 One is a planar ring structure of the inhibitors, which align in the active site of the enzymes and the other is a conserved glutamine residue (the "glutamine switch" mentioned below), which is essential for nucleotide recognition and selectivity.As mentioned above, PDE2 is able to hydrolyze both cAMP and cGMP, whereas some other members of the PDE family are selective for either of the two cyclic nucleotides.The variability in selectivity towards either cAMP or cGMP is thought to be determined by a so-called "glutamine switch".