Phosphorylases polysaccharide synthesis

Because, under any set of conditions, phosphorylase catalyzes synthesis and degradation of polysaccharides, equilibrium is eventually reached. The value of the ratio [inorganic phosphate]/[D-glucosyl phosphate] at equilibrium is remarkably constant over a wide range of concentration of polysaccharide, but varies with pH, because both substances can ionize it varies from 5.7 at pH 6 to 2.7 at pH 7.6. 

Branching enzyme is responsible for the a-1,6-branching of the a-1,4-chain in the synthesis of glycogen. Branching enzyme enhances the rate of polysaccharide (endogenous glycogen) synthesis from glucose-1 -phosphate by phosphorylase. 

As has been described, the combined use of two phosphorylases is a powerful tool to convert one carbohydrate into another with a different structure. The idea of phosphorylase coupling was first examined by Waldmann et al. (1986), but had been employed for the synthesis of cellobiose from sucrose (Kitaoka et al., 1992), laminaribiose from sucrose (Kitaoka et al., 1993), trehalose from maltose (Yoshida et al., 1995) and kojioligosaccharides from trehalose (Chaen et al., 1999). Discovery of new phosphorylases and their application through phosphorylase coupling should be a promising area in polysaccharide and carbohydrate engineering. 

The rate of synthesis, or degradation, of amylose depends on the degree of polymerization of the polysaccharide the action is faster on the species of lower molecular weight. It is thought that inactive enzyme—substrate complexes can form between phosphorylase and internal sections of D-glucosidic chains, and the result of this is an effective diminution in the concentration of enzjune free to attack chain ends. As the inactive complexes are more likely to form with the longer molecules of amylose, the rate of reaction decreases with increasing molecular size of the polysaccharide. 

The maximum velocity of a phosphorylase reaction depends on the temperature and the pH, but is only obtained when the enzyme is saturated with both substrates that is, polysaccharide and inorganic phosphate for degradation, or primer and D-glucosyl phosphate for synthesis. Activities at 30° vary from 2.8 to 14 X 10 moles of D-glucosyl phosphate used up per min. per 10 g, of protein. ... 

It appears that phosphorus is related to the formation of starch in the plant. Thus Hanes has synthesized a linear polysaccharide from a-D-glucopyranosc 1-phosphate (Cori ester) through the action of potato phosphorylase. Dunlap and Beckmann and likewise Cori have found that the B-fraction activates this enzymic synthesis, while the A-frac-tion is inactive. It has not been established whether this effect is due to the branched character of the B-fraction or to the presence of phosphate in its structure. 

For the synthesis of glycogen-type polysaccharides from a-n-glucopyran-osyl phosphate, two enzymes are required. Phosphorylases, in presence of a suitable primer, synthesize linear chains of -( —> 4)-linked n-glucose residues these are then converted into a branched polysaccharide by a branching enzyme. ... 

The continuing interest of Bourne in the chemistry of polysaccharides and associated enzymes originated from the work of Haworth and Peat directed towards the enzymic synthesis and degradation of starch. The impetus for this work was given by the discovery, made by C. S. Hanes in 1940, that a phosphorylase isolated from the potato and pea effects the synthesis, from D-glucosyl phosphate, of starch, later shown (by Haworth, Heath, and Peat) to be amylose. In his first paper (with Haworth and Peat) in 1944, Bourne described the isolation of the Q-enzyme which, in conjunction with phosphorylase, effects the conversion of D-glucosyl phosphate into the major component of whole starch, namely, amylopectin. He had discovered the Q-enzyme in a fraction discarded by previous workers. Already, the quintessence of his mind was revealed in this work meticulous attention to detail, and perception of essentials. 

From Solarium tuberosum, Slabnik and Frydman isolated a unique phosphorylase that has no requirement for the primer addition for formation of an amylopectin-like polysaccharide in a cell-free system. The properties of this enzyme were found to differ from those of the usual potato phosphorylase this new enzyme is assumed to be a glycoprotein, the glycosidic component of which acts as the primer. The activity of this phosphorylase disappeared at 55 , in contrast to the usual phosphorylase activity (which withstands this temperature). There was also good correlation between formation of polysaccharide and appearance of inorganic phosphate in the absence of primer. The polysaccharide formed de novo" was shown to be an eflScient primer for starch synthesis. ... 

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