Improved yields of cyclic nigerosylnigerose from starch by pretreatment with a thermostable branching enzyme

Cyclic nigerosylnigerose (CNN) is produced enzymatically from starch by the combined action of 6-α-glucosyltransferase and 3-α-isomaltosyltransferase. In our previous study, α-1,6-branching chains found in the structure of amylopectin and glycogen were shown to be favorable for CNN formation by the two enzymes. Therefore, we examined whether the introduction of α-1,6-branch points into starch using the action of branching enzyme (BE) could improve the yield of CNN from starch. Thermostable BE from Geobacillus stearothermophilus TC-91 was prepared as a purified recombinant protein. Pretreatment of amylose with BE considerably increased the CNN yield from 5% to 38%. When BE acted on tapioca starch, the CNN yield was elevated from 47% to 60%. Conversely, BE treatment of waxy corn starch containing very little amylose resulted in a negligible increase in CNN yield. In addition, BE exerted a beneficial effect when starch with a lower degree of hydrolysis was used as a substrate. The present results indicate that the addition of α-1,6-glucosidic linkages to starch using BE is an effective strategy to improve the yield of CNN from starch.