Enhancing sucrose synthase activity results in increased levels of starch and ADP-glucose in maize (Zea mays L.) seed endosperms.

Sucrose synthase (SuSy) is a highly regulated cytosolic enzyme that catalyzes the conversion of sucrose and a nucleoside diphosphate into the corresponding nucleoside diphosphate glucose and fructose. In cereal endosperms, it is widely assumed that the stepwise reactions of SuSy, UDPglucose pyrophosphorylase and ADPglucose (ADPG) pyrophosphorylase (AGP) take place in the cytosol to convert sucrose into ADPG necessary for starch biosynthesis, although it has also been suggested that SuSy may participate in the direct conversion of sucrose into ADPG. In this study, the levels of the major primary carbon metabolites, and the activities of starch metabolism-related enzymes were assessed in endosperms of transgenic maize plants ectopically expressing StSUS4, which encodes a potato SuSy isoform.

Cytoplasmic Escherichia coli ADP sugar pyrophosphatase binds to cell membranes in response to extracellular signals as the cell population density increases.

ADP sugar pyrophosphatase (AspP) is a member of the 'Nudix' (Nucleoside diphosphate linked to some other moiety X) hydrolase family of enzymes that catalyzes the hydrolytic breakdown of ADP-glucose (ADPG) linked to glycogen biosynthesis. In a previous work, we showed that AspP activity is strongly enhanced by both glucose-1,6-bisphosphate and nucleotide-sugars, and by macromolecular crowding. In this work, we show that AspP binds to cell membranes as the bacterial population density increases, c.

Occurrence of more than one important source of ADPglucose linked to glycogen biosynthesis in Escherichia coli and Salmonella.

To explore the possible occurrence of sources, other than GlgC, of ADPglucose linked to bacterial glycogen biosynthesis we characterized Escherichia coli and Salmonella DeltaglgCAP deletion mutants lacking the whole glycogen biosynthetic machinery. These mutants displayed the expected glycogen-less phenotype but accumulated ADPglucose. Importantly, DeltaglgCAP cells expressing the glycogen synthase encoding glgA gene accumulated glycogen.

An Escherichia coli mutant producing a truncated inactive form of GlgC synthesizes glycogen: further evidences for the occurrence of various important sources of ADPglucose in enterobacteria.

AC70R1-504 Escherichia coli mutants possess a glgC* gene with a nucleotide change resulting in a premature stop codon that renders a truncated, inactive form of GlgC. Cells over-expressing the wild type glgC, but not those over-expressing the AC70R1-504 glgC*, accumulated high ADPglucose and glycogen levels. AC70R1-504 mutants accumulated glycogen, whereas DeltaglgCAP deletion mutants lacking the whole glycogen biosynthetic machinery displayed a glycogen-less phenotype.

Escherichia coli AspP activity is enhanced by macromolecular crowding and by both glucose-1,6-bisphosphate and nucleotide-sugars.

Escherichia coli ADP-sugar pyrophosphatase (AspP) is a "Nudix" hydrolase that catalyzes the hydrolytic breakdown of ADP-glucose linked to glycogen biosynthesis. Moderate increases of AspP activity in the cell are accompanied by significant reductions of the glycogen content. In vitro analyses showed that AspP activity is strongly enhanced by macromolecular crowding and by both glucose-1,6-bisphosphate and nucleotide-sugars, providing a first set of indicative evidences that AspP is a highly regulated enzyme.

Glycogen phosphorylase, the product of the glgP Gene, catalyzes glycogen breakdown by removing glucose units from the nonreducing ends in Escherichia coli.

To understand the biological function of bacterial glycogen phosphorylase (GlgP), we have produced and characterized Escherichia coli cells with null or altered glgP expression. glgP deletion mutants (DeltaglgP) totally lacked glycogen phosphorylase activity, indicating that all the enzymatic activity is dependent upon the glgP product. Moderate increases of glycogen phosphorylase activity were accompanied by marked reductions of the intracellular glycogen levels in cells cultured in the presence of glucose.

Cloning, expression and characterization of a Nudix hydrolase that catalyzes the hydrolytic breakdown of ADP-glucose linked to starch biosynthesis in Arabidopsis thaliana.

'Nudix' hydrolases are widely distributed nucleotide pyrophosphatases that possess a conserved GX5EX7REUXEEXGU motif where U is usually isoleucine, leucine or valine. Among them, Escherichia coli ADP-sugar pyrophosphatase (ASPP) has been shown to catalyze the hydrolytic breakdown of ADP-glucose linked to bacterial glycogen biosynthesis. Comparisons of the 31 different Nudix-encoding sequences of the Arabidopsis genome with those coding for known bacterial and mammalian ASPPs identified one sequence possessing important divergences in the Nudix motif that, once expressed in E.

An important pool of sucrose linked to starch biosynthesis is taken up by endocytosis in heterotrophic cells.

We have recently shown the occurrence of endocytic sucrose uptake in heterotrophic cells. Whether this mechanism is involved in the sucrose-starch conversion process was investigated by comparing the rates of starch accumulation in sycamore cells cultured in the presence or absence of the endocytic inhibitors wortmannin and 2-(4-morpholynyl-)-8-phenyl-4H-1 benzopyran-4-1 (LY294002). These analyses revealed a two-phase process involving an initial 120 min wortmannin- and LY294002-insensitive starch accumulation period, followed by a prolonged phase that was arrested by the endocytic inhibitors.

Sucrose synthase controls both intracellular ADP glucose levels and transitory starch biosynthesis in source leaves.

The prevailing model on transitory starch biosynthesis in source leaves assumes that the plastidial ADPglucose (ADPG) pyrophosphorylase (AGP) is the sole enzyme catalyzing the synthesis of the starch precursor molecule, ADPG. However, recent investigations have shown that ADPG linked to starch biosynthesis accumulates outside the chloroplast, presumably in the cytosol. This finding is consistent with the occurrence of an 'alternative' gluconeogenic pathway wherein sucrose synthase (SuSy) is involved in the production of ADPG in the cytosol, whereas both plastidial phosphoglucomutase (pPGM) and AGP play a prime role in the scavenging of starch breakdown products.

Most of ADP x glucose linked to starch biosynthesis occurs outside the chloroplast in source leaves.

Sucrose and starch are end products of two segregated gluconeogenic pathways, and their production takes place in the cytosol and chloroplast of green leaves, respectively. According to this view, the plastidial ADP.glucose (ADPG) pyrophosphorylase (AGP) is the sole enzyme catalyzing the synthesis of the starch precursor molecule ADPG.