The three dimensional structure of carbamoyl phosphate synthetase
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abstract
Carbamoyl phosphate synthetase catalyzes the production of carbamoyl phosphate from bicarbonate, glutamine, and two molecules of MgATP. As isolated from E. coil, the enzyme has a total molecular weight of 160K and consists of two polypeptide chains referred to as the large and small subunits. The X-ray crystal structure of CPS has been determined to 2.8 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. 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. The two halves of the large subunit are related by a nearly exact two-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 80 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 unstable enzyme intermediates must therefore be sequentially channeled from one active site to the next through the interior of the protein.
