Chapter 12 (Part 2) Flashcards
What are the 3 types of reversible inhibition?
Competitive, uncompetitive, and mixed, noncompetitive
How can you distinguish the 3 types of reversible inhibition (generally speaking)?
By observing the effects on enzyme kinetics of the enzyme they inhibit
What is the simplest scheme of an enzyme-mediated reaction?
E + S –> ES –> E + P
How does one run a second order enzyme reaction under pseudo first order conditions?
By making substrate concentration in excess over enzyme concentration
Under pseudo first order conditions, which value (k1, k-1, or k2) governs the rate?
K2
When substrate concentration is so high that it is saturating the enzyme, the enzyme is working at its ____ (which variable?).
Maximum velocity (Vmax)
Under pseudo first order conditions, k2 can also be referred to as ___.
Kcat
What does the term Kcat symbolize? What are its units?
The turn over number of the enzyme. Units are inverse seconds. Indicates how fast the enzyme can turn over the substrate
How does one calculate Kcat?
Kcat=Vmax/[E]total
Usually, an enzyme is working with substrate concentrations equal to or (lower, higher) than the Km
Lower
What are the units of the term Kcat/KM?
Inverse molarity times inverse seconds (M-1*s-1)
The (smaller, larger) the Kcat/Km value, the (less, more) catalytically efficient the enzyme
Larger, more OR
Smaller, less
What are 2 examples of limits on how efficient an enzyme can be?
- Kcat/Km can never be greater than the rate at which the ES complex is formed (dictated by K1 rate constant)
- Kcat/Km can never be greater than the rate of diffusion between which 2 objects will encounter each other
Aspirin is an example of an irreversible inhibitor; once the enzyme is blocked, what happens in the cell?
Th enzyme must either be degraded or replaced
What is the inhibition constant (in variables)? Define what it means.
KI. An inhibitor’s ability to bind and inhibit a reaction.
There are 2 mechanisms by which an enzyme can reversibly inhibit a reaction. What are they?
- By preventing formation of the ES complex
2. By preventing conversion of the ES complex to product
There are 2 kinds of inhibition constant: KI and KI’. What is the difference between them?
KI reflects an inhibitor that blocks formation of the ES complex.
KI’ reflects an inhibitor that prevents conversion of the ES complex to product
In competitive inhibition, the inhibitor binds only to the ____, and not the ____.
Free enzyme, ES complex
What does “competitive” mean with regard to competitive inhibition?
The substrate and inhibitor compete for the active site
In competitive inhibition, what does the inhibitor usually resemble?
The substrate
Why is Vmax unchanged in competitive inhibition?
Very high levels of substrate will eventually be able to outcompete the inhibitor
What is the meaning of the apparent Km term?
The Km measured in the presence of inhibitor
What is the alpha factor?
The factor by which substrate concentration must be increased in order to overcome the effects of the inhibitor.
Draw a sample Lineweaver-Burke plot of competitive inhibition, noting in which direction the concentration of inhibitor increases.
Vmax remains the same, slope changes. [I] increases moving from right to left.
In uncompetitive inhibition, the inhibitor binds the _____, but not the _____.
ES complex, free enzyme
(T/F) In uncompetitive inhibition, inhibition can be overcome by adding more substrate.
False. The substrate isn’t competing with the inhibitor, so addition of substrate will not overcome inhibition.
Uncompetitive inhibition usually only occurs in what kind of reactions?
Multisubstrate
(T/F) In uncompetitive inhibition, the inhibitor binds directly to the active site.
False. The inhibitor binds at some there location that causes an allosteric change that inactivates the ES complex
Why does Vmax decrease in uncompetitive inhibition?
As long as there is some inhibitor present, there will always be some inhibited ESI complex
Draw a sample Lineweaver-Burke plot displaying uncompetitive inhibition. Note in which direction the concentration of inhibitor increases.
Parallel lines, [I] increases as you move up vertically from line to line.
Because the inhibitor works on the ES complex in uncompetitive inhibition, the alpha prime factor is really modifying the apparent (substrate, enzyme) concentration
Substrate. The presence of inhibitor makes it look like less substrate is there because it is inactivating the ES complex.
Why does apparent Km decreases as Vmax decreases in uncompetitive inhibition?
Since Vmax decreases, less substrate is needed to reach 0.5Vmax (definition of Km)
In noncompetitive inhibition, what does the inhibitor bind?
Both the free enzyme and ES complex
Will addition of substrate overcome inhibition in noncompetitive inhibition?
No. Similar to uncompetitive inhibition, the inhibitor does not bind in the active site and thus does not compete with the substrate.
Describe how the Km value in noncompetitive inhibition is variable.
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
Draw a sample Lineweaver-Burke plot of noncompetitive inhibition, noting in which direction the concentration of inhibitor increases.
Lines intersect to the left of the y-axis, Vmax decreases, [I] increases from right to left
