[opel65]

Hi chemical forums,

This might seem to be a trivial question but I need to name amino acids used in the urea cycle.

The problem is I don't know what is considered as "used".
What I mean is that N-acetylglutamate is an essential activator of carbamoyl phosphate synthetase-1; this amino acid is not present on the Urea cycle summarization drawing in my textbook which is why I'm confused. The other amino acids like ornithine arginine, aspartate and citrulline are clearly a part of the cycle.

Please help, thanks

[Babcock_Hall]

We need to understand the question better.  Ornithine and arginine are used in one sense of the term, but they are not used up; in a cyclic system such as this one, Orn and Arg are catalysts (the same is true of the intermediates in the TCA cycle).  On the other hand if "used" means supplying the nitrogen and being consumed, then which amino acids do you think fall into this category?

[opel65]

L-Aspartate is a supplier of nitrogen in the citrulline-->argininosuccinate reaction.
The other nitrogen is supploed by free ammonia.
You have to agree with me though, that the answer to this question is hard to define!
Would you consider N-acetylglutamate a part of the cycle? I't an activator ofr carbamoyl phosphate synthetase-1.
I mean, it's something like acetyl CoA in gluconeogenesis which is an essential activator of pyrutave carboxylase.

I still have no idea if I should include it as an amino acid USED in the urea cycle.
It is rather INVOLVED, wouldn't your agree?

[Babcock_Hall]

I would not include N-acetylglutamate (it is part of the cycle in one sense) because it is an activator, not a substrate.  Also, glutamate is the source of the ammonia that is later activated and brought into the urea cycle; therefore, I would include this amino acid as well. 

[opel65]

You mean the conversion of fumarate to malate. Malate to OAA, and OAA included in transamination of glutamate which yields aspartate, an intermediate in the cycle? I can't see any other glutamate there.

[Babcock_Hall]

Glutamate dehydrogenase produces ammonia that is then acted upon by cabamoyl phosphate synthase I.

[aHerraez]

When a question is ambiguous, you make explicit your interpretation(s) in your answer.
Like "if we take USE as such, the answer is this, if we understand USE in this other way, then the answer is that"