Identification of the maize amyloplast stromal 112-kD protein as a plastidic starch phosphorylase.
Amyloplast is the site of starch synthesis in the storage tissue of maize (Zea mays). The amyloplast stroma contains an enriched group of proteins when compared with the whole endosperm. Proteins with molecular masses of 76 and 85 kD have been identified as starch synthase I and starch branching enzyme IIb, respectively. A 112-kD protein was isolated from the stromal fraction by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subjected to tryptic digestion and amino acid sequence analysis. Three peptide sequences showed high identity to plastidic forms of starch phosphorylase (SP) from sweet potato, potato, and spinach. SP activity was identified in the amyloplast stromal fraction and was enriched 4-fold when compared with the activity in the whole endosperm fraction. Native and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses showed that SP activity was associated with the amyloplast stromal 112-kD protein. In addition, antibodies raised against the potato plastidic SP recognized the amyloplast stromal 112-kD protein. The amyloplast stromal 112-kD SP was expressed in whole endosperm isolated from maize harvested 9 to 24 d after pollination. Results of affinity electrophoresis and enzyme kinetic analyses showed that the amyloplast stromal 112-kD SP preferred amylopectin over glycogen as a substrate in the synthetic reaction. The maize shrunken-4 mutant had reduced SP activity due to a decrease of the amyloplast stromal 112-kD enzyme. (+info)
Decreased sucrose content triggers starch breakdown and respiration in stored potato tubers (Solanum tuberosum).
To change the hexose-to-sucrose ratio within phloem cells, yeast-derived cytosolic invertase was expressed in transgenic potato (Solanum tuberosum cv. Desiree) plants under control of the rolC promoter. Vascular tissue specific expression of the transgene was verified by histochemical detection of invertase activity in tuber cross-sections. Vegetative growth and tuber yield of transgenic plants was unaltered as compared to wild-type plants. However, the sprout growth of stored tubers was much delayed, indicating impaired phloem-transport of sucrose towards the developing bud. Biochemical analysis of growing tubers revealed that, in contrast to sucrose levels, which rapidly declined in growing invertase-expressing tubers, hexose and starch levels remained unchanged as compared to wild-type controls. During storage, sucrose and starch content declined in wild-type tubers, whereas glucose and fructose levels remained unchanged. A similar response was found in transgenic tubers with the exception that starch degradation was accelerated and fructose levels increased slightly. Furthermore, changes in carbohydrate metabolism were accompanied by an elevated level of phosphorylated intermediates, and a stimulated rate of respiration. Considering that sucrose breakdown was restricted to phloem cells it is concluded that, in response to phloem-associated sucrose depletion or hexose elevation, starch degradation and respiration is triggered in parenchyma cells. To study further whether elevated hexose and/or hexose-phosphates or decreased sucrose levels are responsible for the metabolic changes observed, sucrose content was decreased by tuber-specific expression of a bacterial sucrose isomerase. Sucrose isomerase catalyses the reversible conversion of sucrose into palatinose, which is not further metabolizable by plant cells. Tubers harvested from these plants were found to accumulate high levels of palatinose at the expense of sucrose. In addition, starch content decreased slightly, while hexose levels remained unaltered, compared with the wild-type controls. Similar to low sucrose-containing invertase tubers, respiration and starch breakdown were found to be accelerated during storage in palatinose-accumulating potato tubers. In contrast to invertase transgenics, however, no accumulation of phosphorylated intermediates was observed. Therefore, it is concluded that sucrose depletion rather than increased hexose metabolism triggers reserve mobilization and respiration in stored potato tubers. (+info)
Starch mobilization in leaves.
Starch mobilization is well understood in cereal endosperms, but both the pathway and the regulation of the process are poorly characterized in other types of plant organs. Arabidopsis leaves offer the opportunity for rapid progress in this area, because of the genomic resources available in this species and the ease with which starch synthesis and degradation can be monitored and manipulated. Progress in understanding three aspects of starch degradation is described: the role of disproportionating enzyme, the importance of phosphorolytic degradation, and new evidence about the involvement of a starch-phosphorylating enzyme in the degradative process. Major areas requiring further research are outlined. (+info)
Tracking interactions that stabilize the dimer structure of starch phosphorylase from Corynebacterium callunae. Roles of Arg234 and Arg242 revealed by sequence analysis and site-directed mutagenesis.
Glycogen phosphorylases (GPs) constitute a family of widely spread catabolic alpha1,4-glucosyltransferases that are active as dimers of two identical, pyridoxal 5'-phosphate-containing subunits. In GP from Corynebacterium callunae, physiological concentrations of phosphate are required to inhibit dissociation of protomers and cause a 100-fold increase in kinetic stability of the functional quarternary structure. To examine interactions involved in this large stabilization, we have cloned and sequenced the coding gene and have expressed fully active C. callunae GP in Escherichia coli. By comparing multiple sequence alignment to structure-function assignments for regulated and nonregulated GPs that are stable in the absence of phosphate, we have scrutinized the primary structure of C. callunae enzyme for sequence changes possibly related to phosphate-dependent dimer stability. Location of Arg234, Arg236, and Arg242 within the predicted subunit-to-subunit contact region made these residues primary candidates for site-directed mutagenesis. Individual Arg-->Ala mutants were purified and characterized using time-dependent denaturation assays in urea and at 45 degrees C. R234A and R242A are enzymatically active dimers and in the absence of added phosphate, they display a sixfold and fourfold greater kinetic stability of quarternary interactions than the wild-type, respectively. The stabilization by 10 mm of phosphate was, however, up to 20-fold greater in the wild-type than in the two mutants. The replacement of Arg236 by Ala was functionally silent under all conditions tested. Arg234 and Arg242 thus partially destabilize the C. callunae GP dimer structure, and phosphate binding causes a change of their tertiary or quartenary contacts, likely by an allosteric mechanism, which contributes to a reduced protomer dissociation rate. (+info)
Mutagenesis of the dimer interface region of Corynebacterium callunae starch phosphorylase perturbs the phosphate-dependent conformational relay that enhances oligomeric stability of the enzyme.
We have used alanine-scanning site-directed mutagenesis of the dimer contact region of starch phosphorylase from Corynebacterium callunae to explore the relationship between a protein conformational change induced by phosphate binding and the up to 500-fold kinetic stabilization of the functional quarternary structure of this enzyme when phosphate is present. Purified mutants (at positions Ser-224, Arg-226, Arg-234, and Arg-242) were characterized by Fourier transform-infrared (FT-IR) spectroscopy and enzyme activity measurements at room temperature and under conditions of thermal denaturation. Difference FT-IR spectra of wild type and mutants in (2)H(2)O solvent revealed small changes in residual amide II band intensities at approximately 1,550 cm(-1), indicating that (1)H/(2)H exchange in the wild type is clearly perturbed by the mutations. Decreased (1)H/(2)H exchange in comparison to wild type suggests formation of a more compact protein structure in S224A, R234A, and R242A mutants and correlates with rates of irreversible thermal denaturation at 45 degrees C that are up to 10-fold smaller for the three mutants than the wild type. By contrast, the mutant R226A inactivates 2.5-fold faster at 45 degrees C and shows a higher (1)H/(2)H exchange than the wild type. Phosphate (20 mM) causes a greater change in FT-IR spectra of the wild type than in those of S224A and 234A mutants and leads to a 5-fold higher stabilization of the wild type than the two mutants. Therefore, structural effects of phosphate binding leading to kinetic stability of wild-type starch phosphorylase are partially complemented in the S224A and R234A mutants. Infrared spectroscopic measurements were used to compare thermal denaturations of the mutants and the wild type in the absence and presence of stabilizing oxyanion. The broad denaturation transition of unliganded wild type in the range 40-50 degrees C is reduced in the S224A and R234A mutants, and this reflects mainly a shift of the onset of denaturation to a 4-5 degrees C higher value. (+info)
Plastidial alpha-glucan phosphorylase is not required for starch degradation in Arabidopsis leaves but has a role in the tolerance of abiotic stress.
To study the role of the plastidial alpha-glucan phosphorylase in starch metabolism in the leaves of Arabidopsis, two independent mutant lines containing T-DNA insertions within the phosphorylase gene were identified. Both insertions eliminate the activity of the plastidial alpha-glucan phosphorylase. Measurement of other enzymes of starch metabolism reveals only minor changes compared with the wild type. The loss of plastidial alpha-glucan phosphorylase does not cause a significant change in the total accumulation of starch during the day or its remobilization at night. Starch structure and composition are unaltered. However, mutant plants display lesions on their leaves that are not seen on wild-type plants, and mesophyll cells bordering the lesions accumulate high levels of starch. Lesion formation is abolished by growing plants under 100% humidity in still air, but subsequent transfer to circulating air with lower humidity causes extensive wilting in the mutant leaves. Wilted sectors die, causing large lesions that are bordered by starch-accumulating cells. Similar lesions are caused by the application of acute salt stress to mature plants. We conclude that plastidial phosphorylase is not required for the degradation of starch, but that it plays a role in the capacity of the leaf lamina to endure a transient water deficit. (+info)
Relationships between structure, function and stability for pyridoxal 5'-phosphate-dependent starch phosphorylase from Corynebacterium callunae as revealed by reversible cofactor dissociation studies.
Using 0.4 m imidazole citrate buffer (pH 7.5) containing 0.1 mm l-cysteine, homodimeric starch phosphorylase from Corynebacterium calluane (CcStP) was dissociated into native-like folded subunits concomitant with release of pyridoxal 5'-phosphate and loss of activity. The inactivation rate of CcStP under resolution conditions at 30 degrees C was, respectively, four- and threefold reduced in two mutants, Arg234-->Ala and Arg242-->Ala, previously shown to cause thermostabilization of CcStP [Griessler, R., Schwarz, A., Mucha, J. & Nidetzky, B. (2003) Eur. J. Biochem.270, 2126-2136]. The proportion of original enzyme activity restored upon the reconstitution of wild-type and mutant apo-phosphorylases with pyridoxal 5'-phosphate was increased up to 4.5-fold by added phosphate. The effect on recovery of activity displayed a saturatable dependence on the phosphate concentration and results from interactions with the oxyanion that are specific to the quarternary state. Arg234-->Ala and Arg242-->Ala mutants showed, respectively, eight- and > 20-fold decreased apparent affinities for phosphate (K(app)), compared to the wild-type (K(app) approximately 6 mm). When reconstituted next to each other in solution, apo-protomers of CcStP and Escherichia coli maltodextrin phosphorylase did not detectably associate to hybrid dimers, indicating that structural complementarity among the different subunits was lacking. Pyridoxal-reconstituted CcStP was inactive but approximately 60% and 5% of wild-type activity could be rescued at pH 7.5 by phosphate (3 mm) and phosphite (5 mm), respectively. pH effects on catalytic rates were different for the native enzyme and pyridoxal-phosphorylase bound to phosphate and could reflect the differences in pK(a) values for the cofactor 5'-phosphate and the exogenous oxyanion. (+info)
Catalytic mechanism of alpha-retaining glucosyl transfer by Corynebacterium callunae starch phosphorylase: the role of histidine-334 examined through kinetic characterization of site-directed mutants.
Purified site-directed mutants of Corynebacterium callunae starch phosphorylase in which His-334 was replaced by an alanine, glutamine or asparagine residue were characterized by steady-state kinetic analysis of enzymic glycosyl transfer to and from phosphate and studies of ligand binding to the active site. Compared with wild-type, the catalytic efficiencies for phosphorolysis of starch at 30 degrees C and pH 7.0 decreased approx. 150- and 50-fold in H334Q (His334-->Gln) and H334N mutants, and that of H334A was unchanged. In the direction of alpha-glucan synthesis, selectivity for the reaction with G1P (alpha-D-glucose 1-phosphate) compared with the selectivity for reaction with alpha-D-xylose 1-phosphate decreased from a wild-type value of approximately 20000 to 2600 and 100 in H334N and H334Q respectively. Binding of G1P to the free enzyme was weakened between 10-fold (H334N, H334Q) and 50-fold (H334A) in the mutants, whereas binding to the complex of enzyme and alpha-glucan was not affected. Quenching of fluorescence of the pyridoxal 5'-phosphate cofactor was used to examine interactions of the inhibitor GL (D-gluconic acid 1,5-lactone) with wild-type and mutant enzymes in transient and steady-state experiments. GL binding to the free enzyme and the enzyme-phosphate complex occurred in a single step. The 50-fold higher constant (K(d)) for GL dissociation from H334Q bound to phosphate resulted from an increased off-rate for the ligand in the mutant, compared with wild-type. A log-log correlation of catalytic-centre activity for phosphorolysis of starch with a reciprocal K(d) value established a linear free-energy relationship (slope=1.19+/-0.07; r2=0.991) across the series of wild-type and mutant enzymes. It reveals that GL in combination with phosphate has properties of a transition state analogue and that the His-334 side chain has a role in selectively stabilizing the transition state of the reaction. (+info)