Argininosuccinate Synthetase

by Kevin Ahern, PhD

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    The next step in the reaction involves the enzyme argininosuccinate synthetase. And in this reaction, you remember there's a two-step process. In the first step of the process, ATP cleaves off a pyrophosphate. We see that happening in that very first step in the top. We see a little molecule in blue, PPi. That's pyrophosphate. That is released from the ATP. That allows the remaining part of the ATP in the form of AMP to combine with the citrulline to make the L-citrulline adenylate complex. In the second part of the process, aspartate displaces the AMP and that’s where we see the AMP leading also shown in blue to create the L-argininosuccinate. The chemical mechanism of the reaction is shown below. Now this two-step process, as I said, involves AMP attachment. And we see the AMP attachment going here, followed by displacement of the AMP by the aspartate. And the displacement is shown here. The product to the reaction argininosuccinate is the substrate for the next step of the reaction. This reaction is the rate limiting step of the cycle. Meaning that it is the slowest and it's the step that other reactions await for it to be completed. The gene expression of the enzyme is reduced by arginine. Now arginine will see seized further ahead in the pathway. And as arginine accumulates, there's no reason to continue making this enzyme. So this regulation by arginine, of this enzyme that feeds and makes arginine is important. On the other hand, the expression of the enzyme is increased by citrulline. And citrulline is further behind that in the cycle and it's literally pushing the thing forward. So this kind of regulation ensures that the cell has the proper amount of the argininosuccinate synthetase enzyme. Defects in this enzyme lead to citrullinemia...

    About the Lecture

    The lecture Argininosuccinate Synthetase by Kevin Ahern, PhD is from the course Amino Acid Metabolism.

    Included Quiz Questions

    1. It is the rate-limiting step of the cycle.
    2. It uses a mechanism with a covalent bond to ADP.
    3. It requires glutamic acid.
    4. All of the answers are true.
    5. None of the answers are true.
    1. All of the answers are true.
    2. None of the answers are true.
    3. It releases fumarate.
    4. It when deficient it leads to ammonia excess.
    5. It produces arginine.
    1. It competes with nitric oxide synthase for arginine.
    2. It is the most commonly deficient enzyme of the cycle.
    3. It produces citrulline and urea from arginine.
    4. All of the answers are true.
    5. None of the answers are true.

    Author of lecture Argininosuccinate Synthetase

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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