Lecture 8 Flashcards
(17 cards)
What are the three main aspects of chemical reactions that enzymes affect?
1) Thermodynamics (direction & energy changes)
2) Structural changes (transition states)
3) Kinetics (reaction speed)
Why do thermodynamically favorable reactions sometimes occur slowly?
Because the formation of transition states requires overcoming an activation energy barrier (ΔG‡), which is often high.
What is the difference between thermodynamics and kinetics in enzyme-catalyzed reactions?
Thermodynamics (ΔG) determines if a reaction is favorable, while kinetics (ΔG‡) determines the speed of the reaction.
What is the transition state in a reaction?
The highest energy intermediate, representing the most unstable structure during the reaction.
How do enzymes increase reaction rates?
By lowering the activation energy (ΔG‡) and stabilizing the transition state.
Does an enzyme change the overall ΔG of a reaction?
No, it only lowers the activation energy but does not affect the overall thermodynamics.
How much can enzymes enhance reaction rates?
Enzymes can speed up reactions by factors of 10⁶ to 10²⁰ times.
What are the two models for enzyme-substrate binding?
1) Lock and Key Model (Emil Fischer, 1894) – substrate fits perfectly into the enzyme.
2) Induced Fit Model (Daniel Koshland, 1958) – enzyme and substrate undergo conformational changes for better binding.
Which model explains both specificity and catalysis?
The Induced Fit Model, because it accounts for enzyme flexibility and transition-state stabilization.
What are the six main ways enzymes enhance reaction rates? Give a slight description.
- Proximity & Orientation – Aligns reactants correctly.
- Substrate Distortion – Strains bonds to facilitate reaction.
- Electrostatic Catalysis – Stabilizes transition state charges.
- Metal Ion Catalysis – Uses metal ions to stabilize charges or generate nucleophiles.
- General Acid-Base Catalysis – Donates or accepts protons.
- Covalent Catalysis – Forms a transient covalent bond with the substrate.
What type of catalysis does chymotrypsin use?
Covalent catalysis and general acid-base catalysis.
What are the three key residues in the catalytic triad of serine proteases?
- Serine (Ser-195) – Acts as a nucleophile.
- Histidine (His-57) – Acts as a general acid/base.
- Aspartate (Asp-102) – Stabilizes histidine via electrostatic interactions.
What is the oxyanion hole in serine proteases?
A pocket in the enzyme that stabilizes the negatively charged tetrahedral intermediates.
What happens in the two tetrahedral intermediates of chymotrypsin catalysis?
The first forms when Ser-195 attacks the peptide bond, and the second forms when water attacks the acyl-enzyme intermediate.
What is the specificity of chymotrypsin, trypsin, and elastase?
Chymotrypsin – Cleaves after aromatic residues (Trp, Tyr, Phe).
Trypsin – Cleaves after positively charged residues (Lys, Arg).
Elastase – Cleaves after small, nonpolar residues (Ala, Ser).
How are enzymes classified?
By the type of reaction they catalyze (hydrolases, oxidoreductases, transferases, etc.).
What class of enzyme are proteases?
Hydrolases – enzymes that break peptide bonds using water.
