FIII: Lecture Slides 58 - 68 Flashcards
What are inactivators?
Act on an enzyme irreversibly
What does inactivation result from?
Inactivation results from a covalent chemical reaction between the inactivator and the enzyme
Why is inactivation irreversible?
Due to the reaction destroying catalytic activity and the enzyme being used up.
What is the simple stoichiometric equation between the inactivator and the enzyme?
2 umol inactivator and 3 umol enzyme = 1 umol of active enzyme left
What are inactivators considered to be?
Extremely toxic (Example: nerve gases have the ability to inactivate acteylcholinesterase which then interferes with nerve impulses)
What are inhibitors?
Bind to enzymes irreversibly
What can an inhibitor do?
Decrease enzyme activity without destroying the catalytic function of enzyme molecule
What happens when the inhibitor concentration is removed?
The enzyme activity is restored
What type of reaction is inhabitation?
Binding equilibrium, as it can be reversed.
Competitive Inhibition
Where the inhibitor binds to the active site on the enzyme, where the substrate would bind (competing with substrate for active site)
Non-competitive Inhibition
Inhibition affects the catalytic rate, the inhibitor binds at an allosteric site separate from the active substrate binding site
What are inhibitors main job?
To regulate enzyme activity in a cell.
What is an example of two drugs that are inhibitors?
Aspirin and Advil:
Both drugs inhibit the cyclooxyrgenase enzyme that make prostaglandins: affect inflammatory response.
When can competitive inhibitors bind?
When the enzyme E is unoccupied (not with a substrate)
What is the inhibitor binding constant?
Ki
What does Ki equal?
[E][I]/[EI]
What is the reaction of an enzyme plus an inhibitor?
E + I —-> (rev) EI
Represented by Ki
What does the formation of the EI complex mean?
The formation means that there is less enzyme available to bind substrate
What can high [S] do?
Outcompete the competitive inhibitor
Why might the substrate and inhibitor have resemblance?
Because they share the same binding site, they may resemble each other in terms of chemical structure.
What happens when you graph enzyme behaviour in presence of competitive inhibitor vs. substrate when you keep [I] constant
The Km of the inhibitor is increased.
What is the equation for Km of Inhibitor?
Km’ = Km(1 + [I]/Ki)
What is Ki?
A characteristic constant for each inhibitor
What is the inhibition factor?
1 + [I]/Ki
What is Ki in terms of Km for competitive inhibition?
The concentration of inhibitor [I] that causes Km to double.
How can competitive inhibition be seen with a Lineweaver-Burk Plot?
- There is no effect on Vmax, so all graphs have same Y-intercept (y-int = 1/Vmax)
- Km’ increases as [I] increases so your x-intercepts get smaller (x-int = -1/Km)
- Slope increases as [I] increases: LB slope = Km’/Vmax
Where can non-competitive inhibitors arise?
Non-competitive inhibition can bind to both Enzyme or Enzyme-Substrate Complex
When a non-competitive inhibitor binds, what two complexes can they form?
They can form an EIS and an EI complex?
What does the formation of EI and EIS mean?
Less ES to undergo catalysis but substrate can still bind to EI without yielding product.
What might the bound inhibitor do?
Disorganize the catalytic component of the enzyme
When might you get mixed inhibition?
If EI and EIS have a different Ki
What happens in a Michael Menten plot of Non-Competitive Inhibition?
- Vmax decreases as [I] increases: V’max = Vmax/1 + [I]/Ki
- Km is unchanged
- If [I] is set equal to Ki, inhibition factor = 2
What is Ki for non-competitive inhibition?
Ki is the concentration of inhibitor [I] that causes Vmax to halve
Non-competitive Inhibition on a Lineweaver Plot:
- No effect on Km so all graphs have same X-inter: -1/km
- Vmax decreases as [I] increases so y -intercept: 1/Vmax
- Slope increases as [I] increases; LB slope = Km/V’max
For mixed inhibition, lines meet above the X-Axis.
