The X-ray crystal structure of Escherichia coli succinic semialdehyde dehydrogenase; structural insights into NADP+ / enzyme interactions

In mammals succinic semialdehyde dehydrogenase (SSADH) plays an essential role in the metabolism of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) to succinic acid (SA). Deficiency of SSADH in humans results in elevated levels of GABA and γ-Hydroxybutyric acid (GHB) which leads to psychomotor retardation, muscular hypotonia, non-progressive ataxia and seizures. In Escherichia coli, two genetically distinct forms of SSADHs had been described which are essential for preventing accumulation of toxic levels of succinic semialdehyde (SSA) in cells. Here we structurally characterise SSADH encoded by the E coli gabD gene and compare these data with the structure of human SSADH. Interestingly, in contrast to the human enzyme in the E. coli SSADH structure, electron density for the complete NADP+ cofactor in the binding sites is clearly evident; these data in particular revealing how the nicotinamide ring of the co-facto is positioned in each active site. Furthermore, our structural data suggest that a deletion of three amino acids in E. coli SSADH permits this enzyme to use NADP+ whereas in contrast the human enzyme utilises NAD+. Finally, the structure of E. coli SSADH gives additional insight into human mutations that result in disease. To cite this data use the following DOI: 10.4225/52/557FAF18B53C8

在哺乳动物中，琥珀酸半醛脱氢酶（succinic semialdehyde dehydrogenase, SSADH）在抑制性神经递质γ-氨基丁酸（γ-aminobutyric acid, GABA）代谢为琥珀酸（succinic acid, SA）的过程中发挥不可或缺的作用。
人体中SSADH的缺乏会导致γ-羟基丁酸（γ-Hydroxybutyric acid, GHB）与GABA水平升高，进而引发精神运动发育迟缓、肌张力低下、非进行性共济失调及癫痫发作。
在大肠杆菌（Escherichia coli）中，已有研究报道了两种遗传特征迥异的SSADH亚型，它们可防止细胞内毒性水平的琥珀酸半醛（succinic semialdehyde, SSA）积累。
本研究对大肠杆菌gabD基因编码的SSADH进行了结构表征，并将所得结构数据与人类SSADH的结构进行对比。
值得注意的是，与人类SSADH不同，大肠杆菌SSADH的结构中可清晰观测到结合位点上完整烟酰胺腺嘌呤二核苷酸磷酸（NADP+）辅酶的电子密度；该数据尤其阐明了辅酶烟酰胺环在每个活性位点的定位方式。
此外，我们的结构数据表明，大肠杆菌SSADH中三个氨基酸的缺失使得该酶能够利用NADP+，而人类SSADH则偏好使用烟酰胺腺嘌呤二核苷酸（NAD+）。
最后，大肠杆菌SSADH的结构还为阐释致病相关的人类SSADH突变提供了新的认知。引用本数据请使用以下DOI：10.4225/52/557FAF18B53C8