TPR complete test 3 #3 C/P section

[akimhaneul]

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I don't understand why D is the answer. I thought that by inhibiting OTC, the cycle would stop working, so ornithine level would drop since it's one of the products of the cycle. Is my logic incorrect?

Thanks!

 

[wizzed101]

A circle by definition has neither start nor end.

Besides, there are many entrances into the urea cycle, just like the Krebs cycle.

 

[akimhaneul]

A circle by definition has neither start nor end.

Besides, there are many entrances into the urea cycle, just like the Krebs cycle.

Do you mind explaining more? I feel like I'm still a bit confused...


Also when you say entrances, do you mean the intermediates entering the cycle from outside?


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[wizzed101]

Yes. All of cycles in biochem are open cycles. For example, arginine is a common amino acid that is fed into the urea cycle as shown.

 

[majikarp]

According to the figure with the cycle, both Ammonia and Ornithine feed into OTC to become Citrulline. Thus, without OTC, Ammonia, Ornithine, and Carbamylphosphate should build up.

To your point about Ornithine being a part of the cycle and the cycle stopping, what you're incorrectly assuming is that OTC is necessary for Ornithine production. It is not necessary according to the figure. For example, Aspartate could come in and lead to the creation of Ornithine. The main point however, is that the Ornithine and Ammonia cannot be converted into their end product Citrulline and have no other pathway.

Although it could be said Ammonia becomes Orotic Acid? But there is no evidence that this reaction can overcome OTC deficiency in the passage so its most likely irrelevant to the question.

 

[wizzed101]

For example, Aspartate could come in and lead to the creation of Ornithine. The main point however, is that the Ornithine and Ammonia cannot be converted into their end product Citrulline and have no other pathway.

Just make sure that you are reading the figure correctly. Asp alone will certainly NOT lead to the creation of Ornithine.