PDE5 is a homodimer containing, in each monomer, a C-term catalytic domain and an N-term regulatory domain. The catalytic domain contains 2 Zn-binding sites (A and B) and an allosteric binding site for cGMP. The regulatory domain contains two allosteric binding sites called GAF a and b, domains responsible for the allosteric binding of cGMP. The occupation of the allosteric site by cGMP is necessary for the specific phosphorylation of Ser-92 by PKG.
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Taken from "Role of phosphodiesterase 5 in synaptic plasticity and memory"
Neuropsychiatric Disease and Treatment 2008;4(2):371-387.