Calcineurin, a Ca²⁺/calmodulin-dependent protein phosphatase, is the common target for two immunophilin–immunosuppressant complexes, cyclophilin A–cyclosporin A (CyPA-CsA) and FKBP–FK506. How the two structurally distinct immunophilin–drug complexes bind the same target has remained unknown. We report the crystal structure of calcineurin (CN) in complex with CyPA-CsA at 2.8-Å resolution. The CyPA-CsA complex binds to a composite surface formed by the catalytic and regulatory subunits of CN, where the complex of FK506 and its binding protein FKBP also binds. While the majority of the CN residues involved in the binding are common for both immunophilin-immunosuppressant complexes, a significant number of the residues are distinct. Unlike FKBP-FK506, CyPA-CsA interacts with Arg-122 at the active site of CN, implying direct involvement of CyPA-CsA in the regulation of CN catalysis. The simultaneous interaction of CyPA with both the composite surface and the active site of CN suggests that the composite surface may serve as a substrate recognition site responsible for the narrow substrate specificity of CN. The comparison of CyPA-CsA-CN with FKBP-FK506-CN significantly contributes to understanding the molecular basis of regulation of CN activity by the immunophilin–immunosuppressant.

Crystal structures of seven phosphodiesterase families (PDE1-5, 7, 9) show a conserved core catalytic domain that contains about 300 amino acids and fourteen α-helices. The catalytic domains of the PDE families 1-4, 7, and 9 have a uniform conformation. However, the H-loop at the active site of PDE5 shows four different conformations upon binding of inhibitors, probably implying a special mechanism for recognition of substrates and inhibitors by PDE5. The active site of all PDE families contains two divalent metal ions: zinc and probably magnesium. The PDE4-AMP and PDE5-GMP structures reveal the conserved interactions of the phosphate groups of the products AMP and GMP, and thus suggest a universal mechanism of nucleophilic attack for all PDE families. The substrate specificity has not been well understood. This review will comment on the early proposal, “glutamine switch”, on basis of the recent biochemical and structural information. The PDE-inhibitor structures have identified a common subpocket for non-selective binding of all inhibitors and potential elements for recognition of familyselective inhibitors. The kinetic analysis on the mutations of PDE7 to PDE4 suggests that the multiple elements must work together to define inhibitor selectivity.

PDE4 (phosphodiesterase-4)-selective inhibitors have attracted much attention as potential therapeutics for the treatment of both depression and major inflammatory diseases, but their practical application has been compromised by side effects. A possible cause for the side effects is that current PDE4-selective inhibitors similarly inhibit isoforms from all four PDE4 subfamilies. The development of PDE4 subfamily-selective inhibitors has been hampered by a lack of structural information. In the present study, we rectify this by providing the crystal structures of the catalytic domains of PDE4A, PDE4B and PDE4D in complex with the PDE4 inhibitor NVP {4-[8-(3-nitrophenyl)-[1,7]naphthyridin-6-yl]benzoic acid} as well as the unliganded PDE4C structure. NVP binds in the same conformation to the deep cAMP substrate pocket and interacts with the same residues in each instance. However, detailed structural comparison reveals significant conformational differences. Although the active sites of PDE4B and PDE4D are mostly comparable, PDE4A shows significant displacements of the residues next to the invariant glutamine residue that is critical for substrate and inhibitor binding. PDE4C appears to be more distal from other PDE4 subfamilies, with certain key residues being disordered. Our analyses provide the first structural basis for the development of PDE4 subfamily-selective inhibitors.

Phosphodiesterases (PDEs) hydrolyze the second messengers cAMP and cGMP. It remains unknown how individual PDE families selectively recognize cAMP and cGMP. This work reports structural studies on substrate specificity. The crystal structures of the catalytic domains of the D674A and D564N mutants of PDE10A2 in complex with cAMP and cGMP reveal that two substrates bind to the active site with the samesynconfiguration but different orientations and interactions. The products AMP and GMP bind PDE10A2 with theanticonfiguration and interact with both divalent metals, in contrast to no direct contact of the substrates. The structures suggest that thesynconfigurations of cAMP and cGMP are the genuine substrates for PDE10 and the specificity is achieved through the different interactions and conformations of the substrates. The PDE10A2 structures also show that the conformation of the invariant glutamine is locked by two hydrogen bonds and is unlikely to switch for substrate recognition. Sequence alignment shows a potential pocket, in which variation of amino acids across PDE families defines the size and shape of the pocket and thus determines the substrate specificity.

Summary: Human leishmaniasis is a major public health problem in many countries, but chemotherapy is in an unsatisfactory state. Leishmania major phosphodiesterases (LmjPDEs) have been shown to play important roles in cell proliferation and apoptosis of the parasite. Thus LmjPDE inhibitors may potentially represent a novel class of drugs for the treatment of leishmaniasis. Reported here are the kinetic characterization of the LmjPDEB1 catalytic domain and its crystal structure as a complex with 3‐isobutyl‐1‐methylxanthine (IBMX) at 1.55 Å resolution. The structure of LmjPDEB1 is similar to that of human PDEs. IBMX stacks against the conserved phenylalanine and forms a hydrogen bond with the invariant glutamine, in a pattern common to most inhibitors bound to human PDEs. However, an extensive structural comparison reveals subtle, but significant differences between the active sites of LmjPDEB1 and human PDEs. In addition, a pocket next to the inhibitor binding site is found to be unique to LmjPDEB1. This pocket is isolated by two gating residues in human PDE families, but constitutes a natural expansion of the inhibitor binding pocket in LmjPDEB1. The structure particularity might be useful for the development of parasite‐selective inhibitors for the treatment of leishmaniasis.

Cyclic nucleotide phosphodiesterases (PDEs) are enzymes controlling cellular concentrations of the second messengers cAMP and cGMP. The crystal structure of the catalytic domain of PDE9A2, a member of a PDE family specifically hydrolyzing cGMP, has been determined at 2.23-Å resolution. The PDE9A2 catalytic domain closely resembles the cAMP-specific PDE4D2 but is significantly different from the cGMP-specific PDE5A1, implying that each individual PDE family has its own characteristic substrate recognition mechanism. The different conformations of the H and M loops between PDE9A2 and PDE5A1 imply their less critical roles in nucleotide recognition. The nonselective inhibitor 3-isobutyl-1-methylxanthine (IBMX) binds to a similar subpocket in the active sites of PDE4, PDE5, and PDE9 and has a common pattern of the binding. However, significantly different orientations and interactions of IBMXs are observed among the three PDE families and also between two monomers of the PDE9A2 dimer. The kinetic properties of the PDE9A2 catalytic domain similar to those of full-length PDE9A imply that the N-terminal regulatory domain does not significantly alter the catalytic activity and the IBMX inhibition.