Enzymes

Question 1

Distinguish between metabolism, a metabolic pathway, and a metabolic reaction.

Answer 1

Metabolism: The sum of all chemical reactions in a cell.

Metabolic pathway: A series of enzyme-catalyzed reactions that build or break down molecules.

Metabolic reaction: A single chemical transformation within a metabolic pathway.

Question 2

Describe the non-enzymatic conversion of 2 H₂O₂ → 2 H₂O + O₂.

Answer 2

• Hydrogen peroxide (H₂O₂) decomposes slowly on its own into water (H₂O) and oxygen (O₂).

• This reaction is spontaneous but very slow without a catalyst.

Question 3

How does catalase catalyze the breakdown of hydrogen peroxide?

Answer 3

• Catalase is an enzyme that speeds up the reaction 2 H₂O₂ → 2 H₂O + O₂.
• It binds H₂O₂ in its active site and stabilizes the transition state, lowering activation energy.
• Produces water and oxygen gas (bubbles) rapidly.

Question 4

Predict the R-groups in the active site of an enzyme that binds sucrose.

Answer 4

Since sucrose is polar, the active site likely contains polar or charged R-groups:
• Serine (-OH)
• Aspartic acid (-COO⁻)
• Glutamine (-CONH₂)

Question 5

Predict the R-groups in the active site of an enzyme that binds ATP.

Answer 5

• ATP has negative phosphate groups, so the active site likely has positively charged R-groups:
• Lysine (-NH₃⁺)
• Arginine (-NH₂⁺)
• Histidine (-NH⁺, depending on pH)

Question 6

Describe the steps of enzyme function using a substrate, enzyme, and product model.

Answer 6

1. Binding – Substrate enters active site, forming an enzyme-substrate complex.
2. Catalysis – Enzyme stabilizes transition state and lowers activation energy.
3. Dissociation – Product is released, and the enzyme is ready to be reused.

Question 7

Distinguish between substrate collision and substrate binding to an enzyme.

Answer 7

Collision – Random movement of substrate molecules towards the enzyme.

Binding – When the substrate fits into the active site, forming a stable enzyme-substrate complex.

Question 8

How would you test the effect of enzyme concentration, substrate concentration, temperature, and pH on enzyme activity?

Answer 8

• [Enzyme] – Increase enzyme concentration and measure reaction rate.
• [Substrate] – Increase substrate concentration and plot rate vs. [substrate] (expect a plateau).
• Temperature – Test enzyme activity at different temperatures (expect an optimum).
• pH – Test reaction rates at different pH levels (expect denaturation at extreme pH values).

Question 9

Distinguish between allosteric inhibition and allosteric activation.

Answer 9

• Allosteric inhibition – A molecule binds to an enzyme at a site other than the active site, causing inactivation (e.g., ATP inhibits phosphofructokinase in glycolysis).
• Allosteric activation – A molecule binds to an allosteric site, increasing enzyme activity (e.g., ADP activating phosphofructokinase).

Question 10

Contrast allosteric regulation and competitive inhibition.

Answer 10

• Allosteric regulation – Molecule binds at an allosteric site, changing enzyme shape (e.g., feedback inhibition).
• Competitive inhibition – Inhibitor competes with substrate for the active site (e.g., methanol competes with ethanol for alcohol dehydrogenase).

Question 11

Graph for enzyme activity vs variables (substrate concentration, temperature, and pH)