Stevens R C, Reinisch K M, Lipscomb W N
Gibbs Chemical Laboratory, Harvard University, Cambridge, MA 02138.
Proc Natl Acad Sci U S A. 1991 Jul 15;88(14):6087-91. doi: 10.1073/pnas.88.14.6087.
The three-dimensional structure of Bacillus subtilis aspartate transcarbamoylase (ATCase; aspartate carbamoyltransferase; carbamoyl-phosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) has been solved by the molecular replacement method at 3.0 A resolution and refined to a crystallographic R factor of 0.19. The enzyme crystallizes in the space group C2 with unit cell dimensions a = 258.5, b = 153.2, and c = 51.9 A and beta = 97.7 degrees. The asymmetric unit is composed of three monomers related by noncrystallographic threefold symmetry. A total of 295 of 304 amino acid residues have been built into the monomer. The last 9 residues in the C terminus were not included in the final model. Each monomer consists of 34% alpha-helix and 18% beta-strand. Three solvent-exposed loop regions (residues 69-84, 178-191, and 212-229) are not well defined in terms of electron density. The catalytic trimer of ATCase from B. subtilis shows great similarity to the catalytic trimer in Escherichia coli ATCase, which was used in constructing the model for molecular replacement. The unliganded trimer from B. subtilis, which is not cooperative, resembles the T (inactive) state slightly more than the R (active)-state form of the E. coli trimer. However, certain regions in the B. subtilis trimer exhibit shifts toward the E. coli R-state conformation.
枯草芽孢杆菌天冬氨酸转氨甲酰酶（ATCase；天冬氨酸氨甲酰基转移酶；氨甲酰磷酸：L-天冬氨酸氨甲酰基转移酶，EC 2.1.3.2）的三维结构已通过分子置换法在3.0埃分辨率下解析，并精修至晶体学R因子为0.19。该酶在空间群C2中结晶，晶胞参数为a = 258.5、b = 153.2和c = 51.9埃，β = 97.7°。不对称单元由通过非晶体学三重对称性相关的三个单体组成。304个氨基酸残基中的295个已构建到单体中。C末端的最后9个残基未包含在最终模型中。每个单体由34%的α螺旋和18%的β链组成。三个溶剂暴露的环区（残基69 - 84、178 - 191和212 - 229）在电子密度方面定义不明确。枯草芽孢杆菌ATCase的催化三聚体与大肠杆菌ATCase中的催化三聚体显示出极大的相似性，后者用于构建分子置换模型。来自枯草芽孢杆菌的未结合配体的三聚体不具有协同性，与大肠杆菌三聚体的R（活性）状态形式相比，更类似于T（无活性）状态。然而，枯草芽孢杆菌三聚体中的某些区域表现出向大肠杆菌R状态构象的转变。
