期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1991
卷号：88
期号：14
页码：6087-6091
DOI：10.1073/pnas.88.14.6087
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：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.
