3.0-A RESOLUTION STUDY OF NUCLEOTIDE COMPLEXES WITH ASPARTATE CARBAMOYLTRANSFERASE

The binding sites of CTP, CDP, 5-BrCTP, and ATP to the allosteric site of aspartate carbamoyltransferase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) have been found in electron-density maps obtained at about 3 A resolution from x-ray diffraction studies of single crystals. The activator ATP binds in the anti conformation, whereas the inhibitor 5-BrCTP binds in the syn conformation. Both activator and inhibitor bind to the same local region of the enzyme. All of the cytidine nucleotides show important interactions of the base with the protein. The triphosphate conformations are similar, whereas the terminal phosphate of CDP occupies the site of the gamma-phosphate of CTP, thus implying a protein-nucleotide interaction at this site. These results are then related to biochemical studies.