The formation of 5-phosphoribosylamine from glutamine and PRPP catalysed by PRPP amino transferase is the regulation point for purine synthesis. The enzyme is an allosteric enzyme, so it can be converted from IMP, GMP and AMP in high concentration binds the enzyme to exerts inhibition while PRPP is in large amount binds to the enzyme which causes activation.  So IMP, GMP and AMP are inhibitors while PRPP is an activator. Between the formation of 5-phosphoribosyl, aminoimidazole and IMP, there is no known regulation step.