the difference between intracellular and extracellular enzymes?

Answer 5

Intracellular enzymes are found within cells and extracellular enzymes are found within outside of cells.

Question 6

Give an example of an intracellular enzyme?

Answer 6

Hydrogen peroxide is produced as a by-product of normal metabolism in the body, however, it is harmful. The enzyme catalase is intracellular and breaks down the hydrogen peroxide into water and oxygen.

Question 7

Give examples of extracellular enzymes.

Answer 7

Example 1: Amylase breaks down starch into maltose. Found in mouth and stomach.
Example 2: Trypsin breaks down proteins into amino acids. Found in small intestine.
Both these enzymes helps with digestion.

Question 8

Why does rate of reaction increase as temperature increase?

Answer 8

To be increasing temperature, it to be giving thermal energy.
This thermal energy is converted to kinetic energy.
The kinetic energy causes both enzyme and substrate to move quicker.
This increases more successful collisions between enzyme and substrate.
Hence, more enzyme-substrate complexes form.
More product is produced.

Question 9

What does an optimum temperature mean for enzymes?

Answer 9

An optimum temperature is the temperature where rate of reaction is at its highest.

Question 10

Why do enzymes start to denature after the optimum temperature?

Answer 10

After the optimum temperature, the kinetic energy (transferred from the thermal energy) causes the bonds in the enzyme to vibrate a lot, and will eventually break. If the bonds in the enzyme break, the tertiary structure of the enzyme changes, and hence, the active site also changes. This means the enzyme is no longer complementary to the substrate (change is shape means they won’t fit) and so the enzyme can no longer catalyse reactions.

Question 11

What does the temperature coefficient, Q10, mean?

Answer 11

The temperature coefficient represents how rate changes with every 10 degrees Celsius increase.

Question 12

What does it mean if the temperature coefficient had a value of 3?

Answer 12

This means the rate of reaction triples, every 10 degrees Celsius.

Question 13

How does pH affect rate of reaction?

Answer 13

Like temperature, pH also has an optimum point. However, a great decrease or increase can also cause an enzyme to denature. If there is a great decrease in pH, this means the solution with enzyme becomes more acidic, so there is more hydrogen ions (protons). A lot of protons can affect the ionic and hydrogen bonds of the enzyme (tertiary structure). This is because the hydrogen ions are able to interact with the charged and polar R groups of amino acids. These interactions causes the tertiary structure of the enzyme to change, and hence, the active site. Since the shape of the active site is changed, no more enzyme-substrate complexes can form, so no reactions can occur. Likewise, having very few hydrogen ions can affect the tertiary structure of an enzyme.

Question 14

The optimum pH for different enzymes in the body varies. Why is this?

Answer 14

Different areas of the body works in environments that may be more or less acidic to help with its function. Pepsin has an optimum pH of 2 and trypsin has an optimum pH of 8.

Question 15

When substrate concentration increases (and enzyme concentration stays the same), what happens to the rate of reaction?

Answer 15

Initially, there will be substrates to occupy an active site- a lot of enzyme-substrate complexes are formed, reactions are catalysed and products are produced. However, as we increase substrate concentration, the rate of reaction will be constant (no increase or decrease). This is because as substrate concentration increases, all active sites will be occupied and there will be no way to increase rate of reaction (assuming enzyme concentration is constant). The maximum amount of enzyme substrate complexes has been formed. In this sense, we say enzyme concentration is the limiting factor as this is stopping an increased ROR. On a substrate rate graph, the graph will eventually plateau as the rate does no change.

Question 16

When enzyme concentration increases (substrate concentration is constant), what happens to the rate of reaction?

Answer 16

The initial rate of reaction is high, because an increasing enzyme concentration (means more active sites) means more enzyme substrate complexes can form. This allows reactions to be catalysed and enzyme product complexes to be form. However, at a point, substrate concentration starts to decrease, because all substrates are being used up in reactions. Hence, even if we were to increase enzyme concentration, the rate of reaction won’t increase as there will be no substrates to catalyse a reaction with. In this sense, substrate concentration is the limiting factor as this is what stops an increased ROR. An enzyme rate graph will show a plateau towards the end.

Question 17

What is a competitive inhibitor? Where does this inhibitor bind to on the enzyme?

Answer 17

A competitive inhibitor is a chemical which has a similar structure to the substrate. Due to this similar structure, the competitive inhibitor is able to bind to the active site.

Question 18

How does a competitive inhibitor affect the rate of reaction?

Answer 18

As the competitive inhibitor competes with the substrate for the same active site, there are less active sites available to the substrate. Hence, less enzyme substrate complexes form, so less reactions are catalysed, which decreases the rate of reaction.

Question 19

When there is a competitive inhibitor present, the rate of reaction generally decreases. How does this change with an increasing substrate concentration?

Answer 19

When there is an increasing (higher) substrate concentration, the substrates outcompetes the inhibitors for the active site. This is because with more substrates, there is a higher chance an enzyme will successfully collide a substrate, rather than an inhibitor. As the substrates outcompetes the inhibitor, V max is restored (to the normal V max if the inhibitors were absent).

Question 20

Where does a non-competitive inhibitor bind to on the enzyme? Hence, why is it called ‘non-competitive’?

Answer 20

The non-competitive inhibitor binds to the allosteric site on the enzyme, not the active site. These inhibitors are not competing with any other molecules to occupy the allosteric site, hence, they are non-competitive.

Question 21

How does the binding of a non-competitive inhibitor affect the rate of reaction?

Answer 21

The binding of the non-competitive inhibitor affects the tertiary structure of the active site. This means the active site is no longer complementary to the substrate. Hence, less enzyme substrate complexes are formed, which decreases the rate of reaction.

Question 22

What is end product inhibition?

Answer 22

End product inhibition is where the product of a reaction acts as an inhibitor to the enzyme that produces it.