Thioredoxin activation of phosphoribulokinase in a bi-enzyme complex from Chlamydomonas reinhardtii chloroplasts

The activation of oxidized phosphoribulokinase either "free" or as part of a bi-enzyme complex by reduced thioredoxins during the enzyme reaction was studied. In the presence of reduced thioredoxin, the product of the reactio n catalyzed by phosphoribulokinase within the bi-enzyme complex does not ap pear in a linear fashion. It follows a mono-exponential pattern that sugges ts a slow dissociation process of the bi-enzyme complex in the assay cuvett e, A plot of the steady state of product appearance against thioredoxin con centration gave a sigmoid curve. On the basis of our experimental results, we propose a minimum model of the activation of phosphoribulokinase by redu ced thioredoxin. Reduced thioredoxin may act on the phosphoribulokinase, ei ther within the complex or in the dissociated metastable form, However, the time required to activate the enzyme as part of the complex is shorter (ab out 20 s) than that required to activate the dissociated form (about 10 min ). This might be of physiological relevance, and we discuss the role of the interactions between phosphoribulokinase and glyceraldehyde-3-phosphate de hydrogenase in the regulation of the Calvin cycle.