Catalytic properties of a hybrid between cyanobacterial large subunits and higher plant small subunits of ribulose bisphosphate carboxylase-oxygenase.

The small subunits of spinach ribulosebisphosphate carboxylase-oxygenase were isolated by mild acid precipitation of the hexadecameric holoenzyme. About one-third of the small subunits remained in the supernatant while the remainder, and all of the large subunits, were precipitated and irreversibly denatured. The spinach small subunits were able to reassemble with the large subunit octamer of ribulosebisphosphate carboxylase-oxygenase from the cyanobacterium, Synechococcus ACMM 323, prepared as described previously (Andrews, T. J., and Ballment, B. (1983) J. Biol. Chem. 258, 7514-7518) to produce a catalytically active, hybrid enzyme. The heterologous small subunits bound an order of magnitude less tightly than homologous small subunits and the specific activity of the hybrid, when fully saturated with foreign small subunits, was about half that of the homologously reassembled or native Synechococcus enzyme. In addition, the Km(CO2) of the hybrid was about twice as high. However, the degree of partitioning between carboxylation and oxygenation was identical for the hybrid, the homologously reassembled, and the native Synechococcus enzymes and clearly less in favor of carboxylation than partitioning by the spinach enzyme. Therefore, this important facet of catalysis by ribulosebisphosphate carboxylase-oxygenase appears to be specified exclusively by the large subunit.