Final answer:
Amidophosphoribosyltransferase (ATase) is the only enzyme regulated in the formation of IMP from PRPP in purine nucleotide biosynthesis, and it is allosterically inhibited by IMP, AMP and GMP.
Step-by-step explanation:
The only enzyme known to be regulated in the steps of forming IMP from PRPP in purine nucleotide biosynthesis is the enzyme amidophosphoribosyltransferase (ATase). This enzyme catalyzes the committed step, which is generally irreversible, and is subject to feedback inhibition by the end products of its pathway, including IMP, AMP, and GMP. ATase is allosterically inhibited by the purine nucleotides (allosteric inhibitors), which act as sensors to signal that enough purine nucleotides have been synthesized. Furthermore, IMP acts as an allosteric activator of ATase, which increases its activity and enhances the pathway's flux when needed.