 In PDEs, which can hydrolyze both cAMP and cGMP, this glutamine is able to rotate freely.In PDEs that are selective for either cAMP or cGMP, this glutamine is constrained by neighboring residues to a position favoring selectivity for either cyclic nucleotide.When cGMP binds to the allosteric GAF-B domain of the PDE, it causes conformational change in the protein structure leading to higher enzyme activity.