Adenosine and guanosine monophosphate synthesis

[Zahra B]

I noticed in my textbook that the synthesis of AMP uses aspartate as a nitrogen source and syntheis of GMP uses glutamine. Why doesn't the synthesis of AMP require asparagine instead of aspartate, since asparagine has a nitrogen group in its side chain? I thought that the reactive part of an AA was the side chain so it doesn't make sense to me.

[BabcockHall]

Remember that cells must balance the biosynthesis of AMP and GMP.  Does that help?

[Zahra B]

No, not really. 

[BabcockHall]

Apologies, I misread your question, and I was pointing the way toward answering a different question.  Aspartate is also used as a donor in the biosynthetic pathway that produces inosine monophosphate (check the two steps in the synthesis of AICAR) and in the urea cycle.  I disagree that a side chain is necessarily more reactive and can think of some examples to illustrate this.  However,  I am not sure what makes one nitrogen donor used in one reaction and a different nitrogen donor used in another.

Edited November 16, 2020 by BabcockHall

[Zahra B]

Thank you for answering. I always thought that the side chains were primarily responsible  for reactions, but since that is not the case, it makes more sense.

[BabcockHall]

The donation of the nitrogen & hydrogen atoms and the concomitant production of fumarate takes place in two steps overall.  One way to think about the second step is that it is simply an elimination reaction.  A proton is lost from what was the beta-carbon of aspartate, and -NH₃(+) is the leaving group, which departs from the alpha-carbon.