Structure of carbamoyl phosphate synthetase: a journey of 96 A from substrate to product.

Carbamoyl phosphate synthetase catalyzes the production of carbamoyl phosphate from bicarbonate, glutamine, and two molecules of MgATP. As isolated from Escherichia coli, the enzyme has a total molecular weight of approximately 160K and consists of two polypeptide chains referred to as the large and small subunits. Here we describe the X-ray crystal structure of this enzyme determined to 2.8 A resolution in the presence of ADP, Mn2+, phosphate, and ornithine. The small subunit is distinctly bilobal with the active site residues located in the interface formed by the NH2- and COOH-terminal domains. Interestingly, the structure of the COOH-terminal half is similar to that observed in the trpG-type amidotransferase family. The large subunit can be envisioned as two halves referred to as the carboxyphosphate and carbamoyl phosphate synthetic components. Each component contains four distinct domains. Strikingly, the two halves of the large subunit are related by a nearly exact 2-fold rotational axis, thus suggesting that this polypeptide chain evolved from a homodimeric precursor. The molecular motifs of the first three domains observed in each synthetic component are similar to those observed in biotin carboxylase. A linear distance of approximately 80 A separates the binding sites for the hydrolysis of glutamine in the small subunit and the ATP-dependent phosphorylations of bicarbonate and carbamate in the large subunit. The reactive and unstable enzyme intermediates must therefore be sequentially channeled from one active site to the next through the interior of the protein.

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