Block C Lecture 2 - Inhibition and Allosteric Regulation Flashcards
What are 4 reasons why enzyme inhibitors are important?
In metabolic regulation
Can be used as drugs
Can be the origin of toxicity
Useful in the study of enzyme mechanisms
(Slide 8)
What is the “lock and key” model of substrate binding?
It describes the binding pocket as a rigid “lock” which is complementary to the substrate. In this hypothesis, there are no conformational changes upon the substrate binding and the protein enzyme is viewed as a rigid substrate
(Slide 9)
What is the “induced fit” model of substrate binding?
It views the binding of the substrate as a structurally interactive process. The substrate substrate binding site is not viewed as being exactly complementary to the substrate, with the binding of the substrate slightly altering the active site structure and the active site slightly alters the substrate structure
(Slide 9)
How does a competitive inhibitor effect Km and Vmax?
At a fixed inhibitor concentration, Km will increase (affinity decreases), but the Vmax remains unchanged by the presence of the inhibitor, as if the substrate concentration is high enough the substrate will always win the competition and bind to the enzyme
(Slide 12)
How can a Lineweaver-Burke plot be used in order to determine if an enzyme is being competitively, non-competitively or irreversibly inhibited?
For competitive inhibition it can be used to see the increase in Km which occurs when an enzyme is being competitively inhibited.
For non-competitive or irreversible inhibition the plot can be used to see the decrease in Vmax when the enzyme is being non-competitively inhibited.
(Slides 14, 18 and 28)
How does non-competitive inhibition effect Km and Vmax values and why?
Km stays the same as the substrate can still bind to the active site. Vmax decreases as the inhibitor effect cannot be counteracted by increasing the concentration of the substrate.
(Slide 15)
What is uncompetitive inhibition?
When the inhibitor only binds to the enzyme-substrate complex at an allosteric site, preventing the enzyme from converting the substrate into the product
(Slide 20)
What does uncompetitive inhibition do to the Km and Vmax and why is this the case?
Km is lowered as the enzyme stabilises the ES complex, increasing the substrate’s affinity for the enzyme, resulting in more ES complexes being formed.
Vmax is lowered as the inhibitor binding prevents the conversion of the substrate into product, decreasing the enzyme’s catalytic activity
(Slide 20)
What is alkaline phosphatase?
An enzyme which catalyses the release of inorganic phosphate from phosphate esters at alkaline pH. It is found in a number of tissue, including the liver, bile ducts, intestine, bone, kidney, placenta and leukocytes
(Slide 22)
Why do uncompetitive inhibitors have little effect on the reaction rate at low substrate concentrations?
As the lower Km of the enzyme offsets the effects of the decreased Vmax
(Slide 23)
What effect does an irreversible inhibitor have on Vmax and Km and why?
Effect of irreversible inhibition is to permanently inactivate the enzyme, meaning the net effect is basically removing the enzyme from the reaction, resulting in a decrease in Vmax, but having no effect on Km
(Slide 27)
How can irreversible and non-competitive inhibition be distinguished (using a graph)?
By plotting Vmax vs [E]t
(Slide 29)
What is diisopropylphosphofluoridate?
A prototype for nerve gas which works by permanently inactivating serine proteases via forming a covalent bond with the active site serine
Note: I just thought this was interesting tbh
(Slide 30)
What is an example of when a cell doesn’t want an enzyme to be working at its maximum rate?
For metabolism (to avoid excessive build-up of intermediates)
(Slide 33)
What are 3 examples of ways which cells can control the activity of their enzymes?
Varying the amount of enzyme protein via control of gene expression
Activating or inhibiting existing enzymes by non-covalent bonding of regulator molecules
Covalently modifying existing enzymes, changing activity
(Slide 33)
Why do enzymes often exhibit a sigmoid response to allosteric effectors?
Binding of the allosteric effector to one subunit of the enzyme influences the binding of subsequent substrate molecules to other subunits (co-operativity).
Many different effectors often act on 1 key regulatory enzyme
(Slide 36)
What does an enzyme having a sigmoid response to an allosteric effector enable?
Better control over the enzyme’s response
(Slide 36)
What do positive and negative allosteric modulators do to Km and vMax?
Positive allosteric modulators decrease Km and increase Vmax whereas negative allosteric modulators do the opposite
(Slides 37 and 38)
