Mutual interactions between α-amylase and amyloglucosidase in the digestion of starch with distinct chain-length distributions at a fully gelatinized state.

Amyloglucosidase (AMG) and α-amylase (AMY) are both involved in starch digestion in human small intestine, whereas their mutual interactions with starch molecules of distinct structures are still unknown. In the current study, starches with different amylose contents (from waxy to high amylose content) were used to investigate the starch-enzyme interactions in the in vitro starch digestibility at a fully gelatinized state. The starch chain-length distributions (CLDs) before and after digestion were obtained by size-exclusion chromatography (SEC), and the in vitro digestograms with a single or combined AMY and AMG were obtained. The results showed that the digestion extent generally followed an order of AMG > AMG + AMY > AMY, suggesting that there was an antagonistic effect between AMG and AMY. Starches with distinct structures were preferred differently by these enzyme combinations. For example, waxy starch could be digested at a much faster rate by AMG + AMY than by a single enzyme. By fitting to the logarithm of slope plot and the combination of parallel and sequential kinetics models, two different starch digestible fractions (slowly vs. rapidly) were identified, and AMG and AMY typically had an antagonistic effect in digesting the rapidly digestible fraction. Finally, the CLD results suggested that a small starch portion with DP 7-2000, not present in the original starch samples, was formed after the digestion. The starch portion with DP > 2000 could be very rapidly digested by AMY, whereas the starch portion with DP 7-2000 could only be less effectively digested by AMY, and the addition of AMG could further reduce this activity. This study provides important information for understanding the starch-enzyme interactions as well as their effects on starch digestibility at a molecular level.