17 Disaccharide Phosphorylases

Abstract

Publisher Summary The term disaccharide phosphorylase is applied to three bacterial enzymes, cellobiose phosphorylase, maltose phosphorylase, and sucrose phosphorylase. Each of the enzymes has been isolated from several sources, and each catalyzes the reversible reaction between the appropriate disaccharide and inorganic phosphate to form glucose 1-phosphate and the respective monosaccharide. All three enzymes can also transfer the glucosyl moiety to acceptors other than phosphate. While considering the mode of action of these enzymes, a decision must be made between two fundamentally different mechanisms. The first mechanism involves the formation of a covalent intermediate (G-E) between the enzyme and the glucosyl moiety of the substrate. The second mechanism, on the other hand, involves no such intermediate. Instead, both substrates initially combine with the enzyme; the displacement reaction then occurs directly without the formation of a covalent intermediate. Sucrose phosphorylase has been purified to near homogeneity from Pseudomonas saccharophila. The purified enzyme appears homogeneous on ultracentrifugation and on starch gel and acrylamide gel electrophoresis. Molecular weight determinations by Sephadex chromatography and from ultracentrifugation data yields a molecular weight of 80,000–100,000. Molecular weight determination by sodium dodecyl sulfate (SDS)-acrylamide gel electrophoresis indicated a molecular weight of 50,000, suggesting that the enzyme is composed of two identical subunits.