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Allosterism – Metabolism and Regulation

by Kevin Ahern, PhD
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    00:00 Now I want to give an example of allosterism here and the example that I will give is a classic one of an enzyme called aspartate transcarbamylase or ATCase. ATCase catalyzes the reaction that you see on the screen. And while that reaction is very important to cells, we won't go through the details of that right here. Suffice it to say that this reaction is the first reaction in the pathway that leads to the production of pyrimidine nucleotides.

    00:32 Now you can see this schematically shown in this figure right here. ATCase is the first enzyme in a pathway, enzymes below there catalyze other reactions and the final product of that pathway is the pyrimidine nucleotide, CTP. You can see from CTP that there's an arrow drawn back up to ATCase. And what CTP is doing, is after it has been produced, if it gets produced in high quantities, it becomes much more likely that CTP will bind to ATCase. ATCase is an allosteric enzyme. It responds to certain molecules and is affected by them. So when CTP binds to ATCase, what does it do? It turns ATCase off. Well if you turn ATCase off, ATCase cannot make a product that's used by the next enzyme and needed by the next enzyme etc.

    01:29 So if you stop the first enzyme in the process, what you do, is you stop the entire pathway from occurring. This phenomenon that I've just described to you is something called feedback inhibition. The end product of the pathway is feeding back and telling the first enzyme “There's plenty of us, don't make anymore”. That's a really simple mechanism.

    01:51 It means that by controlling one enzyme, the cell is able to control the entire pathway.

    01:56 Now ATCase is interesting in a couple of other respects. One is that there are other molecules that if they bind to ATCase will actually activate or turn the enzyme on. One of those molecules, as you can see on the screen, is ATP. ATP, if it's present in abundance, will bind to ATCase and cause the enzyme to move into an active form. Now ATP is an interesting indicator here, because ATP of course, is something that cells use for energy, and high levels of ATP are telling the enzyme, “We've got plenty of energy to do this, go ahead and do it”. Now that's an interesting thing if we think about that because, why are cells making pyrimidine nucleotides. Well they make nucleotides for a variety of reasons, but one of them is that they're making a decision about whether or not to replicate the DNA and divide and the consideration of that requires energy and the energy here is telling the enzyme "We're ready to go."


    About the Lecture

    The lecture Allosterism – Metabolism and Regulation by Kevin Ahern, PhD is from the course Biochemistry: Basics.


    Included Quiz Questions

    1. It is a way to regulate a complete pathway
    2. It occurs when enzymes are covalently modified
    3. It requires ATP, ADP, or AMP
    4. It involves inactivation of the last enzyme of the pathway
    1. CTP
    2. ATP
    3. GTP
    4. cAMP
    5. UTP

    Author of lecture Allosterism – Metabolism and Regulation

     Kevin Ahern, PhD

    Kevin Ahern, PhD


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