Protein liganding to the activator cation of ribulosebisphosphate carboxylase

Spinach leaf ribulosebisphosphate carboxylase forms a quaternary complex with CO2, carboxyarabinitol bisphosphate, and Cr2+ or Co2+. Oxidation of the cation in these complexes produces a protein--cation adduct which is sufficiently stable to be chromatographically isolated after enzyme denaturation. While stoichiometric levels of slowly exchanging cation can be specifically trapped after addition of protein denaturants as well as a vast molar excess of Mg2+, neither CO2 nor carboxyarabinitol bisphosphate remains bound to denatured protein under the conditions employed in these experiments. These observations demonstrate direct innersphere liganding of protein to the exchange-inert cation, which appears to bind at the site normally occupied by the physiologically active cation. Dimeric ribulosebisphosphate carboxylase from Rhodospirillum rubrum also forms a quaternary complex containing stoichiometric amounts of enzyme protomer, CO2, Co2+, and carboxyarabinitol bisphosphate. Lack of a small subunit in the R. rubrum enzyme does not impair binding of the components of the quaternary complex in a nonexchangeable mode. Substantial amounts of protein--cation adduct are recovered upon oxidation and denaturation of the R. rubrum complex, supporting the prediction that the large subunits of the octameric plant enzyme should be the sites of cation binding. The first direct proof for such a hypothesis has been generated by separation of protein subunits derived from a spinach quaternary complex and by the demonstration that the bound cation is associated with the large subunit.