Effects of pH and fructose 2,6-bisphosphate on oxidized and reduced spinach chloroplastic fructose-1,6-bisphosphatase

Changing sugar partitioning in FBPase-manipulated plants

This review offers an overview of the current state of our knowledge concerning the role of fructose-1,6-bisphosphatase (FBPase) in sugar partitioning and biosynthesis, through the analysis of genetically manipulated plants. The existence of two well-characterized isoforms is a consequence of the subcellular compartmentalization of photosynthetic eukaryotes, conditioning their respective regulatory mechanisms and their influence over plant metabolism and photosynthesis. Both isoforms are important, as has been deduced from previous work with different plant species, although there is still much to be done in order to gain a definitive vision of this issue. Despite that, alteration of the FBPase content follows a general pattern, there are some differences that could be considered species-specific. Modifications lead to profound changes in the carbohydrate content and carbon allocation, raising questions as to whether flux of some sugars or sugar precursors from one side to the other of the chloroplast envelope occurs to rebalance carbohydrate metabolism or whether other compensatory, though not fully efficient, enzymatic activities come into play. Due to the pleiotropic nature of modifying the core carbon metabolism, an answer to the above questions would require an exhaustive study involving diverse aspects of plant physiology.

Activation of spinach chloroplast fructose-1,6-bisphosphatase by monovalent cations

Spinach chloroplast fructose 1,6-bisphosphatase, like its counterparts from animal sources, can be activated by monovalent cations such as potassium and ammonium ions. The extents of activation are closely related to the pH values and the concentrations of magnesium ions. The activation effect is most prominent when the concentrations of magnesium ions are high enough to exert inhibitory side effect on the enzyme. Similar to the cases of enzymes from mammalian tissues, activation of the chloroplast enzyme by monovalent cations is, at least partially, due to overcoming of the inhibitory effect by the excess of magnesium ions. The enzyme can also be activated by low concentrations of guanidine hydrochloride, which probably involves a similar mechanism compared with that by monovalent cations.

Enzyme kinetics by mid-infrared spectroscopy: beta-fructosidase study by a one-step assay

An alternate method for enzyme study is proposed. Multidimensional statistical analysis applied on mid-infrared attenuated total reflectance spectra (Cadet et al. (1991) Appl. Spectrosc. 42, 166-172) collected during a kinetic allows a direct and fast quantification of the remaining substrate, as well as a one step enzymatic assay. Furthermore, the combination of these techniques may be used as a structural tool. The method applied to the study of beta-fructosidase is developed in this paper as an example. With appropriate calibration, the method may be extend to any enzyme.

Characterization and kinetic properties of a soya-bean cell-wall phosphatase

A phosphatase from soya-bean cell walls was purified to homogeneity and characterized. It consists of two identical 70-kDa subunits linked by one or several disulphide bridges and, to our knowledge, it does not seem to require metal ions to be fully active. At high substrate concentrations, the enzyme was most efficient at slightly alkaline pH levels, which is at variance with the acid requirements of phosphatases previously established in other plant cell walls; whereas at low substrate concentrations it was more active at acid pH levels. The results of kinetic studies suggested that three ionizable groups of the protein might take part in the reaction. This enzyme is able to hydrolyse various phosphate esters, including PCho and nucleotides. Comparisons between the properties of the enzyme bound to the cell walls to those of the purified enzyme suggest that it is the most common, and perhaps the sole, phosphatase present in soya-bean cell walls.

Effect of temperature and pH on factor XIII from human placenta

1. The activation of the native enzyme was achieved by a proteolytic procedure involving thrombin. 2. The pH profile was independent of the nature of the substrates assayed (casein or dimethylcasein plus putrescine). The optimum pH was between 7.6 and 7.9 and the pK values were 6.5/7 and 8.7/9. A cysteinyl residue appeared to be involved in the pH-dependence activity. 3. In the presence of calcium, the thermostability of enzyme was increased: the temperature at which enzyme lost half of its activity increased up to 7 degrees C. 4. The kinetics of the thermal deactivation of F XIIIa depended on the presence or absence of calcium. 5. In its presence the reaction obeyed second order kinetics, while in its absence, the kinetics were of first order. In the first case, the irreversible thermal deactivation could be described by a two-step mechanism (N----X----D) while in the second case, the deactivation followed the simple model (N----D). 6. Neither divalent cations like Sr2+, Ba2+, Mg2+, nor bovine serum-albumin and polyhydric alcohols were able to increase the thermostability of F XIIIa. 7. Thermal deactivation of F XIIIa did not appear linked to the redox state of enzyme, nor to the modification of SH groups. 8. We observed a good correlation between the loss of activity and the unfolding of the polypeptide chain of F XIIIa during heating. 9. The optimum temperature of F XIIIa activity was 40 degrees C at pH 8 and 45 degrees C at pH 7.

Properties of oxidized and reduced spinach (Spinacia oleracea) chloroplast fructose-1,6-bisphosphatase activated by various agents

Fructose-1,6-bisphosphatase (FBPase) can be reduced and activated by either dithiothreitol or reduced thioredoxin. This activation is pH-dependent. An amino acid group with a pK value of 5.55 is involved in this process. Both enzyme forms can also be stimulated by agents such as fructose 1,6-bisphosphate, Mg2+, Ca2+ and Ca2+/fructose 1,6-bisphosphate. FBPase reduced by dithiothreitol is more strongly activated than the enzyme reduced by thioredoxin. The specificity constant (kcat./Km) is enhanced over 2.5-25-fold and 1.5-2-fold (depending on the agent used) for FBPase reduced by dithiothreitol and thioredoxin respectively. In both cases, no new kinetic properties appeared. The pH-activity profile of the stimulated enzyme is slightly shifted towards the acidic side with respect to the reduced enzyme. A lag phase is observed in the progress curve of both enzymic forms, treated or untreated. Each agent used to stimulate must induce a new conformation of the enzyme, more active than the initial one, characterized by a specificity constant and a relaxation time. This lag phase tends to disappear when the assay temperature is increased. Temperature has the same effect on the activity of oxidized, reduced and stimulated FBPase, but different effects on the stability of the different forms.

Lack of fructose-1,6-bisphosphatase in a range of higher plants that store starch

The aim of this work was to discover whether fructose-1,6-bisphosphatase (FBPase) is present in higher-plant cells that synthesize storage starch. The following were examined: suspension cultures of soybean (Glycine max), tubers of potato (Solanum tuberosum), florets of cauliflower (Brassica oleracea), developing endosperm of maize and of sweet corn (Zea mays), roots of pea (Pisum sativum), and the developing embryos of round and wrinkled varieties of pea. Unfractionated extracts of each tissue readily converted fructose 1,6-bisphosphate to fructose 6-phosphate in assays for both plastidic and cytosolic FBPase. These conversions were not inhibited by 20 microM-fructose 2,6-bisphosphate. Except in extracts of pea embryos and sweet-corn endosperm, treatment with affinity-purified antibodies to pyrophosphate: fructose-6-phosphate 1-phosphotransferase reduced the above fructose 6-phosphate production to the rate found with boiled extracts. The antibody-resistant activity from sweet corn was slight. In immunoblot analyses, antibody to plastidic FBPase did not react positively with any protein in extracts of soybean cells, potato tuber, cauliflower florets, maize endosperm and pea roots. Positive reactions were found for extracts of embryos of both round and wrinkled varieties of peas and endosperm of sweet corn. For pea embryos, but not for sweet-corn endosperm, the Mr of the recognized protein corresponded to that of plastidic FBPase. It is argued that soybean cells, potato tuber, cauliflower florets, maize (var. White Horse Tooth) endosperm and pea roots lack significant activity of plastidic FBPase, but that this enzyme is present in developing embryos of pea. The data for sweet corn (var. Golden Bantam) are not decisive. It is also argued that, where FBPase is absent, carbon for starch synthesis does not enter the amyloplast as triose phosphate.

Failure to corroborate claims that the developing endosperm of wheat (Triticum aestivum) contains significant activities of fructose-1,6-bisphosphatase

This work was done to test claims (Sangwan and Singh, Physiol. Plant. 73: 21-26) that the developing endosperm of wheat (Triticum aestivum L.) contains a cytosolic and a plastidic fructose- 1,6-bisphosphatase (EC 3.1.3.11; FBPase). Repetition of the procedure of Sangwan and Singh with extracts of developing endosperm of Triticum aestivum cv. Mercia produced two peaks of apparent FBPase activity on elution from DEAE-cellulose. Both peaks showed high activity of pyrophosphate:fructose-6-phos-phate 1-phosphotransferase [EC 2.7.1.90; PFK(PP(i) )]. The apparent FBPase activity in both peaks was stimulated by 20 μM fructose-2,6-bisphosphate and inhibited by antibodies to PFK(PP(i) ). Antibody to plastidic FBPase did not react positively in an immunoblot analysis with any protein of M(r) comparable to that of known FBPase in either peak. It is argued that the ability of each peak to convert fructose-1,6-bisphosphate to fructose-6-phosphate was due to PFK(PP(i) ). and that there remains no substantiated evidence for the presence of a plastidic FBPase in the developing endosperm of wheat.

Enzymic capacities of amyloplasts from wheat (Triticum aestivum) endosperm

Lysates of protoplasts from the endosperm of developing grains of wheat (Triticum aestivum) were fractionated on density gradients of Nycodenz to give amyloplasts. Enzyme distribution on the gradients suggested that: (i) starch synthase and ADP-glucose pyrophosphorylase are confined to the amyloplasts; (ii) pyrophosphate: fructose-6-phosphate 1-phosphotransferase and UDP-glucose pyrophosphorylase are confined to the cytosol; (iii) a significant proportion (23-45%) of each glycolytic enzyme, from phosphoglucomutase to pyruvate kinase inclusive, is in the amyloplast. Starch synthase, ADP-glucose pyrophosphorylase and each of the glycolytic enzymes showed appreciable latency when assayed in unfractionated lysates of protoplasts. No activity of fructose-1,6-bisphosphatase was found in amyloplasts or in homogenates of endosperm. Antibody to plastidic fructose-1,6-bisphosphatase did not react positively, in an immunoblot analysis, with any protein in extracts of wheat endosperm. It is argued that wheat endosperm lacks significant plastidic fructose-1,6-bisphosphatase and that carbon for starch synthesis does not enter the amyloplast as a C-3 compound but probably as hexose phosphate.

Enzymes as biosensors. 2. Hysteretic response of chloroplastic fructose-1,6-bisphosphatase to fructose 2,6-bisphosphate

Oxidized chloroplastic fructose-bisphosphatase is almost totally inactive at pH 7.5, that is under pH conditions that prevail in the chloroplast stroma. When preincubated for different time periods with fructose 2,6-bisphosphate and assayed in the absence of this ligand, it displays an activity which is directly related to the duration of the preincubation phase. This implies that fructose 2,6-bisphosphate induces enzyme conformers that appear in sequence and may be competent for catalytic activity. Upon desorption of fructose 2,6-bisphosphate the enzyme may retain its active conformation for a time period whose duration depends on magnesium concentration. It thus appears that reduction of the enzyme is not an obligatory prerequisite for its activity. Fructose 2,6-bisphosphate behaves as a competitive inhibitor of the reduced, active enzyme, with respect to the real substrate. When assayed with the oxidized enzyme, however, it behaves as an activator. Moreover the apparent steady-state rate that may be measured experimentally depends on both fructose 2,6-bisphosphate concentration and the direction of a concentration change. The reaction velocity experimentally measured is thus a meta-steady-state rate and depends on the initial conditions of the system. The fructose-bisphosphatase system thus displays, with respect to fructose 2,6-bisphosphate, a hysteresis loop and may then sense whether the concentration of that ligand is increased or decreased. A model has been proposed which allows one to explain these results. This model is based on the view that the substrate and fructose 2,6-bisphosphate compete for the same site of the enzyme and that this latter ligand stabilizes a conformation competent for enzyme activity. After the ligand has been chased away, the enzyme retains the active conformation for a while and slowly relapses to the initial inactive conformation. The time-scale of this slow relaxation overlaps that of the steady state of product appearance and this generates meta-steady-state kinetics, which is dependent on the initial state and therefore on the history of the system.

Spinach (Spinacia oleracea) chloroplast sedoheptulose-1,7-bisphosphatase. Activation and deactivation, and immunological relationship to fructose-1,6-bisphosphatase

In this paper we study activation by dithiothreitol and reduced thioredoxins and deactivation by oxidized thioredoxins f of sedoheptulose-1,7-bisphosphatase. The behaviour of the enzyme when chromatographed on a thioredoxin-Sepharose column is also described. The enzyme is autoxidizable upon removal of reducing agents, and is activated when reduced by any of the thioredoxins. This mechanism may allow the regulation of the Calvin cycle upon light-dark and dark-light transitions. The formation of a stable complex between enzyme and thioredoxin could explain the inhibitory effect of high thioredoxin concentrations. The use of immunological techniques shows that sedoheptulose-1,7-bisphosphatase and fructose-1,6-bisphosphatase are poorly related immunologically.

pH and kinetic studies of chloroplast sedoheptulose-1,7-bisphosphatase from spinach (Spinacia oleracea)

The aim of this paper is to study some steady-state kinetic properties of sedoheptulose-1,7-bisphosphatase, its pH-dependence and the effect of a substrate analogue, fructose 2,6-bisphosphate. Studies were carried out with sedoheptulose 1,7-bisphosphate and with fructose 1,6-bisphosphate, an alternative substrate. The pK values are identical for both substrates, and fructose 2,6-bisphosphate behaves like a competitive inhibitor. These results suggest that there exists a unique active site for either sedoheptulose 1,7-bisphosphate or fructose 1,6-bisphosphate on the enzyme molecule. Increasing Mg2+ concentrations shifted the optimum pH. As for fructose-1,6-bisphosphatase, we believe that this shift is due to the neutralization of negative charges near the active centre [Cadet, Meunier & Ferté (1987) Eur. J. Biochem. 162, 393-398]. The free species of sedoheptulose 1,7-bisphosphate and fructose 1,6-bisphosphate are not the usual substrates of enzyme, nor is Mg2+. But the kinetics relative to the (Mg2+-substrate4-)2- complex is not consistent with this complex being the substrate. An explanation of this discrepancy is proposed, involving both the negative charges near the active centre and the positive charges of Mg2+. The observed Vmax. of the reduced enzyme is 65% of the theoretical Vmax. for both substrates, but the observed Vmax. relative to sedoheptulose 1,7-bisphosphate is 3 times the one relative to fructose 1,6-bisphosphate. The specificity constant (kcat./Km), 1.62 x 10(6) M-1.s-1 with respect to sedoheptulose 1,7-bisphosphate compared with 5.5 x 10(4) M-1.s-1 with respect to fructose 1,6-bisphosphate, indicates that the enzyme specificity towards sedoheptulose 1,7-bisphosphate is high but not absolute.