title: Bicarbonate activation of adenylyl cyclase via promotion of catalytic active site closure and metal recruitment.
abstract: In an evolutionarily conserved signaling pathway, 'soluble' adenylyl cyclases (sACs) synthesize the ubiquitous second messenger cyclic adenosine 3',5'-monophosphate (cAMP) in response to bicarbonate and calcium signals. Here, we present crystal structures of a cyanobacterial sAC enzyme in complex with ATP analogs, calcium and bicarbonate, which represent distinct catalytic states of the enzyme. The structures reveal that calcium occupies the first ion-binding site and directly mediates nucleotide binding. The single ion-occupied, nucleotide-bound state defines a novel, open adenylyl cyclase state. In contrast, bicarbonate increases the catalytic rate by inducing marked active site closure and recruiting a second, catalytic ion. The phosphates of the bound substrate analogs are rearranged, which would facilitate product formation and release. The mechanisms of calcium and bicarbonate sensing define a reaction pathway involving active site closure and metal recruitment that may be universal for class III cyclases.
authors.affiliation: Department of Biochemistry, Weill Medical College of Cornell University, 1300 York Avenue, New York, New York 10021, USA.
authors.forename: Clemens
authors.initials: C
authors.lastname: Steegborn
authors.forename: Tatiana N
authors.initials: TN
authors.lastname: Litvin
authors.forename: Lonny R
authors.initials: LR
authors.lastname: Levin
authors.forename: Jochen
authors.initials: J
authors.lastname: Buck
authors.forename: Hao
authors.initials: H
authors.lastname: Wu
mesh_terms: D000255:Adenosine Triphosphate / Q000031:analogs & derivatives* / Q000494:pharmacology; D000262:Adenylyl Cyclases / Q000737:chemistry* / Q000378:metabolism*; D000595:Amino Acid Sequence; D000818:Animals; D001639:Bicarbonates / Q000737:chemistry / Q000494:pharmacology*; D001665:Binding Sites / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D020134:Catalytic Domain / Q000187:drug effects*; D018360:Crystallography, X-Ray; D000458:Cyanobacteria / Q000201:enzymology / Q000235:genetics; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D008274:Magnesium / Q000378:metabolism*; D008322:Mammals; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D013873:Thionucleotides / Q000378:metabolism
chemical_list: D001639:Bicarbonates; D013873:Thionucleotides; C017817:adenosine 5'-(1-thio)triphosphate; D000255:Adenosine Triphosphate; D000262:Adenylyl Cyclases; D008274:Magnesium; C002630:alpha,beta-methyleneadenosine 5'-triphosphate; D002118:Calcium