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.

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

No, not really. 

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, 20205 yr by BabcockHall

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.

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.