THE X-RAY STRUCTURE OF SYNECHOCOCCUS RIBULOSE-BISPHOSPHATE CARBOXYLASE OXYGENASE-ACTIVATED QUATERNARY COMPLEX AT 2.2-ANGSTROM RESOLUTION

The structure of the hexadecameric ribulose-bisphosphate carboxylase/o xygenase from Synechococcus PCC6301 has been solved to 2.2-angstrom re solution. Crystallization was in the presence of CO2, Mg+, and 2'-carb oxyarabinitol bisphosphate to form a stable enzyme quaternary complex that mimics one of the intermediate states of the carboxylation reacti on. The structure was solved by molecular replacement using the coordi nates of spinach carboxylase. The deviations in Calpha positions of th e L- and S-subunits are only 0.3 and 2.0 angstrom, respectively, and l ocalized 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 o ccur to allow substrates or products access to and from the active sit e. The active site residues, activating cofactors, and inhibitor are w ell resolved in the electron density map. The disposition of these gro ups around the essential metal provides some indication of their role at different stages of the catalytic cycle.