Crystal forms of CO₂/Mg²⁺-activated and deactivated spinach ribulosebisphosphate carboxylase

The catalytically active form of spinach leaf ribulose-1,5-bisphosphate carboxylase (3-phospho-d-glycerate carboxylase (dimerizing); EC 4.1.1.39) is a ternary complex of enzyme · CO₂ · Mg²⁺. Activation occurs upon incubation of the enzyme with CO₂ and Mg²⁺ at alkaline pH; removal of these ions rapidly dissociates the complex. CO₂/Mg²⁺-activated and nonactivated spinach carboxylases crystallize as tetragonal bipyramids by near-equilibrium vapor diffuson using poly(ethylene glycol) as the precipitant. In contrast, nonequilibrium vapor diffusion with similar solutions and precipitant yielded two different forms of activated and non-activated enzyme. The CO₂/Mg²⁺-activated enzyme crystallized as single, flat-plates by this method, whereas the nonactivated spinach carboxylase grew as tetragonal bipyramids. The flat-plate crystals were catalytically active in the absence of subsequent CO₂/Mg²⁺-activation and retained relatively constant levels of activity for approx. 2000 h of storage at 4°C. The bipyramidal-shaped crystals were competent but inactive without prior activation by CO₂ and Mg²⁺.