Exo-action

Enzymes depolymerizing polysaccharides may have an endo or an exo action pattern, and may hydrolyze, or cleave by elimination. Both the conformation of the polysaccharide and the active site of the enzyme need to be considered in the enzyme-glycan interaction. endo-Enzymes split, by a random type of depolymerization, glycosidic bonds situated internally in... 

Modified substrates may be used in testing the action patterns of purified enzymes. Thus, periodate-oxidized laminaran was used by Smith and coworkers to show the exo action of Basidiomycete (l- 3)-j8-n-glucanase. The oxidized-amylose substrate described earlier (see p. 266) has been used with alpha- and heta-amylases to illustrate how the method is completely general, and applicable to all classes of polysaccharides and poIysaccharidases. ° By use of oxidized pullulan, the endo nature of pullulanase action was confirmed. ... 

The enzyme is readily prepared, in an apparently pure form, from the culture filtrates of this pseudomonad, and has a specific activity comparable to that of crystalline, sweet-potato befa-amylase. The exo action of the enzyme is analogous to that of heta-amylase, but maltotetraose residues, not maltose residues, are removed. The action of the enzyme is halted by branch points, so that limit dextrins are produced from glycogen and amylopectin. The specificity of the enzyme for linkages in the region of branch points has not yet been ascertained. Some applications of this enzyme will be discussed later (see p. 318). 

Glucoamylase from Rhizopus niveus has been shown to have an exo action on (l-> 3)-o - and (1 6)-a-D-glucan linkages as well as on malto-oligo-saccharides. ... 

An a-D-glucosidase purified from flint corn by precipitation with ammonium sulphate, ion-exchange chromatography, and gel-filtrations was homogeneous in ultracentrifugal and disc electrophoretic analysis. The enzyme (6.5 Sy mol. wt. 6.5 X 10 by gel filtration) showed a pH optimum of 3.6 for both maltose and soluble starch. The ratio of velocities of hydrolysis for maltose, phenyl a-D-glucopyranoside, and soluble starch was estimated to be 100 14.3 6.1. The a-o-glucosidase hydrolysed soluble starch by an exo action. 

Phadebas-starch, obtained by reaction of starch with a triazine dye (Cibacron Blue F8GA), continues to be used as a substrate for the assay of a-amylase. Comparison of assays using Phadebas-starch with saccharogenic and amyloclastic procedures showed that the method is reliable and specific for a-amylase, since Phadebas-starch is not hydrolysed by glucoamylase. Further work on Phadebas-starch showed that groups substituted on the helices inhibit the exo-action of jS-amylase, but function only as accidental barriers to the endo-action of a-amylase. ... 

A j8-D-2-acetamido-2-deoxyglucosidase from E. coli has been purified to near homogeneity by electrophoresis on polyacrylamide gel in the presence of sodium dodecyl sulphate and urea. Studies of the substrate specificity of the purified enzyme (pH optimum 7.7, Km (for 4-nitrophenyl 2-acetamido-2-deoxy-l3-D-gluco-pyranoside 0.43 mmol 1 ) confirmed that it has an exo-action. The molecular weight (determined by gel filtration and by gel electrophoresis) of the enzyme in a dissociating medium is 2.6 x 10, showing that it does not contain subunits. 

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