Fig. 25.20 Crystal structure of sildenafil 25.69 (Fig.25.21) in PDE 5. The pyrazolopyrimidinone moiety of the inhibitor is recognized by Gln817 through two parallel hydrogen bonds and binds to the catalytic zinc ion (blue-gray) through a water molecule. It is found in the vicinity of a magnesium ion (light green), which is coordinated by five water molecules and Asp654. A bridging water molecule is shared by Mg2+ and Zn2+.