Department of Molecular Biology, Swedish University of Agricultural Sciences, Uppsala Biomedical Center, Box 590, S-75124 Uppsala, Sweden.
The three-dimensional structure of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) from Rhodospirillum rubrum has been determined at 2.9 A 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 A. 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 beta-sheet with alpha-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 alpha/beta 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 beta-strands in the alpha/beta 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.