Magnetic Resonance Studies on Ribulose Bisphosphate Carboxylase

Abstract

Experiments have been initiated aimed at elucidating the role of the divalent cation required in the RuBP carboxylase-catalyzed reaction. Metal ion is clearly required for activation of the enzyme. However, there is some question concerning an additional role for metal in the catalytic process. Past studies have shown that RuBP and analogous compounds decrease the enhancement of the water proton relaxation rate which arises from binding of a paramagnetic divalent cation, Mn²⁺, to enzyme. Consider, for example, the titration of a RuBP carboxylase-Mn²⁺-CO₂ solution with the transition state analog carboxyribitol bisphosphate. To account for the decrease in observed enhancement from an initial value of 18 to a final value of 1, displacement of water molecules bound in the inner coordination sphere of Mn²⁺ may be invoked. Such displacement could be accomplished if a ligand from CRBP or enzyme replaced a water molecule. Another explanation for the data would be that binding of CRBP results in an altered enzyme conformation which leaves Mn²⁺ with innersphere water ligands that can no longer rapidly exchange with the aqueous medium.