The Malate Valve: Flux Control at the Enzymatic Level

Mehler Reaction: Friend or Foe in Photosynthesis?

“…1 ). It is also possible that activation of the malate valve (Scheibe and Beck, 1994) or increased carbohydrate degradation might increase the supply of NADH to monodehydroascorbate radical reductase and thus affect the redox state of ascorbate in other subcellular compartments. Enhanced availability of hexoses, which are precursors for ascorbate production (Foyer, 1993), might furthermore stimulate ascorbate synthesis (cr.…”

“…By combining these techniques, it is possible to obtain the differences between total photosynthetic electron production on one hand and electron consumption by CO z fixation on the other hand (for references see DiMarco et al, 1990;Cornic and Briantais, 1991). If photorespiration, which is normally a considerable electron sink, is blocked at low Oz concentrations, the magnitude of the sum of alternative electron sinks such as the Mehler reaction, N-and S-assimilation and the malate valve (Robinson, 1988;Scheibe and Beck, 1994), can be assessed. Using these in situ techniques it was found that the utilisation of electrons in alternative pathways did not contribute notably to total photosynthetic electron flux in wheat leaves under non-photorespiratory conditions (Loreto et al, 1994).…”

Torn Between Nature and Lab: A Dying Breed of Plant Scientists?

“…Interestingly, a non-photosynthetic enzyme, NADPdependent malate dehydrogenase showed strong activation by light (Scheibe and Beck 1979). It could prevent over-reduction of NADP by consuming reduction equivalents in the presence of oxaloacetate and was therefore termed "malate valve" (see also Scheibe and Beck 1994). An activator protein, whose nature was unknown (now known as the thioredoxin system, see Sch€ urmann and Buchanan 2008), was concluded to effect light activation of chloroplastic enzymes.…”