The first and committed step in purine biosynthesis is catalyzed by glutamine PRPP amidotransterase. The reaction proceeds with inversion of configuration at C1, yielding the characteristic B configuration that occurs in nucleotides (Figure 14-8). The amidotransferase reaction occurs with the loss of a high-energy bond and is unidirectional. The liberation of pyrophosphate followed by its hydrolysis provides additional energy for phosphoribosylamine synthesis.