Steegborn Clemens, Litvin Tatiana N, Levin Lonny R, Buck Jochen, Wu Hao
Department of Biochemistry, Weill Medical College of Cornell University, 1300 York Avenue, New York, New York 10021, USA.
Nat Struct Mol Biol. 2005 Jan;12(1):32-7. doi: 10.1038/nsmb880. Epub 2004 Dec 26.
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.
在一个进化上保守的信号通路中，“可溶性”腺苷酸环化酶（sACs）响应碳酸氢盐和钙信号合成普遍存在的第二信使环腺苷酸3',5'-单磷酸（cAMP）。在此，我们展示了一种蓝藻sAC酶与ATP类似物、钙和碳酸氢盐复合物的晶体结构，这些结构代表了该酶不同的催化状态。结构显示钙占据第一个离子结合位点并直接介导核苷酸结合。单个离子占据的核苷酸结合状态定义了一种新的、开放的腺苷酸环化酶状态。相比之下，碳酸氢盐通过诱导显著的活性位点闭合并募集第二个催化离子来提高催化速率。结合的底物类似物的磷酸基团发生重排，这将有助于产物的形成和释放。钙和碳酸氢盐传感机制定义了一个涉及活性位点闭合和金属募集的反应途径，这可能对III类环化酶是普遍适用的。
