Regulation of the reductive pentose phosphate cycle

In contrast, the enzymes involved in the reduction of 3-PGA to triose phosphate together catalyse a freely reversible oxidation/reduction, the direction of which, in vivo, is largely determined by the levels of ATP and ADP, NADPH and NADP. In the light, with high levels of ATP and of NADPH the reactions proceed in the direction of triose phosphate driven by the production of 3-PGA and consumption of triose phosphate. In steady-state photosynthesis this provides for a coordination of the activity of parts of the cycle. Any component tending to increase the activity of PRK, for example, will cause the consumption of ATP and production of ADP. This in turn will slow the rate of 3-PGA reduction, leading to decreased synthesis of Rbu-5-P, bringing the cycle back into balance. 

A second novel mechanism of rubisco regulation involves a phosphorylated inhibitor of catalysis which can occupy the catalytic site of the enzyme. The discovery of this inhibitor [30,31] followed the observation that rubisco extracted from 

Phaseolus leaves in the light was significantly more active than from darkened leaves, despite optimal in vitro activation of the enzyme with CO2 and Mg. Several studies have shown that phosphorylated compounds can be effective inhibitors of rubisco in vitro. The results with Phaseolus, however, are the first to document the importance in vivo of a compound which is light-modulated and present in sufficient amounts to reduce dark enzyme activity to close to zero [22], 

Another mechanism of light-dependent enzyme activation has been proposed in which a membrane-bound dithiol-containing factor (light-effect mediator or LEM) reduced by the photosynthetic electron transport system reductively activates regulated enzymes in the chloroplast [28]. Certain facets of this mechanism may be identical to the ferredoxin/thioredoxin system while other aspects are still the subject of debate [18,33], 

In summary, current evidence [39-41] is thus consistent with the view that the ferredoxin/thioredoxin system functions in photosynthetically diverse types of plants as a master switch to restrict the activity of degradatory enzymes and activate biosynthetic enzymes in the light. It is significant that enzymes controlled by the ferredoxin/thioredoxin system (FBPase, SBPase, NADP-G3PDH, and PRK) function in the regenerative phase of the reductive pentose phosphate cycle that is needed to sustain its continued operation - i.e, to regenerate the carbon dioxide acceptor, Rbu-1,5-P2, from newly formed 3-PGA. It seems likely that one of these thioredoxin-linked enzymes limits the regeneration of Rbu-1,5-P2. 

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