Structural and functional comparisons of salivary alpha-glucosidases from the mosquito vectors Aedes aegypti, Anopheles gambiae, and Culex quinquefasciatus

Mosquito vectors of medical importance both blood and sugar feed, and their saliva contains bioactive molecules that aid in both processes. Although it has been shown that the salivary glands of several mosquito species exhibit a-glucosidase activities, the specific enzymes responsible for sugar digestion remain understudied. We therefore expressed and purified three putative recombinant salivary a-glucosidases from the mosquito vectors Aedes aegypti, Anopheles gambiae, and Culex quinquefasciatus and compared their functions and structures. We found that all three enzymes were expressed in the salivary glands of their respective vectors and were secreted into the saliva. The proteins, as well as mosquito salivary gland extracts (SGE), exhibited a-glucosidase activity. We solved the crystal structure of the Ae. aegypti a-glucosidase bound to two calcium ions; comparing structural models between the three species also revealed a high degree of similarity, suggesting similar functional properties. Using the solved Ae. aegypti a-glucosidase structure, we performed molecular docking to compare predicted affinities of several sugars for the Ae. aegypti a-glucosidase. Our modeling efforts suggested that the Ae. aegypti a-glucosidase exhibited preference for polysaccharides over mono- or disaccharides. In agreement, all recombinant enzymes displayed preference for sucrose compared to p-nitrophenyl-a-D-glucopyranoside (NPaGlu) in vitro, and further efforts are underway to compare enzymatic activity for other polysaccharides. In summary, we have described a-glucosidase orthologs from three mosquito vectors that are likely key players in sugar digestion after a sugar meal. Because they are secreted during mosquito feeding, these enzymes maybe useful as markers of exposure for different vectors.