STRUCTURE AND FUNCTION OF CHLOROPLAST PROTEINS .8. SOME PROPERTIES OF RIBULOSE-1,5-DIPHOSPHATE CARBOXYLASE OF ATHIORHODACEAE IN COMPARISON WITH THOSE OF PLANT ENZYME

Ribulose-1,5-diphosphate carboxylase isolated from the heterotrophically grown photosynthetic bacteria, Rhodospirillum rubrum, and Rhodopseudomonas spheroides, were partially purified by (a) Sephadex gel filtration, (b) chromatography on DEAE-cellulose columns, and (c) Mg2+ heat precipitation. The molecular weights of the enzymes were approximately 6.8 × 104 and 2.6 × 105 for R. rubrum and R. spheroides, respectively estimated from the exclusion volumes on columns of Sephadex G-200. Both enzymes exhibited anomalous, non-Michaelian-type reaction curves with respect to NaHCO3 concentrations, characteristic of plant carboxylases. However, Mg2+ did not appreciably promote the enzyme reactions unlike the plant enzyme. In contrast to the plant enzyme also, the bacterial enzymes showed a typical sigmoidal reaction curve with respect to RuDP concentrations in the absence of Mg2+. The addition of Mg2+ normalized the reaction kinetics to the hyperbolic curve. The addition of Mg2+ did not cause a shift in the pH optimum of the bacterial carboxylase reaction, although its addition markedly stabilized the enzyme protein as demonstrable from the enzyme activity vs. temperature curve. Prior treatment of R. rubrum enzyme with rabbit anti-spinach RuDP carboxylase serum did not cause any inhibition on the bacterial carboxylation reaction. © 1969.