Department of Biology, Brandeis University, Waltham, Massachusetts, USA.
Nat Chem Biol. 2011 Oct 30;7(12):950-8. doi: 10.1038/nchembio.693.
Inosine monophosphate dehydrogenase (IMPDH) and guanosine monophosphate reductase (GMPR) belong to the same structural family, share a common set of catalytic residues and bind the same ligands. The structural and mechanistic features that determine reaction outcome in the IMPDH and GMPR family have not been identified. Here we show that the GMPR reaction uses the same intermediate E-XMP* as IMPDH, but in this reaction the intermediate reacts with ammonia instead of water. A single crystal structure of human GMPR type 2 with IMP and NADPH fortuitously captures three different states, each of which mimics a distinct step in the catalytic cycle of GMPR. The cofactor is found in two conformations: an 'in' conformation poised for hydride transfer and an 'out' conformation in which the cofactor is 6 Å from IMP. Mutagenesis along with substrate and cofactor analog experiments demonstrate that the out conformation is required for the deamination of GMP. Remarkably, the cofactor is part of the catalytic machinery that activates ammonia.
肌苷单磷酸脱氢酶 (IMPDH) 和鸟苷单磷酸还原酶 (GMPR) 属于同一结构家族，具有共同的催化残基，并结合相同的配体。但尚未确定决定 IMPDH 和 GMPR 家族反应结果的结构和机制特征。本文表明，GMPR 反应使用与 IMPDH 相同的中间产物 E-XMP*，但在该反应中，中间产物与氨而不是水反应。一个带有 IMP 和 NADPH 的人 GMPR 型 2 的单晶结构偶然捕获了三个不同的状态，每个状态都模拟了 GMPR 催化循环中的一个独特步骤。辅因子存在于两种构象中：一种是准备进行氢化物转移的“内”构象，另一种是与 IMP 相隔 6 Å 的“外”构象。突变以及底物和辅因子类似物实验表明，外构象是 GMP 脱氨所必需的。值得注意的是，辅因子是激活氨的催化机制的一部分。
