Insecticide Activity of Lectins and Secondary Metabolites

Proteins are polymers of amino acids (molecules containing an amino group, a carboxylic group and a hydrophobic or hydrophilic side chain) present in all organisms. Apolar, polar uncharged and electrically charged amino acids are covalently linked through peptide bonds (amide bonds) and the sequence they form in the polypeptide chain (primary structure) determines the tertiary or quaternary structures ultimately presenting some biological activity. Proteins can be formed by one or multiple polypeptides (subunits) with or without a non-amino acid molecule (carbohydrate, ion, lipid, etc) linked to them. Lectins comprise a heterogeneous group of non-immune proteins that interact with carbohydrates. This interaction is behind a number of biological properties, including antimicrobial, antitumoral, hemagglutinating, mitogenic and insecticide activities. The specificity of the carbohydrate binding site is determined by the amino acids forming the lectin molecule, as well as shape and the spatial arrangement of neighboring amino acids; additionally, metal ions may contribute for correct positioning of the amino acid residues for binding to the carbohydrate (Sharon and Lis, 2001). Lectins can be divided into those that bind monosaccharides as well as oligosaccharides, and those that recognize only oligosaccharides (Sharon and Lis, 2007). Depending on carbohydrate specificity, they can be classified as: glucose/mannose, N-acetylglucosamine, galactose, N-acetylgalactosamine, fucose and sialic acid-binding lectins (Wu et al., 2001). The hemagglutinating activity assay (Figure 1A) in presence of free carbohydrates (Figure 1B) has been proved to be a useful tool to characterize lectin specificity. Plant lectins have been isolated from bark, cladodes, flowers, leaves, rhizomes, roots and seeds. They differ from each other with respect to their molecular structures, carbohydratebinding specificities, and biological activities. The compact globular structures, molecular aggregation and glycosylation of lectins in general result in high structural stability (Kawsar et al., 2008; Moreno et al., 2008). In general, lectin isolation procedures include protein extraction steps with aqueous solvent, the production of a lectin-rich fraction, and separation of lectin from protein or non-protein