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      Crystal Structure of Human Soluble Adenylate Cyclase Reveals a Distinct, Highly Flexible Allosteric Bicarbonate Binding Pocket

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          Soluble adenylate cyclases catalyse the synthesis of the second messenger cAMP through the cyclisation of ATP and are the only known enzymes to be directly activated by bicarbonate. Here, we report the first crystal structure of the human enzyme that reveals a pseudosymmetrical arrangement of two catalytic domains to produce a single competent active site and a novel discrete bicarbonate binding pocket. Crystal structures of the apo protein, the protein in complex with α,β-methylene adenosine 5′-triphosphate (AMPCPP) and calcium, with the allosteric activator bicarbonate, and also with a number of inhibitors identified using fragment screening, all show a flexible active site that undergoes significant conformational changes on binding of ligands. The resulting nanomolar-potent inhibitors that were developed bind at both the substrate binding pocket and the allosteric site, and can be used as chemical probes to further elucidate the function of this protein.

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          Most cited references 33

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          Soluble adenylyl cyclase as an evolutionarily conserved bicarbonate sensor.

          Spermatozoa undergo a poorly understood activation process induced by bicarbonate and mediated by cyclic adenosine 3',5'-monophosphate (cAMP). It has been assumed that bicarbonate mediates its effects through changes in intracellular pH or membrane potential; however, we demonstrate here that bicarbonate directly stimulates mammalian soluble adenylyl cyclase (sAC) activity in vivo and in vitro in a pH-independent manner. sAC is most similar to adenylyl cyclases from cyanobacteria, and bicarbonate regulation of cyclase activity is conserved in these early forms of life. sAC is also expressed in other bicarbonate-responsive tissues, which suggests that bicarbonate regulation of cAMP signaling plays a fundamental role in many biological systems.
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            Cytosolic adenylyl cyclase defines a unique signaling molecule in mammals.

            Mammals have nine differentially regulated isoforms of G protein-responsive transmembrane-spanning adenylyl cyclases. We now describe the existence of a distinct class of mammalian adenylyl cyclase that is soluble and insensitive to G protein or Forskolin regulation. Northern analysis indicates the gene encoding soluble adenylyl cyclase (sAC) is preferentially expressed in testis. As purified from rat testis cytosol, the active form of sAC appears to be a fragment derived from the full-length protein, suggesting a proteolytic mechanism for sAC activation. The two presumptive catalytic domains of sAC are closely related to cyanobacterial adenylyl cyclases, providing an evolutionary link between bacterial and mammalian signaling molecules.
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              Mice deficient for soluble adenylyl cyclase are infertile because of a severe sperm-motility defect.

              To acquire the ability to fertilize, spermatozoa undergo complex, but at present poorly understood, activation processes. The intracellular rise of cAMP produced by the bicarbonate-dependent soluble adenylyl cyclase (sAC) has been suggested to play a central role in initiating the cascade of the events that culminates in spermatozoa maturation. Here, we show that targeted disruption of the sAC gene does not affect spermatogenesis but dramatically impairs sperm motility, leading to male sterility. sAC mutant spermatozoa are characterized by a total loss of forward motility and are unable to fertilize oocytes in vitro. Interestingly, motility in sAC mutant spermatozoa can be restored on cAMP loading, indicating that the motility defect observed is not caused by a structural defect. We, therefore, conclude that sAC plays an essential and nonredundant role in the activation of the signaling cascade controlling motility and, therefore, in fertility. The crucial role of sAC in fertility and the absence of any other obvious pathological abnormalities in sAC-deficient mice may provide a rationale for developing inhibitors that can be applied as a human male contraceptive.

                Author and article information

                WILEY-VCH Verlag (Weinheim )
                April 2014
                24 February 2014
                : 9
                : 4
                : 823-832
                [[a] ]Astex Pharmaceuticals Cambridge Science Park, Cambridge, CB4 0QA (UK) E-mail: harren.jhoti@
                Author notes

                Current address: Heptares Therapeutics Ltd, BioPark, Broadwater Road, Welwyn Garden City, AL7 3AX (UK)


                Current address: Gregory Fryer Associates Ltd, 10–12 St Thomas’ Pl, Ely, CB7 4EX (UK)


                Current address: StemCell Sciences UK, Babraham Research Campus, Cambridge, CB22 3AT (UK)

                Supporting information for this article is available on the WWW under

                © 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.

                This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

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