Bell C E, Eisenberg D
UCLA-DOE Laboratory of Structural Biology and Molecular Medicine, Molecular Biology Institute, USA.
Biochemistry. 1996 Jan 30;35(4):1137-49. doi: 10.1021/bi9520848.
Diphtheria toxin (DT), a 58 kDa protein secreted by lysogenic strains of Corynebacterium diphtheriae, causes the disease diphtheria in humans by gaining entry into the cytoplasm of cells and inhibiting protein synthesis. Specifically, the catalytic (C) domain of DT transfers the ADP-ribose group of NAD to elongation factor-2 (EF-2), rendering EF-2 inactive. In order to investigate how the C-domain of DT binds NAD and catalyzes the ADP-ribosylation of EF-2, the crystal structure of DT in complex with NAD has been determined to 2.3 A resolution. This is the first crystal structure of an ADP-ribosyltransferase (ADP-RT) enzyme in complex with NAD and suggests the features of the ADP-RT fold which are important for NAD binding. The conformation of NAD in the complex and the proximity of the Glu148 carboxylate group of the C-domain to the scissile, N-glycosidic bond of NAD suggest plausible modes of catalysis of the ADP-ribosylation reaction. Residues 39-46 of the active-site loop of the C-domain become disordered upon NAD binding, suggesting a potential role for this loop in the recognition of the ADP-ribose acceptor substrate, EF-2. The negatively charged phosphates and two ribose hydroxyls of NAD are not in direct contact with any atoms of the C-domain. Instead, they form an exposed surface which appears to be presented for recognition by EF-2. Structural alignments of the DT-NAD complex with the structures of other members of the ADP-RT family suggest how NAD may bind to these other enzymes.
白喉毒素（DT）是一种由产毒株白喉棒状杆菌分泌的58 kDa蛋白质，它通过进入细胞胞质并抑制蛋白质合成，从而导致人类患白喉病。具体而言，DT的催化（C）结构域将NAD的ADP - 核糖基团转移至延伸因子2（EF - 2），使EF - 2失活。为了研究DT的C结构域如何结合NAD并催化EF - 2的ADP - 核糖基化，已确定了DT与NAD复合物的晶体结构，分辨率为2.3 Å。这是ADP - 核糖基转移酶（ADP - RT）与NAD复合物的首个晶体结构，并揭示了对NAD结合很重要的ADP - RT折叠的特征。复合物中NAD的构象以及C结构域的Glu148羧基与NAD的可裂解N - 糖苷键的接近程度表明了ADP - 核糖基化反应可能的催化模式。C结构域活性位点环的39 - 46位残基在结合NAD后变得无序，表明该环在识别ADP - 核糖受体底物EF - 2中可能发挥作用。NAD带负电荷的磷酸基团和两个核糖羟基不与C结构域的任何原子直接接触。相反，它们形成一个暴露表面，似乎是为了被EF - 2识别而呈现。DT - NAD复合物与ADP - RT家族其他成员结构的比对表明了NAD可能如何与这些其他酶结合。
