Enzymes

Question 2

Q: What happens to the enzyme-catalyzed reaction rate as substrate concentration ([S]) increases?

Answer 2

A: The reaction rate increases until the enzyme is saturated, reaching maximum velocity (Vmax).

Question 3

Q: What is Km in enzyme kinetics?

Answer 3

Km is the substrate concentration at which the reaction reaches half of Vmax.

Low Km: High affinity of the enzyme for the substrate.
High Km: Low affinity of the enzyme for the substrate.

Question 4

Q: At what temperature does enzyme activity typically stop?

Answer 4

A: Enzyme activity stops at 70°C due to denaturation.

Question 5

Q: What is the optimum pH for pepsin?

Answer 5

A: The optimum pH for pepsin is 2.

Question 6

Q: How do slight changes in pH affect enzyme activity?

Answer 6

A: Slight pH changes alter charges on the active site, reducing enzyme activity.

Question 7

Q: What happens to enzymes with extreme pH changes?

Answer 7

A: Extreme pH changes cause denaturation and irreversible loss of enzyme activity.

Question 8

Q: What are the two main types of enzyme inhibitors?

Answer 8

Reversible Inhibitors
Competitive
Allosteric

Irreversible Inhibitors
Inhibitors of cofactors
Inhibitors of apoenzymes

Question 9

Q: What determines the degree of competitive inhibition?

Answer 9

Ratio of inhibitor concentration to substrate concentration.

Relative affinity of the inhibitor and substrate for the enzyme.

Question 10

Q: What is the effect of a competitive inhibitor on Vmax?

Answer 10

Vmax is NOT CHANGED

Question 11

Q: What is the effect of a competitive inhibitor on Km?

Answer 11

A: Km INCREASES because more substrate is needed to reach ½ Vmax.

Question 12

Q: Give an example of a competitive inhibitor used to treat Gout.

Answer 12

Allopurinol:

It structurally resembles hypoxanthine.

It inhibits xanthine oxidase, preventing the conversion of hypoxanthine to uric acid.

Question 13

Q: How does sulfonamide act as a competitive inhibitor?

Answer 13

Sulfonamide resembles P-aminobenzoic acid (PABA).
It inhibits the bacterial synthesis of folic acid, which is essential for bacterial growth.

Question 14

Q: What are Dicumarol and Warfarin used for, and how do they act?

Answer 14

They are anticoagulants.

They structurally resemble vitamin K and inhibit the activation of blood clotting factors.

Question 15

Q: How do Statins work as competitive inhibitors?

Answer 15

Statins inhibit HMG-CoA reductase, the key enzyme in cholesterol synthesis, thereby reducing plasma cholesterol levels.

Question 16

Q: What are allosteric inhibitors?

Answer 16

: Allosteric inhibitors are small organic molecules that bind to a specific site away from the catalytic site (allosteric site) on the enzyme.

Question 17

Q: How do allosteric inhibitors affect enzyme activity?

Answer 17

A: They cause conformational changes in the protein structure, leading to decreased enzyme activity.

Question 18

Q: What is the effect of allosteric inhibitors on Km?

Answer 18

Km increases because the enzyme’s affinity for the substrate decreases.

Question 19

Q: What is the effect of allosteric inhibitors on Vmax?

Answer 19

A: Vmax decreases because the enzyme’s catalytic activity is reduced.

Question 20

Q: How is an allosteric inhibitor different from a competitive inhibitor?

Answer 20

Allosteric inhibitors bind to a site away from the active site.
Competitive inhibitors bind directly at the active site.

Question 21

Q: What is the overall impact of allosteric inhibition on enzyme kinetics?

Answer 21

Km increases → Lower substrate affinity.
Vmax decreases → Reduced catalytic activity.

Question 22

Q: What are the two types of irreversible enzyme inhibitors?

Answer 22

Inhibitors of cofactors (non-protein part).
Inhibitors of the apoenzyme (protein part).

Question 23

Q: How does fluoride act as an irreversible inhibitor?

Answer 23

A: Fluoride chelates Ca²⁺ and Mg²⁺ ions, blocking the action of enzymes that require these ions, e.g., it inhibits enolase (enzyme in glycolysis).

Question 24

Q: Give an example of an enzyme inhibited by fluoride.

Answer 24

A: Enolase (an enzyme of glycolysis) is inhibited by fluoride through chelation of Mg²⁺ ions.

Question 25

Q: How do cyanide and carbon monoxide act as inhibitors?

Answer 25

A: They irreversibly inhibit cytochrome oxidase (enzyme in the respiratory chain) by binding to the iron in the heme group.

Question 26

Q: What enzyme is inhibited by cyanide and carbon monoxide?

Answer 26

A: Cytochrome oxidase, which is critical for cellular respiration in the respiratory chain.

Question 27

Q: What are apoenzyme inhibitors?

Answer 27

A: They are non-specific inhibitors that denature the protein part (apoenzyme) of the enzyme.

Question 28

Q: Give examples of apoenzyme inhibitors.

Answer 28

A: Strong acids, alkalis, alcohols, and salts of heavy metals irreversibly denature proteins, including enzymes.

Question 29

Q: What is the key difference between cofactor inhibitors and apoenzyme inhibitors?

Answer 29

Cofactor inhibitors target the non-protein part of the enzyme (e.g., chelation of ions).
Apoenzyme inhibitors denature the protein part of the enzyme.

Question 30

Q: What are the three main mechanisms for regulating enzyme activity?

Answer 30

Changing the absolute amount of enzyme.
Changing the catalytic activity of the enzyme.
Compartmentation of enzymes.

Question 31

Q: How is the absolute amount of an enzyme controlled?

Answer 31

Rate of enzyme synthesis: Controlled by inducers (stimulate gene expression) and repressors (inhibit gene expression).
Enzyme degradation: Regulated breakdown of enzymes.

Question 32

Q: What are zymogens (proenzymes)?

Answer 32

A: Inactive enzymes that are converted to active enzymes by proteolysis (removal of a polypeptide chain masking the active site).

Question 33

Q: Give an example of zymogen activation.

Answer 33

Digestive enzymes are stored as inactive zymogens inside cells to prevent digestion of cellular proteins. They are activated in the gut to digest food proteins.

Question 34

Q: What is autocatalytic activation?

Answer 34

A: It is when an activated enzyme can activate its own zymogen.

Question 35

Q: How do allosteric modifiers regulate enzymes?

Answer 35

Allosteric activators bind to the allosteric site, causing conformational changes that
increase reaction velocity.

Allosteric inhibitors bind to decrease enzyme activity.

Question 36

Q: What is the effect of allosteric activators on Km?

Answer 36

A: Allosteric activators decrease Km, increasing substrate affinity.

Question 37

Q: Give an example of an allosteric activator and inhibitor for phosphofructokinase-1 (PFK-1).

Answer 37

AMP: Allosteric activator.
ATP: Allosteric inhibitor.

Question 38

Q: What is covalent modification of enzymes?

Answer 38

It is the activation or inactivation of enzymes through phosphorylation or dephosphorylation.

Question 39

Q: Which amino acids are most commonly phosphorylated in covalent modification?

Answer 39

A: Serine or tyrosine
residues in the enzyme’s polypeptide chain.

Question 40

Q: What is the effect of phosphorylation or dephosphorylation?

Answer 40

Phosphorylation can activate or inactivate an enzyme depending on the enzyme.
The reverse process (dephosphorylation) has the opposite effect.

Question 41

Q: Why is compartmentation important in enzyme regulation?

Answer 41

It allows the separation and regulation of metabolic pathways, ensuring proper enzyme localization in specific cellular areas (e.g., cytosol, mitochondria).