Perhaps the most different among the APHs examined structurally is APH-Ia -IIIa). APH-Ia is an atypical APH which phosphorylates only one aminoglycoside, spectinomycin, that is distinct from the other aminoglycoside antibiotics. Its apo, AMP-bound and the ternary structures have been determined, making it the second structurally most studied member of the APH family. Combined, these studies reveal that although members of the APH family share low similarities in sequence and their ligand specificity varies greatly, their overall three-dimensional fold is homologous to each other and to that of ePKs. To further advance the development of APH inhibitors, we describe here the three-dimensional structure of the APH-IIIa and APH-Ia in complex with CKI-7. These inhibitor bound crystal structures of APHs represent the first structures of a eukaryotic protein kinase inhibitor complexed to enzymes that are not eukaryotic protein kinases. Comparison of the inhibitor-bound APH-IIIa and APH-Ia complexes with the nucleotide-bound APH-IIIa and APH-Ia, as well as the CKI-7-bound casein kinase 1 reveals the different inhibitor binding modes as well as topological features that could be exploited in the development of DAPT inhibitors with enhanced affinity and selectivity for APH enzymes. The hydrogen bond between the cyclic nitrogen and a main chain amide in the linker of the enzyme is also present in the CKI-7-bound CK1. The equivalent hydrogen bond observed between N1 of adenine and the tethering segment in the three enzymes is conserved in all adenine-binding to ePKs. This hydrogen bond is not unique to isoquinolinesulfonamide type inhibitors binding to the three enzymes discussed here. A majority of ePK crystal structures complexed with an ATP-competitive inhibitor form at least one hydrogen bond with residues in the linker region, mimicking the ones between N1 and/ or the exocyclic N6 of the adenine and the enzyme. The significance of the hydrogen bond interaction is corroborated by a previous observation in which naphthalene sulfonamide molecules did not display selective inhibition against ePKs until the allcarbon naphthalene ring is substituted with an isoquinoline. In contrast to APH-IIIa in which the aminoethyl tail adopts an extended conformation, this groups adopts the same conformation and is placed in the equivalent area as that in APH-Ia. The aminoethyl tail found in the CK1 structure bends back toward the sulfonyl group and forms an intramolecular interaction between the terminal nitrogen atom and the equatorial sulfonyl oxygen atom. Deviating slightly from the binding mode of CKI-7 to APH-Ia, the contact between the Nb of the aminoethyl and carbonyl of Leu88 located in the linker of the enzyme is achieved via a water molecule, compared to a direct interaction observed in APH-Ia. Hemostasis is one of the most important processes in organisms, and disorders in this system cause deaths under a variety of pathologies. The activation of blood coagulation can be caused by trauma, sepsis, inflammation, obstetric practice and in the course of XL-184 surgical operations, especially operations using extracorporal blood circulation. Hypercoagulation has also been observed during infusion therapy with large volumes of crystalloid plasma substitutes. Oral contraception and artificial vessels or cardiac valves may be sources of minor but permanent activation of coagulation, eventually exhausting the pool of coagulation inhibitors and giving rise to thrombotic events. Thrombotic pathologies are a result of an imbalance in the activity of thrombin, a key enzyme of the coagulation cascade, and its natural inhibitors.