Enzymes

Question 1

Mode of action of enzymes

Answer 1

• active site
• enzyme specificity
• enzyme-substrate complex
• activation energy
• lock-and-key hypothesis
• induced fit hypothesis

Question 2

Factors: Substrate / enzyme concentration description

Answer 2

1. At low substrate concentration, as substrate concentration increase, rate of reaction increases linearly/ proportionately
2. As substrate concentration continues to increase, the increase in rate of reaction slows down
3. At high substrate concentrations, further increase in substrate concentration does not increase rate of reaction. Rate of reaction plateaus off and rate of reaction is at the maximum

Question 3

Factors: Substrate / enzyme concentration explanation

Answer 3

1. At low substrate concentration, as substrate concentration increases, number of effective collisions between enzymes’ active sites and substrates increases because not all active sites are occupied
2. Number of enzyme-substrate complexes formed per unit time increases. Formation rate of products increases proportionately
3. Substrate concentration is limiting the rate of reaction
4. At high substrate concentrations, all active sites of enzymes are occupied at any one time.
5. There is a maximum number of ES complexes formed per unit time, and hence maximum rate of formation of products
6. Other factors like enzyme concentration are now limiting the rate of reaction

Question 4

Factor: Temperature description

Answer 4

1. At very low temperatures, rate of reaction is very slow
2. As temperature increases towards optimum temperature, the rate doubles for every 10 degree celsius rise in temperature. i.e. temperature coefficient Q10=2
3. At optimum temperature, the rate of enzyme reaction is at the medicine
4. At temperature slightly above optimum temperature, the rate of enzyme reaction decreases slowly
5. At very high temperatures, there is a drastic fall in the rate of reaction and it eventually falls to zero

Question 5

Factor: Temperature explanation

Answer 5

1. At very low temperatures, substrate and enzyme molecules have low kinetic energy. Thus, they move very slowly and there are very few effective collisions between enzyme and substrate molecule
2. As temperature increases to optimum temperature, the kinetic energy of molecules increases and molecules move faster. Thus, there are more effective collisions between enzyme and substrate molecules. There is an increase in the number of ES complexes formed per unit time. Hence, substrate molecules at higher energy levels have higher probability to overcome the activation energy barrier to form products
3. At optimum temperature, there is a maximum number of ES complexes formed per unit time
4. At temperatures slightly above optimum temperatures, thermal agitation of enzyme molecules disrupts the weaker bonds such as hydrophobic interactions, h bonds and ionic bonds.
   This distorts the specific 3-dimensional conformation of the enzyme.
   The active site is distorted and no longer complementary to the substrate
   E-S complexes cannot be formed and enzymes are said to be denatured.
   With an increase in percentage of enzymes that are denatured, rate of E-S complex formation will decrease
5. When all enzymes are eventually denatured, rate of reaction then falls to zero

Question 6

Factor: pH description

Answer 6

1. The graph is bell-shaped, symmetrical about the optimum pH
2. Rate is high over a narrow range of pH and peaks at optimum pH
3. At pH values slightly above or below optimum pH, rate of reaction falls drastically

Question 7

Factor: pH explanation

Answer 7

1. At optimum pH, all the ionic and hydrogen bonds between R groups of amino acids are intact. The active sides are complementary to the substrate molecule. as such, maximum number of E-S complex can be formed per unit time
2. A slight change in pH from optimum pH will change the charge found on the acidic and basic R groups of amino acids at the active site. This reduces the binding ability of substrate to active site and hence the rate of formation of E-S complex will decrease
3. A drastic change in pH from optimum pH will disrupt the ionic bonds between the acidic and basic R-groups of the amino acids and h bonds between polar R-groups
   This distorts the specific 3-dimensional conformation of the enzyme
   Thus the active site is distorted and no longer complementary to the substrate
   Substrate molecules can no longer fit the active site of enzyme molecules and E-S complexes cannot be formed. Enzymes are said to be denatured.
   With an increase in percentage of enzymes that are denatured, rate of E-S complex formation will decrease drastically

Question 8

Competitive inhibition

Answer 8

1. Competitive inhibitors are structurally similar in terms of shape, size, charge, orientation to substrate molecules
2. Competitive inhibitors bind to active site of enzyme and competes with the substrate for the active site
3. This reduces the number of active sites available for the substrates to bind and form E-S complex
4. Km (Michaelis Constant) of the enzyme will increase in the presence of competitive inhibitor
5. Vmax can be reached eventually at high substrate concentration
   -> when substrate conc is low, more likely for enzyme to collide with competitive inhibitor molecule to form E-I complex
6. When substrate conc is high, more likely for enzyme molecule to collide with substrate molecules to form E-S complex

Question 9

Non-competitive inhibition

Answer 9

1. Non-competitive inhibitors are not structurally similar, in terms of shape, size, charge and orientation to substrate molecule
2. Non-competitive inhibitors bind at a site away from the active site.
3. This interaction alters the specific 3-D conformation of the enzyme molecule such that
   -> active site is distorted and no longer complementary to substrate, thus not able to binds to the substrate properly
4. Km of the enzyme remains unchanged in the presence of non-competitive inhibitor
5. Vmax is lowered as non-competitive inhibitors reduce the functional enzymes. hence, Vmax will not be restored even if substrate concentration increases.

Question 10

Allosteric activation and inhibition

Answer 10

1. Most allosterically regulated enzymes are constructed from two or more subunits
2. An activator binds to an allosteric site, often located where subunits join
   -> binding of an activator to a regulatory site stabilises the shape that has functional active site
   -> binding of an inhibitor stabilises the inactive form of the enzyme

Question 11

Lock-and-key hypothesis

Answer 11

1. The enzyme acts as a lock and the substrate acts as a key, which fits
   precisely.
2. The active site of the enzyme is perfectly complementary to the substrate in
   terms of shape, size, charge and orientation.
3. The substrate binds to enzyme’s active site to form the enzyme-substrate
   complex.
4. This mode of action is more probable for enzymes that work on only one type of
   substrate

Question 12

Induced fit hypothesis

Answer 12

1. Enzymes may work in a more flexible manner.
2. The active site is not perfectly complementary to the substrate in terms of
   shape, size and orientation.
3. However, upon forming some bonds with the substrate, the enzyme changes its
   shape, which leads to a precise fit to form the enzyme-substrate complex.
4. This mode of action is more probable for enzymes that work on a group of
   closely-related substrates, e.g. lipases.

Question 13

Mode of action of enzymes

Answer 13

Enzymes have unique/ specific three-dimensional conformation with an active site,
which is formed by 3 to 12 amino acids from different parts of a single polypeptide
chain; ® configuration
The active site of an enzyme is complementary to its substrate in terms of shape,
size, charge and orientation determining the enzyme specificity;
When substrate binds to the active site of enzyme with weak bonds such as
hydrogen bonds / ionic bonds / hydrophobic interactions, the enzyme-substrate
complex (E-S complex) is formed;
Enzymes speed up biological reactions as they provide an alternative pathway,
which has a lower EA as compared to the uncatalysed reaction;
Max 1m:
Enzymes lower the activation energy of a reaction by promoting formation of
transition state via: allowing close proximity of reactants /
ensuring correct orientation of reactants to facilitate the reaction taking place /
destabilising the bonds of reactants as enzymes contort reactant molecule /
providing a microenvironment conducive for reaction e.g. creating a water-free zone
for non-polar reactants to react more easily ;
Correct description of lock and key hypothesis / induced fit hypothesis;
not used up in reaction / remain unchanged / reusable / high turnover number /
catalyse many reactions per unit time ;