Role of the small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase in the activation process

Abstract

The large (A) and small (B) subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) from the cyanobacterium Aphanothece halophytica and from the purple sulfur photosynthetic bacterium Chromatium vinosum (strain D) were separated by sucrose density gradient centrifugation at low ionic strength and alkaline pH (9.3), respectively. It was found that subunit B enhances the extent of activation by CO 2 and Mg 2+ at equilibrium of the two homologous enzymes consisting of Aphanothece large subunit and its own small subunit (A aB a) and the Chromatium large subunit and its own small subunit (A cB c). The extent of activation induced by saturating amounts of subunit B was larger with A cB c than A aB a, amounting to 3.7- and 1.8-fold of that by each catalytic core alone, respectively. Subunit B stimulated both the extent of activation at equilibrium and catalysis in a parallel and simultaneous manner with respect to the concentration of B in both homologous enzymes. These results suggest that subunit B interacts with both activation and catalytic sites simultaneously. On the other hand, Chromatium subunit B only slightly stimulated the extent of activation in the hybrid enzyme A aB c. The role of subunit B in enhancing the extent of activation at equilibrium can be substituted by the effect exerted by 6-phosphogluconate. Both homologous enzymes A aB a and A cB c showed a faster deactivation rate when the enzyme was activated in the absence of subunit B. The mechanism by which subunit B promotes activation seems to involve its effect on stabilizing the activated enzyme molecule. From studies on the K m for substrate CO 2 in the hybrid enzyme A aB c a major involvement of subunit B in influencing K m (CO 2) seems unlikely.