Properties of Phosphoribulokinase of Whole Chloroplasts

The ability of intact spinach (Spinacia oleracea) chloroplast preparations to catalyze CO(2) fixation and photophosphorylation was examined. Under conditions optimal for CO(2) fixation, only poor photophosphorylation was observed. Conditions optimal for photophosphorylation were found to be highly inhibitory to the CO(2)-fixing capacity of the intact chloroplast preparation.A method for following the activity of phosphoribulokinase in the intact chloroplast preparation was developed, and conditions for optimal activity were defined. The enzyme was found to be activated 2- to 4-fold by preillumination with a half-time of less than 15 seconds. Activation was inhibited by magnesium ions and selectively by inhibitors of photosynthetic electron transport. We concluded that activation was due to the effect of a photoproduced reductant in a site preceding ferredoxin in the electron transport chain. The photoactivated state of the enzyme decayed in the dark with a half-time of about 8 minutes.