LOCALIZATION AND CHARACTERIZATION OF CALVIN CYCLE ENZYMES IN CHROMATIUM STRAIN D

The three enzymes, ribose 5-phosphate isomerase, phosphoribulokinase, and ribulose 1,5-diphosphate carboxylase, which function sequentially in CO2 fixation via the Calvin cycle, are all released as soluble proteins from autotrophically grown cells of Chromatium. Breakage of cells by different methods, which resulted in differing degrees of membrane fragmentation, gave extracts in which the ratio of the activities was roughly constant. High-speed centrifugation of extracts showed no correlation between the sedimentation of the enzymes and of bacteriochlorophyllcontaining material. The three enzymes were found to migrate separately in sorbitol density gradients; little overlap of activities occurred. Molecular weights of the enzymes were calculated to be approximately 500,000 for the carboxylase, 240,000 for the kinase, and 54,000 for the isomerase. Better than 90% recovery of carboxylase and of isomerase was obtained on the gradients, while recovery of the kinase was 45-55%. Complete separation of the isomerase from carboxylase and kinase was obtained on Sephadex G-200. These data indicate that the enzymes responsible for autotrophic CO2 fixation in Chromatium exist as separate entities in cell extracts. Functional association is not detectable by the methods employed. © 1969, American Chemical Society. All rights reserved.