Polysaccharide phosphorylase

It seems quite possible that not only true phosphorylase but also other transglycosidases may play an important role in the biological syntheses, decompositions and interconversions of disaccharides and polysaccharides. 

The function of the sulfate residue in these polysaccharides is unknown but the suggestion has been made that just as starch is synthesised from D-glucose 1-phosphate by the action of phosphorylase, so the seaweed polysaccharides are formed from the appropriate sugar sulfate by reaction with a sulfatase. ... 

Phosphorylase catalyzes the polymerization of glucose-1-phosphate in order to obtain linear polysaccharide chains with a-(1 4) glycosidic linkages the glycogen... 

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. 

In a second class of regulatory enzymes the active and inactive forms are inter-converted by covalent modifications of their structures by enzymes. The classic example of this type of control is the use of glycogen phosphorylase from animal tissues to catalyse the breakdown of the polysaccharide glycogen yielding glucose-1-phosphate, as illustrated in Fig. 5.37. 

Phospholylases, as summarized in Table 28.1, catalyze the reversible phospho-rolysis of polysaccharides or oligosaccharides, and produce phosphorylated mono-saccharides. Among such enzymes a-glucan phosphorylase (GP, EC 2.4.1.1), sucrose phosphorylase (SP, EC 2.4.1.7) and cellobiose phosphorylase (CBP, EC 2.4.1.20) are of great interest, since they can produce a-glucose 1-phosphate (a-GlP) from three major biomasses starch, sucrose, and cellulose. Only these three are described in this paper, a comprehensive review of other phosphorylases can be found in Kitaoka and Hayashi (2002). 

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 polysaccharide phosphoTylases and the other depolymerizing enzymes differ in both mechanism of action and structure, although it is generally accepted that phosphorylases share the general acid catalysis. 

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. 

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