lecture 12

Question 1

what is the threshold for catalytic perfection

Answer 1

10^8 s-1 m-1

Question 2

define inhibitor

Answer 2

a compound that binds to an enzyme and reduces its activity

Question 3

whats the 4 reasons inhibitors are important

Answer 3

natural inhibitors regulate enzymes
many drugs, poisins and toxins are enzyme inhibitors
they are used to study enzyme mechanisms
used to study metabolic pathways

Question 4

what are the 2 classes of inhibitor and describe them

Answer 4

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.

Question 5

what are the subclasses of reversible inhibitor

Answer 5

competitive or non competitive

Question 6

what are the subclasses of non competitive inhibitors

Answer 6

pure or mixed

Question 7

describe irreversible inhibition

Answer 7

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.

Question 8

describe reversible inhibitors

Answer 8

Then we have Reversible inhibitors which are not covalently bound to the active site. Within the reversible enzymes we have competetive and non competitive.

Question 9

what is competitive inhibition

Answer 9

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.

Question 10

discuss the kinetic changes of the competitive inhibitors

Answer 10

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.

Question 11

examples of competitive inhibitors

Answer 11

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

Question 12

whats non competitive inhibition

Answer 12

Then theres noncompetitive inhibitors. These bind to different sites on the substrate. so the enzyme can bind the substrate, inhibitor or both

Question 13

whats pure non competitive inhibition

Answer 13

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.

Question 14

discuss the kinetic changes of pure non competitive inhibition

Answer 14

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.

Question 15

what does pure non competitive inhibition do to the active site

Answer 15

it changes the structure of the active site such that S still binds, but the transition state is no longer optimal

Question 16

what is mixed non competitive inhibition

Answer 16

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.

Question 17

what does mixed non competitive inhibitors do kinetically

Answer 17

Vmax and Km both change

Question 18

whats a way to fine control enzymes

Answer 18

This is done through feedback and feedforward regulation, this is a strategy to avoid making unnecessary metabolic intermediates when not needed.

Question 19

examples of feedforward for glycogen phosphorylase

Answer 19

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.

Question 20

whats example of feedback regulation of Glycogen phosphorylase

Answer 20

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.

Question 21

whats an allosteric enzyme

Answer 21

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

Question 22

do allosteric enymes follow michaelis mentin

Answer 22

no

Question 23

examples of allosteric enzymes

Answer 23

haemoglobin and glycogen phosphorylase