CYCLIZATION REACTION CATALYZED BY BRANCHING ENZYME

The action of branching enzyme (EC 2.4.1.18) from Bacillus stearotherm ophilus on amylose was analyzed. The enzyme reduced the molecular size of amylose without increasing the reducing power, This result could n ot be explained by the normal branching reaction model, When the produ ct was treated,vith glucoamylase (an exo-type amylase), a resistant co mponent remained. The glucoamylase-resistant component was easily dige sted by an endo-type alpha-amylase or by isoamylase plus glucoamylase. These results suggested that the glucoamylase-resistant component was a cyclic glucan composed of alpha-1,4- and alpha-1,6-glucosidic linka ges. In other words, it was suggested that branching enzyme catalyzed cyclization of the alpha-1,4-glucan chain of the amylose molecule to f orm an alpha-1,6-glucosidic linkage, thereby forming two smaller molec ules. Mass spectrometry also supported the cyclic nature of the produc t.