CRYSTAL-STRUCTURE OF THE TERNARY COMPLEX OF RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE, MG(II), AND ACTIVATOR CO2 AT 2.3-A RESOLUTION

The activated ternary complex, enzyme-CO2-Mg(II), of the dimeric ribulose-1,5-bisphosphate carboxylase/oxygenase from Rhodospirillum rubrum can be prepared in the same crystal from that was used for the crystallographic structure determination of the native nonactivated enzyme [Schneider, G., Branden, C.-I., & Lorimer, G. (1986) J. Mol. Biol. 187, 141-143). The three-dimensional structure of the activated enzyme has been determined to a nominal resolution of 2.3 angstrom by protein crystallographic methods. The activator CO2 forms a carbamate with Lys 191, located at the bottom of the funnel-shaped active site. In both subunits, this labile adduct is stabilized by a Mg(II) ion, bound to the carbamate and the side chains of Asp 193 and Glu 194. One solvent molecule was found within the first coordination sphere of the metal ion. The metal-binding site in ribulose-1,5-bisphosphate carboxylase consists thus of at least three protein ligands, all located on loop 2 of the beta-alpha-barrel. One additional metal ligand, the side chain of the conserved Asn 111, was observed close to the Mg(II) ion in the B-subunit. Other structural differences at the active site between the activated and nonactivated enzyme are limited to side-chain positions. Nevertheless, it is obvious that the hydrogen-bonding pattern in the vicinity of the activator site is completely altered.