CHAPTER 20: Unit 7. Regulation of Enzyme Activity
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| Negative Allosteric Regulation Positive Allosteric Regulation |
| Reference: https://www.khanacademy.org/science/biology/energy-and-enzymes/enzyme-regulation/a/enzyme-regulation |
| The rates of the enzyme-catalyzed reactions are controlled by regulatory enzymes. These types of enzymes can increase the rate of reaction when it is needed, and they can decrease the rate of reaction when it is needed as well.These regulatory enzymes are:Allosteric EnzymesFeedback ControlCovalent Modifications |
| 1. Allosteric EnzymesAllosteric enzymes are enzymes that can bind with a regulator molecule that is different from the substrate. Such regulator binds at specific site in the enzyme called allosteric site which is different from the active site which bind to the substrate only.There are positive allosteric regulation and negative allosteric regulation as can be seen from the figure above.In the positive allosteric regulation, the positive regulator changes the shape of the active site in the way the active site will have identical shape as the shape of the substrate and reaction will occur.In the negative allosteric regulation, the negative regulator changes the shape of the active site in the way the active site will have different shape as the shape of the substrate and reaction will not occur. |
| 2. Feedback ControlThe end product accumulations which is negative regulator can affect the activity site of the enzyme. The end product molecules bind to the allosteric site of the enzyme at or closer to the beginning of the reaction series. Such binding changes the shape of the enzyme’s active site in the way the new coming substrate can not fit into the active site and thus the reaction stops, and the intermolecular products molecules will not be formed.When the end products accumulations are low, the end product molecule will leave the allosteric site and the active site will be reactivated and the substrate can bind to the active site and the reaction occurs. |
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| Reference: https://socratic.org/questions/what-is-feedback-inhibition |
| An example of the feedback control regulation is given in the figure below: |
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| Reference: https://slideplayer.com/slide/4638524/ |
| 3. Covalent ModificationThe activity of some enzymes will depend on the formation or the cleavage of bonds on the enzyme’s polypeptides chain. Such dependence is called covalent modification which can regulate the enzyme activity.Two examples will be given below: |
a.) Zymogens (Proenzyme) Some enzymes become active as soon as they are produced and fold into tertiary or quaternary structures.Zymogens (also called Proenzyme) are produced as inactive enzymes and stored for further usage or need. When |
| Reference: https://www.slideshare.net/MaheshMahendrakar/enzyme-regulation-zymogen |
| The zymogen (proinsulin) has two chains A and B connected via S – S bonds as shown in the figure.When the zymogen is needed to be activated, the S – S bonds between the chains A and B are broken and active sites become free and zymogen (proinsulin) is converted into insulin (active enzyme).The videos below illustrate the enzyme’s activity regulation: |