Three-dimensional structure of ribulose-1,5-bisphosphate carboxylase/oxygenase from Rhodospirillum rubrum at 2.9 Å resolution

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Abstract
The three‐dimensional structure of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) from Rhodospirillum rubrum has been determined at 2.9 Å resolution by X‐ray crystallographic methods. The MIR‐electron density map was substantially improved by two‐fold non‐crystallographic symmetry averaging. The polypeptide chains in the dimer were traced using a graphics display system with the help of the BONES option in FRODO. The dimer has approximate dimensions of 50 x 72 x 105 Å. The enzyme subunit is a typical two‐domain protein. The smaller, N‐terminal domain consists of 137 amino acid residues and forms a central, mixed five‐stranded β‐sheet with α‐helices on both sides of the sheet. The larger C‐terminal domain consists of 329 amino acid residues. This domain has an eight‐stranded parallel α/β barrel structure as found in triosephosphate isomerase and a number of other functionally non‐related proteins. The active site in Rubisco determined by difference Fourier techniques and fitting of active site residues to the electron density map, is located at the carboxy‐end of the β‐strands in the α/β barrel of the C‐terminal domain. There are few domain–domain interactions within the subunit. The interactions at the interface between the two subunits of the dimer are tight and extensive. There are tight contacts between the two C‐terminal domains, which build up the core of the molecule. There are also interactions between the N‐terminal domain of one subunit and the C‐terminal domain of the second subunit, close to the active site.