Thermodynamics and kinetics of sugar phosphate binding to D-ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO)

It has been demonstrated by microcalorimetric and fast reaction techniques that D-ribulose 1,5-bisphosphate carboxylase/oxygenase (RUBISCO) from spinach shows strong and weak binding sites for the substrate D-ribulose 1,5-bisphosphate (RuBP), for the effector 6-D-phosphogluconate (6-PG), and for the so called ‘transition state analogue’ 2-carboxy-D-arabinitol 1,5-bisphosphate (CABP). The stoichiometry n, the dissociation constant Kd and the enthalpy change ΔHb associated with the strong binding of RuBP and CABP to RUBISCO were measured by isothermal differential titration calorimetry. In addition, differential scanning calorimetry showed an increase of the thermal stability of RUBISCO in the presence of RuBP, 6-PG and especially CABP. The kinetics of binding of RuBP and 6-PG to RUBISCO were measured by stopped flow and iodine laser temperature jump experiments using the fluorescence probe 2-(p-toluidinyl)naphthalene-6-sulfonate. The kinetics of the reversible bimolecular binding reactions of RuBP and 6-PG revealed a fast and a slow phase corresponding to the strong and weak ligand binding phenomena observed in equilibrium measurements. The association and dissociation rate constants k+ and k- for these processes were determined. The dissociation constants Kd calculated from the kinetic constants are in good agreement with Kd values obtained from calorimetric and fluorescence titration studies.