Enhancement of the reductive activation of chloroplast fructose-1,6-bisphosphatase by modulators and protein perturbants

Plastidic phosphoglycerate kinase from Phaeodactylum tricornutum: on the critical role of cysteine residues for the enzyme function

Chloroplastidic phosphoglycerate kinase (PGKase) plays a key role in photosynthetic organisms, catalyzing a key step in the Calvin cycle. We performed the molecular cloning of the gene encoding chloroplastidic PGKase-1 in the diatom Phaeodactylum tricornutum. The recombinant enzyme was expressed in Escherichia coli, purified and characterized. Afterward, it showed similar kinetic properties than the enzyme studied from other organisms, although the diatom enzyme displayed distinctive responses to sulfhydryl reagents. The activity of the enzyme was found to be dependent on the redox status in the environment, determined by different compounds, including some of physiological function. Treatment with oxidant agents, such as diamide, hydrogen peroxide, glutathione and sodium nitroprusside resulted in enzyme inhibition. Recovery of activity was possible by subsequent incubation with reducing reagents such as dithiothreitol and thioredoxins (from E. coli and P. tricornutum). We determined two midpoint potentials of different regulatory redox centers, both values indicating that PGKase-1 might be sensitive to changes in the intracellular redox environment. The role of all the six Cys residues found in the diatom enzyme was analyzed by molecular modeling and site-directed mutagenesis. Results suggest key regulatory properties for P. tricornutum PGKase-1, which could be relevant for the functioning of photosynthetic carbon metabolism in diatoms.

Non-reductive modulation of chloroplast fructose-1,6-bisphosphatase by 2-Cys peroxiredoxin

2-Cys peroxiredoxin (2-Cys Prx) is a large group of proteins that participate in cell proliferation, differentiation, apoptosis, and photosynthesis. In the prevailing view, this ubiquitous peroxidase poises the concentration of H2O2 and, in so doing, regulates signal transduction pathways or protects macromolecules against oxidative damage. Here, we describe the first purification of 2-Cys Prx from higher plants and subsequently we show that the native and the recombinant forms of rapeseed leaves stimulate the activity of chloroplast fructose-1,6-bisphosphatase (CFBPase), a key enzyme of the photosynthetic CO2 assimilation. The absence of reductants, the strict requirement of both fructose 1,6-bisphosphate and Ca2+, and the response of single mutants C174S and C179S CFBPase bring forward clear differences with the well-known stimulation mediated by reduced thioredoxin via the regulatory 170's loop of CFBPase. Taken together, these findings provide an unprecedented insight into chloroplast enzyme regulation wherein both 2-Cys Prx and the 170's loop of CFBPase exhibit novel functions.

The oxygen evolving enhancer protein 1 (OEE) of photosystem II in green algae exhibits thioredoxin activity

A thioredoxin-like chloroplast protein of the fructosebisphosphatase-stimulating f-type, but with an unusually high molecular mass of 28 kDa has previously been identified and purified to homogeneity in a fractionation scheme for resolution of the acid- and heat-stable, regular-size (12kDa) thioredoxins of the unicellular green algae, Scenedesmus obliquus. An apparently analogous protein of 26 kDa was described in a cyanobacterium, Anabaena sp., but no such large thioredoxin species f exists in the thioredoxin profiles of higher plants. The structure of the 28 kDa protein, which had been envisaged to represent a precursor, or fusion product of the two more specialized, common chloroplast thioredoxins f and m has now been determined by amino acid sequencing. Although it exhibits virtually all the properties and enzyme-modulating activities of a thioredoxin proper this algal protein, surprisingly, does not belong to the thioredoxin family of small redox proteins but is identical with OEE (oxygen evolving enhancer) protein 1, an auxiliary component of the photosystem II manganese cluster. Extracts of Chlorella vulgaris and Chlamydomonas reinhardtii also contain heat-stable protein fractions of 23-26 kDa capable of specifically stimulating chloroplast fructosebisphosphatase in vitro. In contrast, OEE protein 1 from spinach is not able to modulate FbPase or NADP malate dehydrogenase from spinach chloroplasts. A dual function of the OEE protein in algal photosynthesis is envisaged.

ADP-glucose pyrophosphorylase from potato tuber: site-directed mutagenesis of homologous aspartic acid residues in the small and large subunits

Asp142 in the homotetrameric ADP-glucose pyrophosphorylase (ADP-Glc PPase) enzyme from Escherichia coli was demonstrated to be involved in catalysis of this enzyme [Frueauf, J.B., Ballicora, M.A. and Preiss J. (2001) J. Biol. Chem., 276, 46319-46325]. The residue is highly conserved throughout the family of ADP-Glc PPases, as well as throughout the super-family of sugar-nucleotide pyrophosphorylases. In the heterotetrameric ADP-Glc PPase from potato (Solanum tuberosum L.) tuber, the homologous residue is present in both the small (Asp145) and the large (Asp160) subunits. It has been proposed that the small subunit of plant ADP-Glc PPases is catalytic, while the large subunit is modulatory; however, no catalytic residues have been identified. To investigate the function of these conserved Asp residues in the ADP-Glc PPase from potato tuber, we used site-directed mutagenesis to introduce either an Asn or a Glu. Kinetic analysis in the direction of synthesis or pyrophosphorolysis of ADP-Glc showed a significant decrease (more than four orders of magnitude) in the specific activity of the SD145NLwt, SD145NLD160N, and SD145NLD160E mutants, while the effect was smaller (approximately two orders of magnitude) with the SD145ELwt, SD145ELD160N, and SD145ELD160E mutants. By contrast, mutation of the large subunit alone did not affect the specific activity but did alter the apparent affinity for the activator 3-phosphoglycerate, showing two types of apparent roles for this residue in the different subunits. These results show that mutation of Asp160 of the large subunit does not affect catalysis, thus the large subunit is not catalytic, and that the negative charge of Asp145 in the small subunit is necessary for enzyme catalysis.

Alteration of inhibitor selectivity by site-directed mutagenesis of Arg(294) in the ADP-glucose pyrophosphorylase from Anabaena PCC 7120

Previous alanine scanning mutagenesis of ADP-glucose pyrophosphorylase from Anabaena PCC 7120 indicated that Arg(294) plays a role in inhibition by orthophosphate [J. Sheng, J. Preiss, Biochemistry 36 (1997) 13077]. In this study, analysis of several site-directed mutants in the presence of different metabolic effectors showed that the primary inhibitor for two of the mutant proteins, R294A and R294Q, was no longer orthophosphate but rather NADPH, which was a reversal in the pattern of inhibitor selectivity from the wild-type. Despite the differences in charge and size, analysis of the purified R294K, R294E, and R294Q mutant enzymes demonstrated similar decreases in orthophosphate affinity as the R294A mutant, while most of the other kinetic values were similar to those reported for the wild-type. All these results suggest that the positive charge of Arg(294) is not specifically involved in orthophosphate binding and that it is important in determining inhibitor selectivity.

Rapeseed chloroplast thioredoxin-m. Modulation of the affinity for target proteins

The stroma of higher plant chloroplasts contains two thioredoxins (Trx) with different specificity for the reduction of protein disulfide bonds. Based upon electrostatic features of domains that participate in the thiol/disulfide exchange, we prepared mutants of rapeseed Trx-m bearing opposite charges at a single position and subsequently analyzed their action on the activation of rapeseed chloroplast fructose 1,6-phosphate (CFBPase). The replacement of Pro-35 with lysine and glutamic residues enhanced and impaired, respectively, the stimulation of CFBPase relative to the wild-type and the P35A mutant. Furthermore, the shielding of electrostatic interactions with high concentrations of KCl greatly increased and concurrently made indistinguishable the affinity of all variants for CFBPase. The capacity to stimulate the enzyme activity likewise was enhanced concertedly by fructose-1,6-bisphosphate and Ca(2+) but, at variance with the action of KCl, remained sensitive to charges in the side chain of mutants. These results were consistent with a mechanism in which intermolecular electrostatic interactions and intramolecular non-covalent interactions control the formation of the non-covalent complex between reduced Trx and oxidized CFBPase and, in so doing, modulate the thiol/disulfide exchange.

Mechanism of reductive activation of potato tuber ADP-glucose pyrophosphorylase

The potato tuber (Solanum tuberosum L.) ADP-glucose pyrophosphorylase activity is activated by a incubation with ADP-glucose and dithiothreitol or by ATP, glucose- 1-phosphate, Ca2+, and dithiothreitol. The activation was accompanied by the appearance of new sulfhydryl groups as determined with 5, 5'-dithiobis(2-nitrobenzoic acid). By analyzing the activated and nonactivated enzymes on SDS-polyacrylamide gel electrophoresis under nonreducing conditions, it was found that an intermolecular disulfide bridge between the small subunits of the potato tuber enzyme was reduced during the activation. Further experiments showed that the activation was mediated via a slow reduction and subsequent rapid conformational change induced by ADP-glucose. The activation process could be reversed by oxidation with 5, 5'-dithiobis(2-nitrobenzoic acid). Incubation with ADP-glucose and dithiothreitol could reactivate the oxidized enzyme. Chemical modification experiments with [14C]iodoacetic acid and 4-vinylpyridine determined that the intermolecular disulfide bridge was located between Cys12 of the small subunits of the potato tuber enzyme. Mutation of Cys12 in the small subunit into either Ala or Ser eliminated the requirement of DTT on the activation and prevented the formation of the intermolecular disulfide of the potato tuber enzyme. The mutants had instantaneous activation rates as the wild-type in the reduced state. A two-step activation model is proposed.

Role of electrostatic interactions on the affinity of thioredoxin for target proteins. Recognition of chloroplast fructose-1, 6-bisphosphatase by mutant Escherichia coli thioredoxins

Chloroplast thioredoxin-f functions efficiently in the light-dependent activation of chloroplast fructose-1, 6-bisphosphatase by reducing a specific disulfide bond located at the negatively charged domain of the enzyme. Around the nucleophile cysteine of the active site (-W-C-G-P-C-), chloroplast thioredoxin-f shows lower density of negative charges than the inefficient modulator Escherichia coli thioredoxin. To examine the contribution of long range electrostatic interactions to the thiol/disulfide exchange between protein-disulfide oxidoreductases and target proteins, we constructed three variants of E. coli thioredoxin in which an acidic (Glu-30) and a neutral residue (Leu-94) were replaced by lysines. After purification to homogeneity, the reduction of the unique disulfide bond by NADPH via NADP-thioredoxin reductase proceeded at similar rates for all variants. However, the conversion of cysteine residues back to cystine depended on the target protein. Insulin and difluoresceinthiocarbamyl-insulin oxidized the sulfhydryl groups of E30K and E30K/L94K mutants more effectively than those of wild type and L94K counterparts. Moreover, the affinity of E30K, L94K, and E30K/L94K E. coli thioredoxin for chloroplast fructose-1,6-bisphosphatase (A0.5 = 9, 7, and 3 microM, respectively) increased with the number of positive charges, and was higher than wild type thioredoxin (A0.5 = 33 microM), though still lower than that of thioredoxin-f (A0.5 = 0.9 microM). We also demonstrated that shielding of electrostatic interactions with high salt concentrations not only brings the A0.5 for all bacterial variants to a limiting value of approximately 9 microM but also increases the A0.5 of chloroplast thioredoxin-f. While negatively charged chloroplast fructose-1,6-bisphosphatase (pI = 4.9) readily interacted with mutant thioredoxins, the reduction rate of rapeseed napin (pI = 11.2) diminished with the number of novel lysine residues. These findings suggest that the electrostatic interactions between thioredoxin and (some of) its target proteins controls the formation of the binary noncovalent complex needed for the subsequent thiol/disulfide exchange.

Characterization of cysteine residues involved in the reductive activation and the structural stability of rapeseed (Brassica napus) chloroplast fructose-1,6-bisphosphatase

In higher plants, light enhances the activity of chloroplast fructose-1,6-bisphosphatase via a cascade of thiol/disulfide exchanges. We have examined the structural and functional role of seven conserved cysteine residues in the rapeseed (Brassica napus) enzyme by site-directed mutagenesis. After lysis of Escherichia coli cells, C53S and C191S variants partitioned mainly in the insoluble fraction whereas C96S, C157S, C174S, C179S, and C307S mutants were soluble. Homogeneous preparations of the latter hydrolyzed fructose 1,6-bisphosphate at similar rates in the presence of 10 mM Mg2+ but only C157S, C174S and C179S mutants were both efficient catalysts at 1 mM Mg2+ and nearly insensitive to dithiothreitol. These results demonstrate the contribution of Cys53 and Cys191 to the stability of the enzyme and the participation of Cys157, Cys174 and Cys179 in the reductive process responsive of the light-dependent regulation. Given that mutations at Cys96 and Cys307 neither destabilize the enzyme nor affect the reductive modulation, their function remains unknown.

Chloroplast fructose-1,6-bisphosphatase: modification of non-covalent interactions promote the activation by chimeric Escherichia coli thioredoxins

Although all thioredoxins contain a highly conserved amino acid sequence responsible for thiol/disulfide exchanges, only chloroplast thioredoxin-f is effective in the reductive stimulation of chloroplast fructose-1,6-bisphosphatase. We set out to determine whether Escherichia coli thioredoxin becomes functional when selected modulators alter the conformation of the target enzyme. Wild type and chimeric Escherichia coli thioredoxins match the chloroplast counterpart when the activation of chloroplast fructose 1,6-biphosphatase is performed in the presence of fructose 1,6-bisphosphate, Ca2+, and either trichloroacetate or 2-propanol. These modulators of enzyme activity do change the conformation of chloroplast fructose-1,6-bisphosphatase whereas bacterial thioredoxins remain unaltered. Given that fructose 1,6-bisphosphate, Ca2+, and non-physiological perturbants modify non-covalent interactions of the protein but do not participate in redox reactions, these results strongly suggest that the conformation of the target enzyme regulates the rate of thiol/disulfide exchanges catalyzed by protein disulfide oxidoreductases.

Completion of the thioredoxin reaction mechanism: kinetic evidence for protein complexes between thioredoxin and fructose 1,6-bisphosphatase

The activation of chloroplast fructose 1,6-bisphosphatase from spinach and soybean leaves by the two chloroplast thioredoxins isolated from the same plants has been studied. The thioredoxin saturation characteristics (Vmax: 0.15-103.2 mumol Pi/min per mg enzyme; K0.5: 0.0048-0.84 microM; Hill coefficient n: 1.02-3.80) indicate that in addition to the reductive activation by thioredoxin specific complex formation between thioredoxin and fructose 1,6-bisphosphatase is responsible for fine regulation of the enzyme activity. This complex formation has been inserted into the thioredoxin mechanism and the physiological consequences discussed. Obviously, physiologically relevant investigations of the thioredoxin-dependent regulation of fructose 1,6-bisphosphatase activity can only be performed in homologous enzyme-thioredoxin combinations. Dithiothreitol and E. coli thioredoxin are no complete substitutes in regulatory studies.

High level expression in Escherichia coli, purification and properties of chloroplast fructose-1,6-bisphosphatase from rapeseed (Brassica napus) leaves

In chloroplasts, the light-modulated fructose-1,6-bisphosphatase catalyzes the formation of fructose 6-bisphosphate for the photosynthetic assimilation of CO2 and the biosynthesis of starch. We report here the construction of a plasmid for the production of chloroplast fructose-1,6-bisphosphatase in a bacterial system and the subsequent purification to homogeneity of the genetically engineered enzyme. To this end, a DNA sequence that coded for chloroplast fructose-1,6-bisphosphatase of rapeseed (Brassica napus) leaves was successively amplified by PCR, ligated into the Ndel/EcoRI restriction site of the expression vector pET22b, and introduced into Escherichia coli cells. When gene expression was induced by isopropyl-β-D-thiogalactopyranoside, supernatants of cell lysates were extremely active in the hydrolysis of fructose 1,6-bisphosphate. Partitioning bacterial soluble proteins by ammonium sulfate followed by anion exchange chromatography yielded 10 mg of homogeneous enzyme per 1 of culture. Congruent with a preparation devoid of contaminating proteins, the Edman degradation evinced an unique N-terminal amino acid sequence [A-V-A-A-D-A-T-A-E-T-K-P-]. Gel filtration experiments and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that the (recombinant) rapeseed chloroplast fructose-1,6-bisphosphatases was a tetramer [160 kDa] comprised of four identical subunits. Like other chloroplast fructose-1,6-bisphosphatases, the recombinant enzyme was inactive at 1 mM fructose 1,6-bisphosphate and 1 mM Mg(2+) but became fully active after an incubation in the presence of either 10 mM dithiothreitol or 1 mM dithiothreitol and chloroplast thioredoxin. However, at variance with counterparts isolated from higher plant leaves, the low activity observed in absence of reductants was not greatly enhanced by high concentrations of fructose 1,6-bisphosphate (3 mM) and Mg(2+) (10 mM). In the catalytic process, all chloroplast fructose-1,6-bisphosphatases had identical features; viz., the requirement of Mg(2+) as cofactor and the inhibition by Ca(2+). Thus, the procedure described here should prove useful for the structural and kinetic analysis of rapeseed chloroplast fructose-1,6-bisphosphatase in view that this enzyme was not isolated from leaves.