Properties of Phosphoribulokinase of Whole Chloroplasts

Abstract

The ability of intact spinach (Spinacia oleracea) chloroplast preparations to catalyze CO₂ fixation and photophosphorylation was examined. Under conditions optimal for CO₂ fixation, only poor photophosphorylation was observed. Conditions optimal for photophosphorylation were found to be highly inhibitory to the CO₂-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.