Inhibitors & Allosteric Enzymes

Question 1

What’s the big difference between a true noncompetitive inhibitor and a mixed noncompetitive inhibitor?

Answer 1

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

Question 2

What changes would you see on a Lineweaver-Burk Plot with a true noncompetitive inhibitor vs a mixed noncompetitive inhibitor?

Answer 2

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

Question 3

What is an uncompetitive inhibitor?

Answer 3

uncompetitive inhibitors ONLY bind to the ES complex and prevent the product from being formed (Stuck in the ES complex form)

Question 4

How do uncompetitive inhibitors affect a Lineweaver-Burk Plot?

Answer 4

It creates a parallel line because Vmax and Km decrease proportionally (Shifting the line-up and left)

Question 5

How do uncompetitive inhibitors affect Vmax and Km?

Answer 5

For uncompetitive inhibitors both Vmax and Km decrease

Question 6

What are Irreversible Inhibitors?

Answer 6

Irreversible inhibitors form covalent bonds with the enzyme, permanently inactivating it.

Question 7

What is a suicide substrate?

Answer 7

A suicide substrate is a type of irreversible inhibitor that is chemically modified by the enzyme during the normal catalytic process.

Question 8

What are the two states that Allosteric Enzymes can be found in?

Answer 8

T-state: Low affinity for the substrate (inactive form)
R-state: High affinity for the substrate (active form)

Question 9

What do Allosteric Enzymes look like graphically?

Answer 9

Positive cooperativity results in a sigmoidal curve when plotting reaction velocity (V) against substrate concentration [S].

Question 10

What do Allosteric Activators and Inhibitors do to the graph?

Answer 10

Activators shift the curve leftward, inhibitors shift the curve rightward

Question 11

For ATCase who are the Allosteric Effectors?

Answer 11

ATP stabilizes the R (relaxed) state (Decreases Km)
CTP stabilizes the T (tense) state (Increases Km)

Question 12

What is the structure of ATCase?

Answer 12

ATCase consists of 2 catalytic trimers and 3 regulatory dimers.

Question 13

Why is CTP not considered a “competitive” inhibitor?

Answer 13

Because competitive inhibitors always bind to the catalytic site rather than the allosteric site

Question 14

What’s the difference between a K system and a V system?

Answer 14

In a K system, the effector primarily alters the Km
In a V system, the effector primarily alters the Vmax

Question 15

What is the big difference between heterotrophic and homotrophic effectors?

Answer 15

Homotrophic effects occur when the substrate itself acts as an effector.
Heterotrophic effects involve effectors that are not the substrate.

Question 16

What are examples of heterotrophic and homotrophic effectors when it comes to ATCase?

Answer 16

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

Question 17

When there is little substrate what form can enzymes be found in?

Answer 17

The T (Tense) form (High Km)

Question 18

What is the L ratio? How does it relate to sigmoidicity?

Answer 18

The equilibrium constant between (Tensed state/ Relaxed state) = L. A higher L means more sigmoidicity

Question 19

What is the C ratio?

Answer 19

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

Question 20

What effect do activators and inhibitors have on the R and T states of these enzymes?

Answer 20

Binding of activator shifts in favor of R (less sigmoid)
Binding of inhibitor shifts in favor of T (more sigmoid)

Question 21

What are the Key Differences Between the Concerted and Sequential Models?

Answer 21

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.

Question 22

What is negative cooperativity?

Answer 22

Negative cooperativity occurs when the binding of the first ligand (substrate) decreases the affinity of the enzyme for subsequent ligands.