Department of Molecular Biology, Biomedical Center, Uppsala, Sweden.
The structure of the hexadecameric ribulose-bisphosphate carboxylase/oxygenase from Synechococcus PCC6301 has been solved to 2.2-A resolution. Crystallization was in the presence of CO2, Mg2+, and 2'-carboxyarabinitol bisphosphate to form a stable enzyme quaternary complex that mimics one of the intermediate states of the carboxylation reaction. The structure was solved by molecular replacement using the coordinates of spinach carboxylase. The deviations in C alpha positions of the L- and S-subunits are only 0.3 and 2.0 A, respectively, and localized at specific regions of the two polypeptides. One region that shows significant divergence of the peptide backbone is loop 6 of the beta barrel in the L-subunit. Two other elements, the C terminus, and a highly conserved loop of the N-terminal domain of a second L-subunit, interact with loop 6 in the quaternary complex. These three regions, plus two other flexible segments, completely enfold the bisphosphate inhibitor. Significant alteration in their spatial relationship must occur to allow substrates or products access to and from the active site. The active site residues, activating cofactors, and inhibitor are well resolved in the electron density map. The disposition of these groups around the essential metal provides some indication of their role at different stages of the catalytic cycle.