Starch phosphorylase

Starch is stored in plant cells in the form of granules in the stroma of plas-tids (plant cell organelles) of two types chloroplasts, in which photosynthesis takes place, and amyloplasts, plastids that are specialized starch accumulation bodies. When starch is to be mobilized and used by the plant that stored it, it must be broken down into its component monosaccharides. Starch is split into its monosaccharide elements by stepwise phosphorolytic cleavage of glucose units, a reaction catalyzed by starch phosphorylase (Figure 7.23). This is formally an a(1 4)-glucan phosphorylase reaction, and at each step, the prod-... 

FIGURE 7.23 The starch phosphorylase reaction cleaves glucose residues from amy-lose, producing a-D-glucose-l-phosphate. 

This enzyme [EC 2.4.1.1], also called phosphorylase, catalyzes the reaction of [(l,4)-o -D-glucosyl] with orthophosphate to produce [(l,4)-o -D-glucosyl] -i and a-D-glucose 1-phosphate. The name to be used with this enzyme is dependent on the naturally occurring substrate for example, glycogen phosphorylase, starch phosphorylase, maltodextrin phosphorylase, etc. 

SELENOPHOSPHATE SYNTHETASE STARCH PHOSPHORYLASE SUCCINYL-CoA SYNTHETASE SUCROSE PHOSPHORYLASE TUBULIN.TYROSINE LIGASE ORTHOPHOSPHATE CONTINUOUS ASSAY Orthovanadate,... 

NORMAL ERROR CURVE STATISTICS (A Primer STAPHYLOCOCCAL NUCLEASE STARCH PHOSPHORYLASE... 

For a glucan to be a substrate for starch phosphorylase, it must be longer than maltotetraose. Shorter oligosaccharides, however, can be used by glucosyltransferases such as D-enzyme (EC 2.4.1.25) in the reaction... 

These enzymes increase the degree of polymerization of short oligosaccharides, converting them into suitable substrates for starch phosphorylase. 

St. Pierre, B., and Brisson, N. 1995. Induction of the plastidic starch phosphorylase gene in potato storage sink tissue Effect of sucrose and evidence for coordinated regulation of phosphorylase and starch biosynthetic genes. Planta. 195, 339-344. 

The conformations in Figure 5.65 again suggest an S i mechanism. Positional isotope exchange in apparently unreacted glucosyl phosphate would not be expected for this mechanism, unless for some reason the glycosylation of the acceptor was reversible in the ternary complex. The evidence from potato starch phosphorylase favours a double displacement mechanism there is no... 

Glucose-1-phosphate is the product when ttfl, 4) glycosidic bonds at nonreducing ends are broken by starch phosphorylase. Branch points in starch are removed by debranching enzyme. The products of starch digestion, glucose and glucose-1-phosphate, are then converted to triose phosphate and exported to the cytoplasm. In photosynthesiz-ing cells, most triose phosphate is converted to sucrose. 

Glycogen in animal tissues and in microorganisms (and starch in plants) can be mobilized for use within the same cell by a phosphorolytic reaction catalyzed by glycogen phosphorylase (starch phosphorylase in... 

It has been reported that preparations of starch grains display very high synthetase activity if they are isolated in 0.5 M sucrose, presumably because they are well preserved after such treatment. However, the possibility that the high reaction rate may be due to the combined action of nucleotide pyrophosphatase and starch phosphorylase has not been adequately ruled out in that study. 

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