Molecular structure of Bacillus subtilis aspartate transcarbamoylase at 3.0 A resolution.

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（aspartate transcarbamoylase，简称ATCase；又称天冬氨酸氨基甲酰转移酶；氨甲酰磷酸:L-天冬氨酸氨基甲酰转移酶，EC 2.1.3.2）的三维结构通过分子置换法解析至3.0埃分辨率，并经晶体学精修至R因子为0.19。该酶的晶体空间群为C2，晶胞参数为a=258.5、b=153.2、c=51.9埃，β角为97.7°。不对称单元由三个通过非晶体学三重对称相关的单体构成。304个氨基酸残基中共有295个被构建至单体模型中，C端最后9个残基未纳入最终结构模型。每个单体包含34%的α螺旋与18%的β折叠链。三个溶剂暴露的环区（残基69-84、178-191及212-229）的电子密度定义不明确。枯草芽孢杆菌ATCase的催化三聚体与大肠杆菌ATCase的催化三聚体具有高度结构相似性，后者被用于构建本次研究的分子置换模型。该枯草芽孢杆菌的未配体三聚体不具备协同效应，其构象与大肠杆菌ATCase的T（失活）态更为接近，但部分区域仍向大肠杆菌ATCase的R（活性）态构象发生偏移。