Inhibitors & Allosteric Enzymes Flashcards
What’s the big difference between a true noncompetitive inhibitor and a mixed noncompetitive inhibitor?
True Noncompetitive Inhibitors bind equally well to the enzyme and the ES complex (They don’t affect ES Binding affinity)
Mixed Noncompetitive Inhibitors bind to both the enzyme and the ES complex but with different affinities meaning MNI’s cause the Vmax to go down but Km can go down or up
What changes would you see on a Lineweaver-Burk Plot with a true noncompetitive inhibitor vs a mixed noncompetitive inhibitor?
Because TNI’s don’t affect Km only the Vmax, only the Y-intercept will change and not the X-axis
For MNIs, they can alter both the X-intercept and the Y-intercept
What is an uncompetitive inhibitor?
uncompetitive inhibitors ONLY bind to the ES complex and prevent the product from being formed (Stuck in the ES complex form)
How do uncompetitive inhibitors affect a Lineweaver-Burk Plot?
It creates a parallel line because Vmax and Km decrease proportionally (Shifting the line-up and left)
How do uncompetitive inhibitors affect Vmax and Km?
For uncompetitive inhibitors both Vmax and Km decrease
What are Irreversible Inhibitors?
Irreversible inhibitors form covalent bonds with the enzyme, permanently inactivating it.
What is a suicide substrate?
A suicide substrate is a type of irreversible inhibitor that is chemically modified by the enzyme during the normal catalytic process.
What are the two states that Allosteric Enzymes can be found in?
T-state: Low affinity for the substrate (inactive form)
R-state: High affinity for the substrate (active form)
What do Allosteric Enzymes look like graphically?
Positive cooperativity results in a sigmoidal curve when plotting reaction velocity (V) against substrate concentration [S].
What do Allosteric Activators and Inhibitors do to the graph?
Activators shift the curve leftward, inhibitors shift the curve rightward
For ATCase who are the Allosteric Effectors?
ATP stabilizes the R (relaxed) state (Decreases Km)
CTP stabilizes the T (tense) state (Increases Km)
What is the structure of ATCase?
ATCase consists of 2 catalytic trimers and 3 regulatory dimers.
Why is CTP not considered a “competitive” inhibitor?
Because competitive inhibitors always bind to the catalytic site rather than the allosteric site
What’s the difference between a K system and a V system?
In a K system, the effector primarily alters the Km
In a V system, the effector primarily alters the Vmax
What is the big difference between heterotrophic and homotrophic effectors?
Homotrophic effects occur when the substrate itself acts as an effector.
Heterotrophic effects involve effectors that are not the substrate.
What are examples of heterotrophic and homotrophic effectors when it comes to ATCase?
The binding of the substrate aspartate to ATCase exhibits homotropic cooperativity (One aspartate molecule binding causes others to bind to)
ATP is a positive heterotropic effector
CTP is a negative heterotropic effector
When there is little substrate what form can enzymes be found in?
The T (Tense) form (High Km)
What is the L ratio? How does it relate to sigmoidicity?
The equilibrium constant between (Tensed state/ Relaxed state) = L. A higher L means more sigmoidicity
What is the C ratio?
This is the ratio of the dissociation constants for the substrate in the R and T forms. (KR/KT) = C. Higher c means less sigmoidicity
What effect do activators and inhibitors have on the R and T states of these enzymes?
Binding of activator shifts in favor of R (less sigmoid)
Binding of inhibitor shifts in favor of T (more sigmoid)
What are the Key Differences Between the Concerted and Sequential Models?
In the Concerted Model:
All subunits of an enzyme are either in the T state or R state
Cooperative binding arises because the binding of the substrate stabilizes the R form.
In the Sequential Model:
Substrate binding induces a change in one subunit from the T to the R form.
This conformational change is THEN transmitted to adjacent subunits, increasing their affinity for the substrate.
What is negative cooperativity?
Negative cooperativity occurs when the binding of the first ligand (substrate) decreases the affinity of the enzyme for subsequent ligands.
