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Biochemistry 303: Competitive Inhibition


What is meant by competitive inhibition?
The inhibitor binds in the active site of the enzyme, thus competing with the substrate for the hotspot. Enzymes have varying affinities for inhibitors, and of course the most effective inhibitor would be more attractive to the enzyme than the regular substrate.

What effect does a competitive inhibitor have on a Michaelis Menten plot?
Moves line to right, slows down rxn, can lower Vmax. Curve stays hyperbolic.

What effect does a competitive inhibitor have on a Lineweaver-Burke plot?
Makes the line steeper, moves Km closer to 1/[S] axis

Does it affect Km or Vmax?
Km changes. It becomes apparent Km, modified by alpha = 1 + [I]/Ki
Vm stays the same. The line in the Michaelis Menten curve gradually approaches same Vmax when [S] approaches infinity.
"can affect enzyme's Km, Kcat, or both"

pages 214-217 in text: Competitive Inhibition:
-is the most common form of reversible enzyme inhibition
-resembles substrate in size & chem properties
-lacks exact electronic structure that allows it to react
-"The presence of a reversible inhibitor does not affect the enzyme's Kcat, so as [S] approaches infinity, Vo approaches Vmax".
-substrate analogs are good CI's
- the best inhibitors are transition state analogs
-to catalyze a rxn, an enzyme must bind to and stabilize the rxns TS -- this somehow supports the statement that TS analogs are best inhibitors

WHY DO SOME TS ANALOGS ACT AS COMPETITIVE ENZYME INHIBITORS?
-well, why?
-TS analogs make good drugs
-triose phosphate isomerase
-HIV treatments designed by considering how TS analogs inhibit viral enzymes

How does a transition state analog affect a Lineweaver-Burke plot? Changes Km by a factor of alpha but doesn't change Vmax. Line is steeper. This is competitive inhibition, still.

Kcat - not affected by reversible inhibitor

Ki = inhibition constant = [E][I]/[EI] = dissociation constant for the enzyme-inhibitor complex
-lower Ki means tighter EI binding
-useful for assessing inhibitory power of different substances
-used to measure drug potency
-drugs work by positive feedback

Product inhibition = when the product P of a reaction occupies the enzyme's active site, and prevents binding of more substrate.

suicide inhibitor = binds to the enzyme and won't let go

reversible inhibition = noncovalent bonding of inhibitor to enzyme
irreversible = bonds covalently and changes enzyme

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