Influence of the activation state on the sedimentation properties of ribulose bisphosphate carboxylase from Alcaligenes eutrophus.

Ribulose-1,5-bisphosphate carboxylase from the chemolithotrophic hydrogen bacterium Alcaligenes eutrophus was maximally active in the presence of 50 mM HCO3- plus 10 mM Mg2+. Deactivation occurred upon removal of these ions. Reactivation was achieved by incubation of the enzymes with HCO3- plus Mg2+. The concentration of HCO3- (CO2) required for half-maximal activation was 1.84 nM (0.064 mM). Sedimentation velocity studies revealed that activation/deactivation is associated with drastic changes in the sedimentation properties of the enzyme. While the inactive form had a sedimentation coefficient, s20,w, of 17.5 S, the s20,w gradually decreased as the enzyme was reactivated and the fully reactivated form exhibited an s20,w of 14.3 S. A structural analogue of ribulose 1,5-bisphosphate, xylulose 1,5-bisphosphate, caused a deactivation of the enzyme concomitant with an increase in the sedimentation velocity. It is suggested that the alterations in the hydrodynamic properties accompanying the activation/deactivation process are due to considerable conformational changes that affect the molecular volume and/or the shape of the enzyme. Dissociation/association events were not involved in the changes. The s20,w of about 18 S, generally reported for the large hexadecameric ribulose bisphosphate carboxylases, appears to be characteristic of the inactive form.