Metabolism of the reserve polysaccharide of Streptococcus mitior (mitis): is there a second alpha-1,4-glucan phosphorylase?

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The alpha-1,4-glucan phosphorylase (alpha-1,4-glucan: orthophosphate glucosyltransferase; EC 2.4.1.1) associated with the particulate cell fraction of Streptococcus mitior strain S3 was compared with the soluble maltodextrin phosphorylase that had been previously isolated from the same organism (Walker et al., 1969). The particulate enzyme was more sensitive to the glycogen content of the cell than the soluble euzyme; its activity was highest when the cells were grown under conditions favoring high glycogen storage. Substrate specificities of the two high activity towards endogenous glycogen, whereas low-molecular-weight maltodextrins were the preferred substrates for the soluble phosphorylase. The purification of the particulate phosphorylase included incubation of the particulate fraction in 160 mM sodium phosphate-10 mM sodium citrate-0.1% (wt/vol) Triton X-100 buffer (pH 6.7) and ion-exchange chromatography on diethylamino-ethyl- Sephadex A-50. The purified enzyme was fully soluble. The value for the purification factor was variable and depended on (i) the substrate used and (ii) whether the synthetic or the degradative reaction was being measured. The solubilization resulted in considerable changes in the properties of the phosphorylase: the pH optimum for activity was raised from 6.0 to 7.0-7.5 and the substrate specificity was altered. Consequently, the purified enzyme bore greater similarity to the soluble maltodextrin phosphorylase. The reported results are best explained in terms of a single phosphorylase, the specificity which is determind by its binding state in the cell. The enzyme acts as a glycogen phosphorylase in the particulate state and as a maltodextrin phosphorylase when soluble. The equilibrium between the two forms is related to the glycogen content of the cells.

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