Enzymes Flashcards
Q: What happens to the enzyme-catalyzed reaction rate as substrate concentration ([S]) increases?
A: The reaction rate increases until the enzyme is saturated, reaching maximum velocity (Vmax).
Q: What is Km in enzyme kinetics?
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.
Q: At what temperature does enzyme activity typically stop?
A: Enzyme activity stops at 70°C due to denaturation.
Q: What is the optimum pH for pepsin?
A: The optimum pH for pepsin is 2.
Q: How do slight changes in pH affect enzyme activity?
A: Slight pH changes alter charges on the active site, reducing enzyme activity.
Q: What happens to enzymes with extreme pH changes?
A: Extreme pH changes cause denaturation and irreversible loss of enzyme activity.
Q: What are the two main types of enzyme inhibitors?
Reversible Inhibitors
Competitive
Allosteric
Irreversible Inhibitors
Inhibitors of cofactors
Inhibitors of apoenzymes
Q: What determines the degree of competitive inhibition?
Ratio of inhibitor concentration to substrate concentration.
Relative affinity of the inhibitor and substrate for the enzyme.
Q: What is the effect of a competitive inhibitor on Vmax?
Vmax is NOT CHANGED
Q: What is the effect of a competitive inhibitor on Km?
A: Km INCREASES because more substrate is needed to reach ½ Vmax.
Q: Give an example of a competitive inhibitor used to treat Gout.
Allopurinol:
It structurally resembles hypoxanthine.
It inhibits xanthine oxidase, preventing the conversion of hypoxanthine to uric acid.
Q: How does sulfonamide act as a competitive inhibitor?
Sulfonamide resembles P-aminobenzoic acid (PABA).
It inhibits the bacterial synthesis of folic acid, which is essential for bacterial growth.
Q: What are Dicumarol and Warfarin used for, and how do they act?
They are anticoagulants.
They structurally resemble vitamin K and inhibit the activation of blood clotting factors.
Q: How do Statins work as competitive inhibitors?
Statins inhibit HMG-CoA reductase, the key enzyme in cholesterol synthesis, thereby reducing plasma cholesterol levels.
Q: What are allosteric inhibitors?
: Allosteric inhibitors are small organic molecules that bind to a specific site away from the catalytic site (allosteric site) on the enzyme.
Q: How do allosteric inhibitors affect enzyme activity?
A: They cause conformational changes in the protein structure, leading to decreased enzyme activity.
Q: What is the effect of allosteric inhibitors on Km?
Km increases because the enzyme’s affinity for the substrate decreases.
Q: What is the effect of allosteric inhibitors on Vmax?
A: Vmax decreases because the enzyme’s catalytic activity is reduced.
Q: How is an allosteric inhibitor different from a competitive inhibitor?
Allosteric inhibitors bind to a site away from the active site.
Competitive inhibitors bind directly at the active site.
Q: What is the overall impact of allosteric inhibition on enzyme kinetics?
Km increases → Lower substrate affinity.
Vmax decreases → Reduced catalytic activity.
Q: What are the two types of irreversible enzyme inhibitors?
Inhibitors of cofactors (non-protein part).
Inhibitors of the apoenzyme (protein part).
Q: How does fluoride act as an irreversible inhibitor?
A: Fluoride chelates Ca²⁺ and Mg²⁺ ions, blocking the action of enzymes that require these ions, e.g., it inhibits enolase (enzyme in glycolysis).
Q: Give an example of an enzyme inhibited by fluoride.
A: Enolase (an enzyme of glycolysis) is inhibited by fluoride through chelation of Mg²⁺ ions.
Q: How do cyanide and carbon monoxide act as inhibitors?
A: They irreversibly inhibit cytochrome oxidase (enzyme in the respiratory chain) by binding to the iron in the heme group.
Q: What enzyme is inhibited by cyanide and carbon monoxide?
A: Cytochrome oxidase, which is critical for cellular respiration in the respiratory chain.
Q: What are apoenzyme inhibitors?
A: They are non-specific inhibitors that denature the protein part (apoenzyme) of the enzyme.
Q: Give examples of apoenzyme inhibitors.
A: Strong acids, alkalis, alcohols, and salts of heavy metals irreversibly denature proteins, including enzymes.
Q: What is the key difference between cofactor inhibitors and apoenzyme inhibitors?
Cofactor inhibitors target the non-protein part of the enzyme (e.g., chelation of ions).
Apoenzyme inhibitors denature the protein part of the enzyme.
Q: What are the three main mechanisms for regulating enzyme activity?
Changing the absolute amount of enzyme.
Changing the catalytic activity of the enzyme.
Compartmentation of enzymes.
Q: How is the absolute amount of an enzyme controlled?
Rate of enzyme synthesis: Controlled by inducers (stimulate gene expression) and repressors (inhibit gene expression).
Enzyme degradation: Regulated breakdown of enzymes.
Q: What are zymogens (proenzymes)?
A: Inactive enzymes that are converted to active enzymes by proteolysis (removal of a polypeptide chain masking the active site).
Q: Give an example of zymogen activation.
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.
Q: What is autocatalytic activation?
A: It is when an activated enzyme can activate its own zymogen.
Q: How do allosteric modifiers regulate enzymes?
Allosteric activators bind to the allosteric site, causing conformational changes that
increase reaction velocity.
Allosteric inhibitors bind to decrease enzyme activity.
Q: What is the effect of allosteric activators on Km?
A: Allosteric activators decrease Km, increasing substrate affinity.
Q: Give an example of an allosteric activator and inhibitor for phosphofructokinase-1 (PFK-1).
AMP: Allosteric activator.
ATP: Allosteric inhibitor.
Q: What is covalent modification of enzymes?
It is the activation or inactivation of enzymes through phosphorylation or dephosphorylation.
Q: Which amino acids are most commonly phosphorylated in covalent modification?
A: Serine or tyrosine
residues in the enzyme’s polypeptide chain.
Q: What is the effect of phosphorylation or dephosphorylation?
Phosphorylation can activate or inactivate an enzyme depending on the enzyme.
The reverse process (dephosphorylation) has the opposite effect.
Q: Why is compartmentation important in enzyme regulation?
It allows the separation and regulation of metabolic pathways, ensuring proper enzyme localization in specific cellular areas (e.g., cytosol, mitochondria).
