A novel Bacillus pullulanase. Its properties and application in the glucose syrups industry.

Abstract

The majority of starches used in the manu-facture of glucose syrups contain 75-85% amylopectin.1) Amylopectin is a highly branched polysaccharide consisting of linear chains of 1, 4-α-linked D-glucose residues, joined together by 1, 6-α-glucosidic linkages. The branch points occur on average every 20-25 D-glucose units, so that amylopectin contains 4-5% of 1, 6-α-glucosidic linkages.2-4) The 1, 6-α-glucosidic linkages act as a kind of barrier to the action of exo-acting, saccharif ying amylases such as glucoamylases or maltogenic /3-amylases. Endo-acting α-amylases are able to by-pass the branch points, 5) but in general are not capable cf hydrolyzing the 1, 6-α-glucosidic linkage. Recent work by Kobayashi and co-workers has shown that at least one α-amylase (from Thermoactinomyces vulgaris) can hydro-lyze 1, 6-α-glucosidic linkages, in addition to 1, 4-α-glucosidic linkages.6) Glucoamylases can slowly hydrolyze 1, 6-α-glucosidic linkages in amylopectin and partially hydrolyzed amylopectin, 7) but the action of maltogenic exo-amylases ceases as a branch point is approached.8) It is therefore obvious that the efficiency of the saccharification reaction could be improved by incorporating a specific amylopectin debranchina enzyme in the system. Debranching enzyme such as isoamylase [EC 3.2.1.68, glycogen 6-glucanohydrolase] and pullulanase [EC 3.2.1.41, pullulan 6-glucanohy-drolase] have been known for many years, 9) but their use in the glucose syrups industry is far from widespread. Pullulanases from Klebsiella pneumoniae, 10, 11) Streptomyces sp., 12) and Bacillus cereus var, mycoides13) and isoamylases from Pseudomonas amyloderamosa14, 15) and Cy-tophaga sp.16) are not sufficiently thermostable to be used at 60°C. Moreover the Pseudomonas isoamylase was the only debranching enzyme sufficiently acidophilic to be used at a pH of around 4. 5.17, 18) After an extensive screening programme, our research laboratories succeeded in isolating a species of Bacillus which produced a thermostable, acidophilic pullulanase which was free from glucoamylase, β-amylase and a-amylase side activities. Some of the properties of this enzyme will now be described.