Chapter 12 (Part 2)

Question 1

What are the 3 types of reversible inhibition?

Answer 1

Competitive, uncompetitive, and mixed, noncompetitive

Question 2

How can you distinguish the 3 types of reversible inhibition (generally speaking)?

Answer 2

By observing the effects on enzyme kinetics of the enzyme they inhibit

Question 3

What is the simplest scheme of an enzyme-mediated reaction?

Answer 3

E + S –> ES –> E + P

Question 4

How does one run a second order enzyme reaction under pseudo first order conditions?

Answer 4

By making substrate concentration in excess over enzyme concentration

Question 5

Under pseudo first order conditions, which value (k1, k-1, or k2) governs the rate?

Answer 5

K2

Question 6

When substrate concentration is so high that it is saturating the enzyme, the enzyme is working at its ____ (which variable?).

Answer 6

Maximum velocity (Vmax)

Question 7

Under pseudo first order conditions, k2 can also be referred to as ___.

Answer 7

Kcat

Question 8

What does the term Kcat symbolize? What are its units?

Answer 8

The turn over number of the enzyme. Units are inverse seconds. Indicates how fast the enzyme can turn over the substrate

Question 9

How does one calculate Kcat?

Answer 9

Kcat=Vmax/[E]total

Question 10

Usually, an enzyme is working with substrate concentrations equal to or (lower, higher) than the Km

Answer 10

Lower

Question 11

What are the units of the term Kcat/KM?

Answer 11

Inverse molarity times inverse seconds (M-1*s-1)

Question 12

The (smaller, larger) the Kcat/Km value, the (less, more) catalytically efficient the enzyme

Answer 12

Larger, more OR

Smaller, less

Question 13

What are 2 examples of limits on how efficient an enzyme can be?

Answer 13

1. Kcat/Km can never be greater than the rate at which the ES complex is formed (dictated by K1 rate constant)
2. Kcat/Km can never be greater than the rate of diffusion between which 2 objects will encounter each other

Question 14

Aspirin is an example of an irreversible inhibitor; once the enzyme is blocked, what happens in the cell?

Answer 14

Th enzyme must either be degraded or replaced

Question 15

What is the inhibition constant (in variables)? Define what it means.

Answer 15

KI. An inhibitor’s ability to bind and inhibit a reaction.

Question 16

There are 2 mechanisms by which an enzyme can reversibly inhibit a reaction. What are they?

Answer 16

1. By preventing formation of the ES complex

2. By preventing conversion of the ES complex to product

Question 17

There are 2 kinds of inhibition constant: KI and KI’. What is the difference between them?

Answer 17

KI reflects an inhibitor that blocks formation of the ES complex.
KI’ reflects an inhibitor that prevents conversion of the ES complex to product

Question 18

In competitive inhibition, the inhibitor binds only to the ____, and not the ____.

Answer 18

Free enzyme, ES complex

Question 19

What does “competitive” mean with regard to competitive inhibition?

Answer 19

The substrate and inhibitor compete for the active site

Question 20

In competitive inhibition, what does the inhibitor usually resemble?

Answer 20

The substrate

Question 21

Why is Vmax unchanged in competitive inhibition?

Answer 21

Very high levels of substrate will eventually be able to outcompete the inhibitor

Question 22

What is the meaning of the apparent Km term?

Answer 22

The Km measured in the presence of inhibitor

Question 23

What is the alpha factor?

Answer 23

The factor by which substrate concentration must be increased in order to overcome the effects of the inhibitor.

Question 24

Draw a sample Lineweaver-Burke plot of competitive inhibition, noting in which direction the concentration of inhibitor increases.

Answer 24

Vmax remains the same, slope changes. [I] increases moving from right to left.

Question 25

In uncompetitive inhibition, the inhibitor binds the _____, but not the _____.

Answer 25

ES complex, free enzyme

Question 26

(T/F) In uncompetitive inhibition, inhibition can be overcome by adding more substrate.

Answer 26

False. The substrate isn’t competing with the inhibitor, so addition of substrate will not overcome inhibition.

Question 27

Uncompetitive inhibition usually only occurs in what kind of reactions?

Answer 27

Multisubstrate

Question 28

(T/F) In uncompetitive inhibition, the inhibitor binds directly to the active site.

Answer 28

False. The inhibitor binds at some there location that causes an allosteric change that inactivates the ES complex

Question 29

Why does Vmax decrease in uncompetitive inhibition?

Answer 29

As long as there is some inhibitor present, there will always be some inhibited ESI complex

Question 30

Draw a sample Lineweaver-Burke plot displaying uncompetitive inhibition. Note in which direction the concentration of inhibitor increases.

Answer 30

Parallel lines, [I] increases as you move up vertically from line to line.

Question 31

Because the inhibitor works on the ES complex in uncompetitive inhibition, the alpha prime factor is really modifying the apparent (substrate, enzyme) concentration

Answer 31

Substrate. The presence of inhibitor makes it look like less substrate is there because it is inactivating the ES complex.

Question 32

Why does apparent Km decreases as Vmax decreases in uncompetitive inhibition?

Answer 32

Since Vmax decreases, less substrate is needed to reach 0.5Vmax (definition of Km)

Question 33

In noncompetitive inhibition, what does the inhibitor bind?

Answer 33

Both the free enzyme and ES complex

Question 34

Will addition of substrate overcome inhibition in noncompetitive inhibition?

Answer 34

No. Similar to uncompetitive inhibition, the inhibitor does not bind in the active site and thus does not compete with the substrate.

Question 35

Describe how the Km value in noncompetitive inhibition is variable.

Answer 35

If KI>KI’, then the inhibitor binds the free enzyme better, so alpha>alpha prime, and Km increases

If KI’>KI, then the inhibitor binds the ES complex better, so alpha prime>alpha, and Km decreases

If KI is almost equal to KI’, then Km will not change

Question 36

Draw a sample Lineweaver-Burke plot of noncompetitive inhibition, noting in which direction the concentration of inhibitor increases.

Answer 36

Lines intersect to the left of the y-axis, Vmax decreases, [I] increases from right to left