This reaction is strongly regulated by IMP,GMP,and AMP.it has been found that PRPP aminotransferase from human placenta exists in two forms with molecular weights of 133,000 and 270,000. The enzyme activity is correlated with the smaller form. In the presence of 5'-nucleotides the small active form is converted to the large form, whereas PRPP causes the large form to shift to the active form. This can be viewed as presented on the right.