Knapp J E, Mitchell D T, Yazdi M A, Ernst S R, Reed L J, Hackert M L
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, TX 78712, USA.
J Mol Biol. 1998 Jul 24;280(4):655-68. doi: 10.1006/jmbi.1998.1924.
The dihydrolipoamide succinyltransferase (E2o) component of the 2-oxoglutarate dehydrogenase multienzyme complex is composed of 24 subunits arranged with 432 point group symmetry. The catalytic domain (CD) of the E2o component catalyzes the transfer of a succinyl group from the S-succinyldihydrolipoyl moiety to coenzyme A. The crystal structure of the Escherichia coli E2oCD has been solved to 3.0 A resolution using molecular replacement phases derived from the structure of the catalytic domain from the Azotobacter vinelandii dihydrolipoamide acetyltransferase (E2pCD). The refined model of the E. coli E2oCD consists of residues 172 to 404 and has an R-factor of 0.205 (Rfree=0.249) for 9696 reflections between 20.0 and 3.0 A resolution. Although both E2oCD and E2pCD form 24mers, subtle changes in the orientations of two helices in E2oCD increase the stability of the E2oCD 24mer in comparison to the less stable A. vinelandii E2pCD 24mer. Like E2pCD and chloramphenicol acetyltransferase (CAT), the active site of E2oCD is located in the middle of a channel formed at the interface between two 3-fold related subunits. Two of the active-site residues (His375 and Thr323) have a similar orientation to their counterparts in E2pCD and CAT. A third catalytic residue (Asp379) assumes a conformation similar to the corresponding residue in E2pCD (Asn614), but different from its counterpart in CAT (Asp199). Binding of the substrates to E2oCD is proposed to induce a change in the conformation of Asp379, allowing this residue to form a salt bridge with Arg184 that is analogous to that formed between Asp199 and Arg18 in CAT. Computer models of the active site of E2o complexed with dihydrolipoamide and with coenzyme A led to the identification of the probable succinyl-binding pocket. The residues which form this pocket (Ser330, Ser333, and His348) are probably responsible for E2o's substrate specificity.
2-氧代戊二酸脱氢酶多酶复合物的二氢硫辛酰胺琥珀酰转移酶（E2o）组分由24个亚基组成，排列具有432点群对称性。E2o组分的催化结构域（CD）催化琥珀酰基从S-琥珀酰二氢硫辛酰部分转移至辅酶A。利用源自维涅兰德固氮菌二氢硫辛酰胺乙酰转移酶（E2pCD）催化结构域结构的分子置换相位，已将大肠杆菌E2oCD的晶体结构解析至3.0 Å分辨率。大肠杆菌E2oCD的精制模型由172至404位残基组成，对于20.0至3.0 Å分辨率之间的9696个反射，其R因子为0.205（Rfree = 0.249）。尽管E2oCD和E2pCD均形成24聚体，但与稳定性较低的维涅兰德固氮菌E2pCD 24聚体相比，E2oCD中两个螺旋取向的细微变化增加了E2oCD 24聚体的稳定性。与E2pCD和氯霉素乙酰转移酶（CAT）一样，E2oCD的活性位点位于两个三重相关亚基之间界面处形成的通道中间。两个活性位点残基（His375和Thr323）与其在E2pCD和CAT中的对应残基具有相似的取向。第三个催化残基（Asp379）呈现出与E2pCD中相应残基（Asn614）相似的构象，但与其在CAT中的对应残基（Asp199）不同。底物与E2oCD的结合被认为会诱导Asp379构象发生变化，使该残基与Arg184形成盐桥，这类似于CAT中Asp199与Arg18之间形成的盐桥。与二氢硫辛酰胺和辅酶A复合的E2o活性位点的计算机模型导致了可能的琥珀酰结合口袋的鉴定。形成该口袋的残基（Ser330、Ser333和His348）可能决定了E2o的底物特异性。
