Interfacial kinetics reveal enzymatic resistance mechanisms behind granular starch with smooth surfaces

Heterogeneous (interfacial) enzyme catalyzed hydrolysis of starch granules occurs in various biological systems, including plants, animal/human digestion, and microorganisms. Factors such as granular size, surface area, pores, and smoothness play crucial roles in influencing this process. However, limited understanding persists regarding the high enzymatic resistance of starch granules with smooth surfaces. In this study, we investigated the hydrolysis mechanism of glucoamylase (GA) on three different types of starch granules with smooth surfaces, extracted from Curcuma zedoaria (zedoary) rhizomes, Solanum tuberosum (potato) tubers, and Manihot esculenta (tapioca or cassava) roots. We compared the Langmuir adsorption, interfacial kinetics, and the multi-level structure of the three starches. Our data demonstrate that the lower enzymatic resistance observed in tapioca starch stems from the higher density of enzymatic attack sites (kinΓmax) recognized by GA on tapioca starch (1.0 nmol/g) compared to potato (0.6 nmol/g) and zedoary (0.3 nmol/g) starch granules. The high kinΓmax for tapioca starch was significantly influenced by its relatively lower B-type crystallinity, which is disrupted by the presence of short fa chains (degree of polymerization (DP) < 12) and long amylose chains. Furthermore, the relatively higher proportion of longer chains (fb1 and fb2 chains) on the surface of tapioca starch also contributed to higher kinΓmax for GA, resulting in lower enzymatic resistance. These findings enhance our understanding of how the structure of starch granules affects enzymatic catalysis, particularly in granular starches with smooth surfaces devoid of pores. Such insights are crucial for elucidating the digestion and utilization of starch granules.