Digestive alpha-amylases of the flour moth Ephestia kuehniella - adaptation to ...

The digestive tract of lepidopteran insects is extremely alkaline. In the present work, molecular adaptation of amylolytic enzymes to this environment was investigated in the flour moth Ephestia kuehniella, an important stored‐product pest. Three digestive α‐amylases [Ephestia kuehniellaα‐amylase isoenzymes 1–3 (EkAmy1–3)] with an alkaline pH optimum were purified from larvae and biochemically characterized. These isoenzymes differ significantly in their sensitivity to α‐amylase inhibitors of plant origin that are directed against herbivores as antifeedants. Such functional variability renders the amylolytic system less vulnerable to suppression by plant defensive molecules. Moreover, we found that expression of α‐amylases is upregulated in larvae feeding on a diet enriched with an α‐amylase inhibitor. The α‐amylases are secreted into the larval midgut by an exocytotic mechanism, as revealed by immunogold microscopy. The cDNA sequence of EkAmy3 was determined, and a homology model of EkAmy3 was built in order to analyze the structural features responsible for adaptation to alkaline pH. First, the overall fold was found to be stabilized by remodeling of ion pairs. Second, molecular simulations supported by activity measurements showed that EkAmy3 does not bind a Cl^–, owing to an Arg‐to‐Gln mutation in a conserved binding site. The Cl^–‐binding residues are in contact with the catalytic residues, and this change might help to fine‐tune the catalytic pK_a values to an alkaline pH optimum. We conclude that lepidopteran α‐amylases are evolutionarily adapted in terms of structure and expression dynamics for effective functioning in the digestive system.