[The light-induced activation of NADP(+)-dependent glyceraldehyde-3-phosphate dehydrogenase : IX. The reaction in isolated chloroplasts]

Properties of a multimeric protein complex from chloroplasts possessing potential activities of NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase and phosphoribulokinase

A homogeneous multimeric protein isolated from the green alga, Scenedesmus obliquus, has both latent phosphoribulokinase activity and glyceraldehyde-3-phosphate dehydrogenase activity. The glyceraldehyde-3-phosphate dehydrogenase was active with both NADPH and NADH, but predominantly with NADH. Incubation with 20 mM dithiothreitol and 1 mM NADPH promoted the coactivation of phosphoribulokinase and NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase, accompanied by a decrease in the glyceraldehyde-3-phosphate dehydrogenase activity linked to NADH. The multimeric enzyme had a Mr of 560,000 and was of apparent subunit composition 8G6R. R represents a subunit of Mr 42,000 conferring phosphoribulokinase activity and G a subunit of 39,000 responsible for the glyceraldehyde-3-phosphate dehydrogenase activity. On SDS-PAGE the Mr-42,000 subunit comigrates with the subunit of the active form of phosphoribulokinase whereas that of Mr-39,000 corresponds to that of NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase. The multimeric enzyme had a S20,W of 14.2 S. Following activation with dithiothreitol and NADPH, sedimenting boundaries of 7.4 S and 4.4 S were formed due to the depolymerization of the multimeric protein to NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase (4G) and active phosphoribulokinase (2R). It has been possible to isolate these two enzymes from the activated preparation by DEAE-cellulose chromatography. Prolonged activation of the multimeric protein by dithiothreitol in the absence of nucleotide produced a single sedimenting boundary of 4.6 S, representing a mixture of the active form of phosphoribulokinase and an inactive dimeric form of glyceraldehyde-3-phosphate dehydrogenase. Algal thioredoxin, in the presence of 1 mM dithiothreitol and 1 mM NADPH, stimulated the depolymerization of the multimeric protein with resulting coactivation of phosphoribulokinase and NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase. Light-induced depolymerization of the multimeric protein, mediated by reduced thioredoxin, is postulated as the mechanism of light activation in vivo. Consistent with such a postulate is the presence of high concentrations of the active forms of phosphoribulokinase and NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase in extracts from photoheterotrophically grown algae. By contrast, in extracts from the dark-grown algae the multimeric enzyme predominates.

Algal glyceraldehyde-3-phosphate dehydrogenases. Conversion of the NADH-linked enzyme of Scenedesmus obliquus into a form which preferentially uses NADPH as coenzyme

Scenedesmus obliquus contains two glyceraldehyde-3-phosphate dehydrogenases (EC 1.2.1.-) one of which uses NADH as its preferred coenzyme (D-enzyme) and the other NADPH (T-enzyme). On incubation of the D-enzyme with cysteine and a 1,3-diphosphoglycerate-generating system the specific activity with NADH as coenzyme decreased whilst that with NADPH increased by a factor of 10. The components of the generating system had no effect on the D-enzyme individually and it is concluded that 1,3-diphosphoglycerate was probably responsible for the change in nucleotide specificity. The coenzyme specificity of the T-enzyme was not affected by such treatment. A similar type of activation occurred to a lesser extent on incubation of the D-enzyme with 2,3-diphosphoglycerate. The NADPH-dependent activity of the D-enzyme could also be promoted by incubation with NADPH. However, in this case the activation was less than that seen with either 1,3- or 2,3-diphosphoglycerate. The change in coenzyme specificity of the D-enzyme occurred in parallel with changes in sedimentation behaviour. Initially, a single boundary of S20,w equals 14.5 S was present, but on conversion to NADPH-dependent activity by incubation with the 1,3-diphosphoglycerate-generating system, new boundaries of 7.5 S and 5.5 S appeared. The first of these corresponds in sedimentation coefficient to the native T-enzyme. On removal of 1,3-diphosphoglycerate the 7.5 S boundary disappeared accompanied by an increase in that of 14.5 S, whilst the 5.5 S boundary persisted. These changes are consistent with the reversible conversion of the D-enzyme into a form similar to the native T-enzyme in response to cysteine and 1,3-diphosphoglycerate. These effects may be explained if acylation of the active site of the D-enzyme by 1,3-diphosphoglycerate results in displacement of the bound nucleotide, thus promoting nucleotide exchange. These findings are consistent with the kinetic mechanism established for other glyceraldehyde-3-phosphate dehydrogenases. Similar activation was seen in extracts of other species of the Chlorophyta but not in other photosynthetic organisms. The significance of this type of activation of enzyme activity to the metabolism of these species of algae is discussed.

The reversible depolymerization of spinach chloroplast glyceraldehyde-phosphate dehydrogenase. Interaction with nucleotides and dithiothreitol

The ligand-dependent dissociation of spinach chloroplast glyceraldehyde-phosphate dehydrogenase (Mr 600000) to protomers of Mr about 145000, previously shown by us in 1973, has been further characterized by the technique of velocity sedimentation in sucrose gradients. The process exhibits cooperativity and is accompanied by an increase of the apparent NADP+-dependent activity (reactivation) from a ratio of 0.1-0.2 to a ratio of 1 to 2 with respect to the NAD+-dependent activity. In addition to NADP+ and NADPH, most nucleotide triphosphates and, to some extent, Pi, act as dissociating agents. The enzyme is depolymerized and progressively inactivated in the presence of 2'-AMP. 2. Incubation with 20 mM dithiothreitol or 8-10 mM GTP increases the apparent NADP/H)-dependent activity, although addition of a small amount of a dissociating compound, such as 0.06 mM NADP+, is required for depolymerization. 3. NAD+, NADH and, to a lesser extent, glyceraldehyde 3-phosphate, NMN and cyclic AMP act as inhibitors of the dissociation and reactivation, however induced. They also favour the reassociation of protomers to tetramers. 4. The NADP(H)-linked activity is probably a property of the protomers only. The system described here resembles in many respects the light-dependent regulation of the NADP(H)-linked activity in vivo.

The effect of SO 3 (--) on the activity of ribulose-1,5-diphosphate carboxylase in isolated spinach chloroplasts

SO 3 (--) inhibits the activity of ribulose-1,5-diphosphate carboxylase in isolated spinach chloroplasts. It shows a non-competitive inhibition pattern with respect to ribulose-1,5-diphosphate and Mg(++) but a competitive one with respect to HCO 3 (-) . The K i -values are 14 mM SO 3 (--) and 9.5 mM SO 3 (-) respectively for the non-competitive inhibition but only 3.0 mM SO 3 (--) in the case of competitive inhibition with HCO 3 (--) as a substrate. Thus it is concluded that the competitive inhibition type will predominate at low SO 3 (--) and low internal CO2 concentrations.

Red-light-enhanced conversion of tritiated gibberellin A9 into other gibberellin-like substances in homogenates of etiolated barley leaves

Irradiation of homogenates of etiolated barley leaves with red light resulted in an increase in the levels of gibberellin (GA)-like substances as compared to dark controls. When homogenates were fed with [(3)H]-GA9 there was as incorporation of the radioactivity into a number of other GA's: this process occurred to a greater extent in red light than in darkness, and could be inhibited by boiling the extract prior to addition of the [(3)H]-GA9.

[Differences in the light-activation of NADP-dependent glyceraldehyde-3-phosphate dehydrogenase and of ribulose-5-phosphate kinase between plants containing the Calvin and those containing the C4-dicarboxylic acid pathway of photosynthetic carbon reduction]

1. Preceding experiments had shown that irradiance of intact leaves or of isolated chloroplasts causes a reversible increase in the activity of NADP-GPD (Ziegler and Ziegler, 1965) as well as of ribulose-5-phosphate kinase (Latzko and Gibbs, 1969). Examination of several species which carry out the Calvin type of photosynthetic CO2 fixation (Vicia faba, Spinacia oleracea, Nicotiana tabacum, Avena sativa) now revealed that the dark level of NADP-GPD activity ranges between 300-400 μmol NADPH/mg chlorophyll·h; irradiance causes an activation to an turnover rate of 900-1600 μmol NADPH/mg chlorophyll·h. 2. The dark-level of ribulose-5-phosphate kinase in these Calvin type plants corresponds to about 400 \gmmol PO4---/mg chlorophyll\sdh. It rises to 900\2-1300 \gmmol PO4---/mg chlorophyll\sdh after irradiance. 3. In all species examined which carry out the C4-dicarboxylic acid type of CO2 fixation (Zea mays, Cyperus rotundus, Portulacca oleracea, Saccharum officinarum) the dark-level of NADP-GPD as well as of ribulose-5-phosphate kinase is already as high as the light-level of Calvin type plants. In these species irradiance either activates both enzymes only to a small extent (Saccharum officinarum, Portulacea oleracea) or it activates only one of the two enzymes to an exceptional high activity (NADP-GPD in Zea mays, ribulose-5-phosphate kinase in Cyperus rotundus), while the activity of the other one remains nearly constant. 4. The dark-level of NADP-GPD in young Zea mays (2 leaves expanded) is as high as in adult plants; moreover its further activation by light corresponds to that in adult plants. In contrast, the dark-activity of ribulose-5-phosphate kinase in young Zea mays corresponds to the lower level found in Calvin type plants and is activated by irradiance in the same manner as it is in the latter plants. 5. The activity of ribose-5-phosphate isomerase is not influenced by light.

[The light-induced activation of NADP(+)-dependent glyceraldehyde-3-phosphate dehydrogenase : XI. The kinetic of the reversible reaction in lemna gibba]

1. The increase in the activity of the NADP(+)-dependent glyceraldehyde-3-phosphate dehydrogenase by irradiance and its reversal by darkness were measured. A T-values (activity after irradiance: activity in the dark after complete loss of activation) were plotted against increasing time of irradiation as well as against increasing time of darkening. 2. The A T-values for irradiance as well as for darkening yield sigmoidal types of curves, and are similar to those corresponding to the integral of a curve of the type y=e (-x) (2)(Gauss-curve). The (theoretical) sigmoidal curve was transformed (by double variable transformation) to optimally cover the experimental data. By differentiation Gauss-curves are obtained which express the change of enzyme activity per unit of time. The maximal points of these curves correspond to the half maximum of activation (not activity!) and to the highest change of activity per unit of time. 3. Figs. 3, 4 and 5 show that the increase in activity after onset of irradiation starts earlier than does the decrease after onset of darkening. Conditions which may cause the preservation of the activated enzyme during about 100 sec after darkening are discussed. However, the decrease is much more rapid later on than is the increase in the light; the change in activity (change of A T) per unit of time is smaller in the activation process than it is in the inactivation process. Thus the Gauss-curve becomes much sharper in the latter case. 4. Constant irradiance (irradiation with constant E eduring a given time period) at different points of dark-inactivation causes an activation velocity which increases with the time of the preceding dark period (Fig. 6). Therefore the Gauss-curve becomes sharper with increasing time of the preceding dark period (Fig. 7). 5. Increasing irradiance (irradiation with constant E eduring an increasing time period) at 3 different points of inactivation by variation of the time of predarkening also results in an increase in activation velocity with increase of time of predarkening (Figs. 8 and 9). 6. The model which best fits earlier experimental results as well as the kinetic studies is one in which activation is accomplished by association of four subunits of the enzyme (E o) to the activated tetrameric form (E akt). NADPH, created during the non-cyclic electron flow, could be the allosteric effector. Thus the activation velocity (change in activity per unit of time) is dependent on the concentration of E o. Both increase with increasing time of darkness. In contrast the time period of activation at constant irradiation is independent of the concentration of E o and is only dependent on the amount of the effector (NADPH), which increases with increasing time (increasing irradiance).