Energy balances Enzyme catalysis

Synthesis is regulated at several other places as well. After IMP is made the pathway splits to build either AMP or GMP. Enzyme XII, which catalyzes the first step from IMP to AMP, is itself slowed down by excess amounts of AMP. Similarly, the catalysis of the first step from IMP to GMP is inhibited by excess GMP. (Unlike King Midas, the enzymes can tell when they have too much of a good thing.) Finally, Enzyme XII uses GTP as an energy pellet because, if a lot of GTP is around, more A nucleotides (AMR, ADP and ATP) are needed to keep the supply in balance. The final step in the synthesis of GMP uses ATP as an energy source for similar reasons. ... 

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