 EHNA is in close proximity to Gln859 at the active site, which could donate two hydrogen bonds to N1 and N6 of the nitrogen atoms in the adenine ring of EHNA.On the other site of the binding pocket the Asp811 could donate another hydrogen bond to N7 in the adenine ring in order to stabilize the bond inhibitor.This hypothesis is supported by the fact that the Asp811Ala mutant has decreased activity toward cAMP, whereas activity toward cGMP is unchanged.