Changing Kinetic Properties of Fructose-1,6-bisphosphatase from Pea Chloroplasts during Photosynthetic Induction

A simple procedure for purifying the major chloroplast fructose-1,6-bisphosphatase from spinach (Spinacia oleracea) and characterization of its stimulation by sub-femtomolar mercuric ions

A rapid procedure for the purification of the redox-regulated chloroplast fructose-1,6-bisphosphatase [EC 3.1.3.11] from spinach leaf extract to homogeneity is described. No thiol-reducing agents were present during the purification and the enzyme is > 99% in the oxidized form. A rapid procedure to reduce and activate the Fru-1,6-P2ase by dithiothreitol in the absence of thioredoxin is described. Reduction activates the enzyme up to several hundred-fold when assayed at pH 8.0 with 2 mM Mg2+. The activity of the purified oxidized enzyme is unusually sensitive to changes in Mg2+ and H+ concentration. Tenfold changes in Mg2+ or H+ concentration lead to > 100-fold increases in activity. The recoveries of fructose-1,6-bisphosphatase activity as determined by the activity of the oxidized enzyme at pH 8.0/20 mM Mg2+; pH 9.0/2 mM Mg2+; pH 8/2 mM Mg2+ plus 0.1 mM Hg(II) or of the reduced enzyme at pH 8.0/2 mM Mg2+ are similar (approximately 40%) indicating that the major proportion of these activities in a leaf extract is catalyzed by the same enzyme. Moreover, antibodies raised against the purified enzyme inhibit all of the above activities in crude leaf extracts. The kinetic properties of the purified enzyme suggest that the oxidized Mg(2+)-dependent enzyme can play no significant role in photosynthetic carbon assimilation. A survey of some kinetic properties of Fru-1,6-P2ase activity in extracts of various photosynthetic organisms reveals that all 11 species examined possess a redox- and pH/Mg(2+)-stimulated Fru-1,6-P2ase, whereas Fru-1,6-P2ase in extracts of Taxus baccata (a gymnosperm), Chlorella vulgaris (a green alga), and the cyanobacterium Nostoc muscorum were not activated by Hg(II). The heat stability that proved useful in the purification of the spinach enzyme was conserved in both angiosperms and gymnosperms. The oxidized enzyme (which normally has no thiol groups accessible to 5,5'-dithio-bis[2-nitrobenzoic acid]) but not the reduced enzyme can be stimulated many hundred-fold by addition of extraordinarily low concentrations of Hg(II) to a complete assay mixture. With the aid of EDTA as a Hg(II) buffer, half-maximal stimulation was achieved at 2 x 10(-16) M free Hg(II). Methylmercury also stimulates the enzyme many hundred-fold at very low concentrations. The concentration for half-maximal stimulation by methylmercury was determined with a cyanide buffer to be approximately 10(-16) M. This, together with the high affinity of the enzyme for Hg(II), suggests that Hg(II) stimulates the enzyme by binding to an enzyme thiol group that be comes exposed in the catalytically active enzyme, thereby stabilizing the oxidized enzyme in an active conformation. By contrast, in the absence of Fru-1,6-P2 and either Mg2+ or Ca2+, Hg(II) (even at 2 x 10(-16) M) rapidly inactivates the oxidized Fru-1,6-P2ase. This inactivation is similar to the inactivation of Fru-1,6-P2ase that occurred at high pH (> 9) and which is also prevented by Fru-1,6-P2 and either Mg2+ or Ca2+. Although the Hg(II)- and high pH-inactivated oxidized enzyme has no activity, both forms of the enzyme can be activated by reduction. The usefulness of buffers to maintain low, defined Hg(II) and organic mercurial concentrations is discussed.

Changing Activity of Glucose-6-Phosphate Dehydrogenase from Pea Chloroplasts during Photosynthetic Induction

Light inactivation of glucose 6-phosphate dehydrogenase is rapid and occurs before photosynthetic O(2) evolution is measureable in intact chloroplasts. Likewise, dark activation is rapid. The major light induced change in the kinetic parameters of glucose 6-phosphate dehydrogenase is in maximal velocity.

The photosynthetic induction response in wheat leaves: net CO2 uptake, enzyme activation, and leaf metabolites

The photosynthetic induction response was studied in whole leaves of wheat (Triticum aestivum L.) following 5-min, 30-min and 10-h dark periods. After the 5-min dark treatment there was a rapid burst in the rate of photosynthesis upon illumination (half of maximum after 30s), followed by a slight decrease after 1.5 more min and then a gradual rise to the maximum rate. During this initial burst in photosynthesis, there was a rapid rise in the level of 3-phosphoglycerate (PGA) and a high PGA/triose-phosphate (triose-P) ratio was obtained. In addition, after the 5-min dark treatment, ribulose-1,5-bisphosphate carboxylase (Rubisco, EC 4.1.1.39), ribulose-5-phosphate kinase (EC 2.7.1.19) and chloroplastic fructose-1,6-bisphosphatase (EC 3.1.3.11) maintained a relatively high state of activation, and maximum activation occurred within 1 min of illumination. The results indicate there is a high capacity for CO2 fixation in the cycle upon illumination but attaining maximum rates requires an increase in the ribulose-1,5-bisphosphate (RuBP) pool (adjustment in triose-P utilization for carbohydrate synthesis versus RuBP synthesis). With both the 30-min and 10-h dark pretreatments there was only a slight rise in photosynthesis upon illumination, followed by a lag, then a gradual increase to steady-state (half-maximum rate after 6 min). In contrast to the 5-min dark treatment, the level of PGA was low and actually decreased initially, whereas the level of RuBP increased and was high during induction, indicating that Rubisco is limiting. This regulation via the carboxylase was not reflected in the initial extractable activity, which reached a maximum by 1 min after illumination. The light activation of chloroplastic fructose-1,6-bisphosphatase in leaves darkened for 30 min and 10 h prior to illumination was relatively slow (reaching a maximum after 8 min). However, this was not considered to limit carbon flux through the carbon-fixation cycle during induction since RuBP was not limiting. When photosynthesis approached the maximum steady-state rate, a high PGA/triose-P ratio and a high PGA/RuBP ratio were obtained. This may allow a high rate of photosynthesis by producing a favorable mass-action ratio for the reductive phase (the conversion of PGA to triose phosphate) while stimulating starch and sucrose synthesis.

Photosynthetic carbon metabolism in isolated pea chloroplasts: metabolite levels and enzyme activities

We report here that enzyme activation precedes the rise in metabolite levels, which appear to limit photosynthetic CO2 fixation during induction in pea leaf chloroplasts. Therefore light activation may be required for the build-up of photosynthetic intermediates and hence for photosynthesis in isolated chloroplasts. Analysis of metabolite levels and the known kinetic properties of the chloroplast enzymes indicates that the reductive pentose phosphate cycle is subject to control which fluctuates between several points during induction and when CO2 fixation is maximal. The transketolase-aldolase-catalyzed reactions around sedoheptulose-biphosphatase appear to provide a simple and effective primary control for photosynthetic CO2 fixation. When substrate levels and enzyme active site concentrations are taken into account, there is insufficient glyceraldehyde 3-phosphate dehydrogenase, aldolase, and transketolase activity to support photosynthetic CO2 fixation at observed rates. These results suggest that there may be direct transfer of glyceraldehyde 3-phosphate among these enzymes in the pea chloroplast.

Effects of Arsenite, Sulfite, and Sulfate on Photosynthetic Carbon Metabolism in Isolated Pea (Pisum sativum L., cv Little Marvel) Chloroplasts

Photosynthetic CO(2)-fixation in isolated pea (Pisum sativum L., cv Little Marvel) chloroplasts during induction is markedly inhibited by 0.4 millimolar sulfite. Sulfate at the same concentration has almost no effect. The (14)CO(2)-fixation pattern indicates that the primary effect of sulfite is inhibition of the reaction catalyzed by ribulose bisphosphate carboxylase and a stimulation of export of intermediates out of the chloroplasts. Inhibition of light modulation of stromal enzyme activity does not appear to account for the toxicity of SO(2) in this Pisum variety. Arsenite at 0.2 millimolar concentrations inhibits light activation and inhibits photosynthetic CO(2) fixation. The (14)CO(2)-fixation pattern indicates that the primary effect of arsenite is inhibition of light activation of reductive pentose phosphate pathway enzyme activity.