lecture 12 Flashcards
what is the threshold for catalytic perfection
10^8 s-1 m-1
define inhibitor
a compound that binds to an enzyme and reduces its activity
whats the 4 reasons inhibitors are important
natural inhibitors regulate enzymes
many drugs, poisins and toxins are enzyme inhibitors
they are used to study enzyme mechanisms
used to study metabolic pathways
what are the 2 classes of inhibitor and describe them
First is the irreversable inhibitor which binds covanlently to the enzyme at the active site. These irreversbale react with specific amino acids in the chain. These being so strong that the inhibitor is not released.
what are the subclasses of reversible inhibitor
competitive or non competitive
what are the subclasses of non competitive inhibitors
pure or mixed
describe irreversible inhibition
binds covanlently to the enzyme at the active site. These irreversbale react with specific amino acids in the chain. These being so strong that the inhibitor is not released.
describe reversible inhibitors
Then we have Reversible inhibitors which are not covalently bound to the active site. Within the reversible enzymes we have competetive and non competitive.
what is competitive inhibition
These compete directly with the substrate for the active site.
So the enzyme has two possibilites,
E+S <-> ES and E+I<-> EI. Competitive binds to the active site, competing with the substrate for the site.
discuss the kinetic changes of the competitive inhibitors
These inhibitors do not change the Vmax of the enzyme as an infinite amount of [S] can theoretically be used to eventually usurp the effects of the inhibitor. However the competitive inhibitors do mean the more [S] is needed to get to Km (half V max).
For competitive we see a change in the X-intercept on a lineweaver-burk plot, but not a change in the y-intercept.
examples of competitive inhibitors
Examples of competitive are things like transition state analogues, which emminate the transition state of the enzyme and the transition state that would be made with the substrate, this means the enzyme cannot further react.
a specific drug is anastrozole
whats non competitive inhibition
Then theres noncompetitive inhibitors. These bind to different sites on the substrate. so the enzyme can bind the substrate, inhibitor or both
whats pure non competitive inhibition
In pure non competitive inhibition the binding of the inhibitor has no effect on the binding of the substrate. This means the enzyme will bind equally well with the inhibitor, the substrate or both.
discuss the kinetic changes of pure non competitive inhibition
Vmax decreases but Km stays the same, So on a lineweaver-burk the y-intercept changes but the x-intercept does not. Also the slope of the line changes.
what does pure non competitive inhibition do to the active site
it changes the structure of the active site such that S still binds, but the transition state is no longer optimal
what is mixed non competitive inhibition
The non-competitive inhibition is not always pure, it can be mixed inhibition. This is where the inhibitor again binds away from the active site, but this time there may be a conformational change in the enzyme resulting in changes in enzyme ability to bind to the substrate.
what does mixed non competitive inhibitors do kinetically
Vmax and Km both change
whats a way to fine control enzymes
This is done through feedback and feedforward regulation, this is a strategy to avoid making unnecessary metabolic intermediates when not needed.
examples of feedforward for glycogen phosphorylase
High levels of AMP may show low levels of ATP to the Gpase enzyme. The high AMP may show we need energy. AMP has a binding site to Gpase.
FF activation can occur as AMP allosterically activates the activity of Gpase. Promoting activity.
Another FF method is cellular signalling. For example a kinase may do PTM to the Gpase. In this case the kinase add P to serine residue, which again activates Gpase activity.
This is allosteric regulation by PTM.
whats example of feedback regulation of Glycogen phosphorylase
Say we have lots of Glucose-6-P going to make ATP. This G6P can bind to Gpase to do feedback inhibition. Which then reduces Gpase activity, reducing ATP production.
Other molecules like caffine and purines are metabolism products. If theres lots of these around we may not need more ATP. This caffine say is a non competitive which can bind to a site on the enzyme and reduce Gpase activity.
whats an allosteric enzyme
allosteric enzymes are those that have mutiple subunits and show cooperativity. also those enzymes that have another ligand binding site which is not the active site
do allosteric enymes follow michaelis mentin
no
examples of allosteric enzymes
haemoglobin and glycogen phosphorylase
