Unravelling the mechanism of enzymatic resistance in different high amylose starch granules

This study investigates the phenomenon that, in contrast to amylopectin-rich starch granules, high amylose starch (HAS) granules typically exhibit high hydrolytic resistance manifested as low density of enzyme attack sites on the starch granule surface. However, among the various types of examined HAS granules, we identified differences in enzymatic resistance. We associated this effect as a result of variations in specific rate of the enzymatic reaction, with intermediate affinity leading to the highest enzymatic efficacy characteristic for the Sabatier principle. However, a surprisingly low hydrolytic resistance, in fact the lowest among the investigated HAS, was found for amylose-only barley starch granules (97 % amylose). This behavior was connected with an optimal affinity of the applied model hydrolase, glucoamylase (GA), leading to the highest hydrolytic efficacy among the HASs. Exploration of surface structures by Fourier transform infrared (FTIR) spectroscopy revealed that larger starch granules with less ordered surface structures tended to exhibit optimum affinity for GA, and hence get degraded with higher catalytic activity and being less resistant. The findings improve our perception of the mechanisms that underlie hydrolytic enzyme resistance of HASs, providing implications for understanding and rationally design of enzymatically modified resistant starches.