Carbamoyl phosphate synthetase: an amazing biochemical odyssey from substrate to product

Carbamoyl phosphate synthetase (CPS) catalyzes one of the most remarkable r eactions ever described: in biological chemistry, in which carbamoyl phosph ate is produced from one molecule of bicarbonate, two molecules of Mg2+ ATP , and one molecule of either glutamine or ammonia. The carbamoyl phosphate so produced is utilized in the synthesis of arginine and pyrimidine nucleot ides. It is also employed in the urea cycle in most terrestrial vertebrates . Due to its large size, its important metabolic role, and the fact that it is highly regulated, CPS has been the focus of intensive investigation for nearly 40 years. Numerous enzymological, biochemical, and biophysical stud ies by a variety of investigators have led to a quite detailed understandin g of CPS. Perhaps one of the most significant advances on this topic within the last 2 years has been the successful X-ray crystallographic analysis o f CPS from Escherichia coli. Quite unexpectedly, this structural investigat ion revealed that the three active sites on the protein are widely separate d from one another. Furthermore, these active sites are connected by a mole cular tunnel with a total length of approximately 100 Angstrom, suggesting that CPS utilizes this channel to facilitate the translocation of reaction intermediates from one site to another. In this review, we highlight the re cent biochemical and X-ray crystallographic results that have led to a more complete understanding of this finely tuned instrument of catalysis.