Chapter 3 Enzymes

Question 1

Define enzymes:

Answer 1

Enzymes are globular proteins that catalyse reactions inside cells (intracellular enzymes) or are secreted to
catalyse reactions outside cells (extracellular enzymes).

Question 2

Enzymes mode of action:

Answer 2

1. The active site, a small region on the enzyme, is where specific substrate molecules bind.
2. Enzyme-substrate complex forms.
3. Activation energy is lowered and rate of reaction is increased.
4. Reaction occurs.
5. Enzyme is regenerated as it was (unchanged).

Question 3

Describe the lock and key hypothesis:

Answer 3

According to this model, the enzyme (lock) and substrate (key) are perfectly complementary in shape. The substrate fits into the enzyme’s active site precisely.

Question 4

Describe the induced fit hypothesis:

Answer 4

This theory posits that enzymes are somewhat flexible and can adapt their shape slightly to fit the substrate better. When a substrate enters an enzyme’s active site, the enzyme structure may adjust to envelop the substrate more effectively.

Question 5

What are the factors affecting the rate of enzyme-catalysed reactions?

Answer 5

temperature
pH
enzyme concentration
substrate concentration
inhibitor concentration

Question 6

Describe Vmax and Km (Michaelis-Mensten constant):

Answer 6

Vmax – the maximum rate of the reaction, when all the enzyme’s active sites are saturated with substrate.
Km (also known as the Michaelis constant) – the substrate concentration at which the reaction rate is 50% of the Vmax.

Question 7

What is enzyme affinity?

Answer 7

Enzyme affinity is a measure of how well a substrate fits the enzyme active site.

Question 8

What does Km values say about affinity?

Answer 8

The value of Km is inversely related to the affinity of the enzyme for its substrate. Low Km values for an enzyme correspond to high affinity for substrate.

Question 9

Describe competitive reversible inhibition:

Answer 9

Competitive inhibitors bind to the active site of an enzyme, directly competing with the substrate. It can be reversed by increasing the substrate concentration.

Question 10

Describe non-competitive reversible inhibition:

Answer 10

These inhibitors bind to an allosteric site (a site other than the active site) of the enzyme, causing a conformational change at active site (altered tertiary structure) that reduces enzyme activity. It can be reversed when inhibitor detaches from allosteric site.

Question 11

Advantages of immobilised enzymes:

Answer 11

1. Can be re-used.
2. Product is enzyme free.
3. More tolerant to temperature changes. (do not denature so easily)
4. More tolerant to pH changes. (do not denature so easily)