Thioredoxin activation of phosphoribulokinase in a chloroplast multi-enzyme complex

The activation process of spinach phosphoribulokinase by thioredoxin f has been studied with the enzyme in a free, isolated state, or integrated in a multi-enzyme complex. The time periods required for enzyme activation are always smaller and the maximal enzyme velocities are always greater when chloroplast phosphoribulokinase is included in the multi-enzyme complex than when it is in the isolated state. Comparative kinetic studies show that phosphoribulokinase extracted from the complex behaves exactly as in the isolated state. The reduced form of the kinase, whatever it has been included in the complex or isolated from the chloroplasts, are deactivated by oxidized thioredoxins. In the absence of thioredoxin f however, the reduced form of the isolated enzyme undergoes spontaneous oxidation whereas the reduced kinase included in the multi-enzyme complex is stable. 'Unspecific' proteins such as bovine serum albumin do not provide any protection of the kinase against autooxidation, whereas 'homologous' specific proteins such as ribulose-1,5-bisphosphate carboxylase/oxygenase dramatically decrease the rate of this autooxidation process. These results therefore support the view that interactions between phosphoribulokinase and the other components of the multi-enzyme complex play an important role in the modulation of the activity of this enzyme. The possible part of these interactions in the control of the Calvin cycle is discussed.