| dc.description.abstract | Aspartate transcarbamoylase (ATCase, EC 2.1.3.2) is a ubiquitous enzyme which catalyzes the first unique step in pyrimidine biosynthesis in both prokaryotes and eukaryotes. From bacteria to animals, enzyme complexes occur in the pyrimidine pathway providing architectural variety. The Escherichia coli ATCase consists of two catalytic trimers bridged by three regulatory dimers. Catalysis occurs at the interface of adjacent catalytic monomers, and catalytic trimers are active apart from the holoenzyme. Each of the catalytic monomers are divided functionally and structurally into two domains, that are individually responsible for binding substrates carbamoyl phosphate and aspartate.
The ATCase sequences of the yeast saccharomyces cerevisiae, the slime mold Dictyostelium discoideum, and hamster as deduced from DNA sequences were obtained from various sources. The secondary and tertiary structures of these eukaryotic enzymes were predicted based on the E. coli crystal structure as determined by Dr. W. N. Lipscomb. The E. coli ATCase catalytic monomer is composed of 310 amino acids while genomic cistrons of yeast and slime mold and the hamster cDNA encode ATCases of 311, 310, and 308 amino acids, respectively. The positional identity relative to E. coli sequence is 43% for yeast and 44% for both hamster and slime mold. The predicted secondary structure of all three (yeast, slime mold, hamster enzyme) agreed favorably with the actual secondary structure of E. coli ATCase. Previous analysis of the tertiary structure of hamster ATCase was predicted to be very similar to the E. coli catalytic trimer, with maximum displacements confined to exterior loops. Computational analysis of the tertiary structure of slime mold accomplished in this study have suggested that the insertions and deletions are also present on the exterior loops.
A comparison of the primary sequences of the E. coli enzyme and the three eukaryotic enzymes (yeast, slime mold, and hamster) revealed significant differences in the amino acids present in the exterior loops of E. coli. Comparisons between the three eukaryotes show the differences in only two regions which corresponded to amino acids 110 to 119 and 198 to 218 of E. coli, both of which are found at the surface of the bacterial enzyme structure. | en |
