Chapter 5 Flashcards
What are the three key properties of enzymes?
- Highly specific – Catalyze only specific reactions.
- Lower activation energy – Speed up reactions without being consumed.
- Catalyze reactions in both directions – Reach equilibrium faster.
What determines enzyme specificity?
The precise interaction between the enzyme and substrate, determined by shape and chemical properties of the active site.
Give an example of enzymes with different levels of specificity.
Papain – Cleaves any peptide bond (low specificity).
Thrombin – Cleaves only after arginine residues (high specificity).
What are the two types of enzyme cofactors?
- Coenzymes – Small organic molecules, often vitamin-derived (e.g., NAD⁺, FAD).
- Metal ions – Inorganic cofactors (e.g., Zn²⁺, Mg²⁺).
What is the difference between a holoenzyme and an apoenzyme?
Holoenzyme = Enzyme + cofactor (active).
Apoenzyme = Enzyme without cofactor (inactive).
What does Gibbs free energy (ΔG) determine about a reaction?
ΔG < 0 → Reaction is spontaneous (exergonic).
ΔG > 0 → Reaction is non-spontaneous (endergonic).
ΔG = 0 → Reaction is at equilibrium.
How does ΔG relate to the equilibrium constant (K′eq)?
ΔG°′= −RT ln K′eq
A large K′eq means negative ΔG°′, favoring product formation.
What are the key characteristics of an enzyme’s active site?
Three-dimensional cleft formed by distant residues.
Small part of the enzyme volume.
Microenvironment optimized for catalysis.
Substrate binding via hydrogen bonds, ionic bonds, and van der Waals interactions.
What is the induced fit model of enzyme binding?
Enzyme changes shape upon substrate binding, improving interaction.
More accurate than the lock-and-key model.
What does a Michaelis-Menten plot show?
Hyperbolic curve – Describes enzyme velocity (V₀) vs. substrate concentration ([S]).
Vmax – Maximum velocity when enzyme is saturated.
Km (Michaelis constant) – [S] at ½Vmax, measures substrate affinity.
What does a low Km indicate?
High substrate affinity (enzyme binds substrate tightly).
What is a Lineweaver-Burk plot, and why is it useful?
Double reciprocal plot of 1/V₀ vs. 1/[S].
Converts Michaelis-Menten curve into a straight line.
Used to determine Vmax and Km.
What are the two types of multi-substrate reactions?
- Sequential reactions – All substrates must bind before any product is released.
- Ordered: Substrates bind in a specific sequence.
- Random: Substrates bind in any order. - Double-displacement (Ping-Pong) reactions – One product is released before the second substrate binds.
How do allosteric enzymes differ from Michaelis-Menten enzymes?
Sigmoidal (S-shaped) curve, not hyperbolic.
Show cooperative binding (binding of one substrate affects others).
How do competitive, uncompetitive, and noncompetitive inhibitors affect enzyme kinetics?
1️⃣ Competitive Inhibition
Binding Site: Active Site
Effect on Km: Increases (substrate must outcompete inhibitor)
Effect on Vmax: No change (can still reach Vmax with enough substrate)
Lineweaver-Burk Plot Shift: X-intercept shifts left
Can be Overcome by More Substrate? Yes
2️⃣ Uncompetitive Inhibition
Binding Site: Enzyme-Substrate (ES) Complex
Effect on Km: Decreases (substrate binding is enhanced by inhibitor binding)
Effect on Vmax: Decreases (inhibitor prevents product formation)
Lineweaver-Burk Plot Shift: Parallel shift
Can be Overcome by More Substrate? No
3️⃣ Noncompetitive Inhibition
Binding Site: Allosteric Site (not the active site)
Effect on Km: No change (substrate binding is unaffected)
Effect on Vmax: Decreases (enzyme is inactivated)
Lineweaver-Burk Plot Shift: Y-intercept increases
Can be Overcome by More Substrate? No
What are the main types of irreversible enzyme inhibitors?
- Group-Specific Reagents – React with specific enzyme side chains.
- Affinity Labels – Mimic substrates, covalently bind to active site.
- Mechanism-Based Inhibitors – Enzyme activates the inhibitor, causing self-inactivation.
Why are irreversible inhibitors useful in medicine?
They permanently inactivate enzymes, useful for drugs like penicillin (targets bacterial transpeptidase).
