Plastidic glucose-6-phosphate dehydrogenases are regulated to maintain activity in the light

Abstract

Glucose-6-phosphate dehydrogenase (G6PDH) can initiate the glucose 6-phosphate (G6P) shunt around the Calvin-Benson cycle. In order to understand the regulation of flux through this pathway, we have characterized the biochemical parameters and redox regulation of the three functional plastidic isoforms of Arabidopsis G6PDH. When purified, recombinant proteins were measured, all three exhibited significant substrate inhibition by G6P but not NADP+, making the determination of enzyme kinetic parameters complex. We found that the half saturation concentration of G6PDH isoform 1 is increased under reducing conditions. The other two isoforms exhibit less redox regulation, however, isoform 2 is strongly inhibited by NADPH. Redox regulation of G6PDH1 can be partially reversed by hydrogen peroxide or protected against by presence of its substrate, G6P. Overall, our results support the conclusion that G6PDH can have significant activity throughout the day and can be dynamically regulated to allow or prevent flux through the glucose 6-phosphate shunt.

Transcriptional Regulation of the Glucose-6-Phosphate/Phosphate Translocator 2 Is Related to Carbon Exchange Across the Chloroplast Envelope

“…The import of G6P via GPT2 could be problematic because it could also be acted on by glucose-6-phosphate dehydrogenase (G6PDH) in the chloroplast (Preiser et al, 2019). G6PDH is generally considered to be deactivated in the light when the chloroplast stroma is reduced.…”

Source of 12C in Calvin–Benson cycle intermediates and isoprene emitted from plant leaves fed with 13CO2

“…In this model, some part of the cytosolic G6P may be acted on by G6PDH and eventually reenter the CBC as either Ru5P or refixed CO 2 diluting the label in the CBC. The oxidative pentose phosphate pathway should be active, at least to some degree in the light [57].…”

“…In response to stress and in CBC enzyme loss-of-function mutants, a plastidial shunt also occurs. The plastidial shunt may be regulated by stromal G6P concentration and/or H 2 O 2 [57]. Both the plastidial and cytosolic shunts may lead to high cyclic electron flow rates [72].…”

“…We do not think this is a result of changes in gene expression or protein synthesis but rather that stress stimulates the activity of plastidial G6PDH, which is normally not active in the light. We have recently shown that plastidial G6PDH can be activated in the light, for example by the accumulation of H 2 O 2 [57], causing the plastidial shunt. However, this is not a source of 12 C since carbon going through the plastidial shunt will start with a degree of label the same as what gets reinjected into the CBC.…”

“…The large ATP deficit at high temperature would require a significant rate of cyclic electron flow and it is well-known that high temperature stimulates cyclic electron flow [77][78][79][80][81]. The mechanism of this stimulation could involve an accumulation of H 2 O 2 , which is known to stimulate cyclic electron flow [82] and the plastidial G6PDH [57].…”