Cyclodextrins are not the major cyclic alpha-1,4-glucans produced by the initial action of cyclodextrin glucanotransferase on amylose.

The initial action of cyclodextrin glucanotransferase (CGTase, EC 2.4.1.19) from an alkalophilic Bacillus sp. A2-5a on amylose was investigated. Synthetic amylose was incubated with purified CGTase then terminated in the very early stage of the enzyme reaction. When the reaction mixture was treated with glucoamylase and the resulting glucoamylase-resistant glucans were analyzed with high performance anion exchange chromatography, cyclic alpha-1,4-glucans, with degree of polymerization ranging from 9 to more than 60, in addition to well known alpha-, beta-, and gamma-cyclodextrin (CD), were detected. The time-course analysis revealed that larger cyclic alpha-1, 4-glucans were preferentially produced in the initial stage of the cyclization reaction and were subsequently converted into smaller cyclic alpha-1,4-glucans and into the final major product, beta-CD. CGTase from Bacillus macerans also produced large cyclic alpha-1, 4-glucans except that the final major product was alpha-CD. Based on these results, a new model for the action of CGTase on amylose was proposed, which may contradict the widely held view of the cyclization reaction of CGTase.