**2. Definition, classification, and general features of lectins**

Lectins are proteins or glycoproteins of the nonimmune origin with specific binding affinity for the carbohydrate moiety of glycoconjugates [1]. Lectins comprise a structurally diverse class of proteins characterized by their ability to selectively bind carbohydrate moieties of the glycoproteins of the cell surface. Lectins may be obtained from plant, microbial, or animal sources and may be soluble or membrane bound [2]. In nature, lectins play a role in biological recognition phenomena involving cells and proteins and thereby protect plants against external pathogens such as fungi and other organisms. The ability to bind and agglutinate red blood cells is well known and used for blood typing; hence, the lectins are commonly called hemagglutinins [3].

The term lectin is derived from the Latin word *legere* meaning "to choose" or "select" and has been generalized to encompass all nonimmune carbohydrate-specific agglutinins regardless of blood type specificity or source. Lectins were initially found and described in plants, but in subsequent years, multiple lectins were isolated from microorganisms and also from animals [4]. Interestingly, plant and animal lectins show no primary structural homology, but they demonstrate similar preferential binding to carbohydrates [5]. This suggests that animal and plant lectin genes may have coevolved, thus highlighting the importance of lectin-carbohydrate interactions in living systems [6].

Based on the amino acid sequences of available lectins, it is deduced that the carbohydratebinding property of most lectins resides in a polypeptide sequence, which is termed as "carbohydrate-recognition domain" [7]. The binding with simple or complex carbohydrate conjugates is reversible and non-covalent. The specificity of lectins toward carbohydrates can be defined on the basis of "hapten inhibition test," in which various sugars or saccharides are tested for their capacity to inhibit the property of hemagglutination of erythrocytes [8].

Lectins have been classified according to different features such as source (animal, vegetal, fungal, viral), carbohydrate affinity (mannose, glucose, galactose, fucose, sialic acid), number, and specificity of carbohydrate recognition domains (merolectins, hololectins, chimerolectins, and superlectins) [9]. However, current classification is based on 3D structure and is related to 48 families (**Table 1**) [10].


and in this group, the legume lectins have been related to insecticidal and insectistatic activities. In addition, *Phaseolus vulgaris* (PHA), *Glechoma hederacea* (Gleheda), *Canavalia ensiformis* (ConA), *Griffonia simplicifolia* (GSII), and *Pisum sativum* (PSA) lectins and other legume and Lamiaceae lectins have been studied by the Protein Research Group (PRG) in Colombia. It was evidenced that plant legume lectin domains have structural features characterized by a high percentage of β-sheet structures associated with dimeric or tetrameric assembly, presenting several specific sugar recognition sites, including mannose. In addition to these features, these lectins can interact with the digestive system of insect pests and produce a

Lectins are proteins or glycoproteins of the nonimmune origin with specific binding affinity for the carbohydrate moiety of glycoconjugates [1]. Lectins comprise a structurally diverse class of proteins characterized by their ability to selectively bind carbohydrate moieties of the glycoproteins of the cell surface. Lectins may be obtained from plant, microbial, or animal sources and may be soluble or membrane bound [2]. In nature, lectins play a role in biological recognition phenomena involving cells and proteins and thereby protect plants against external pathogens such as fungi and other organisms. The ability to bind and agglutinate red blood cells is well known and used for blood typing; hence, the lectins

The term lectin is derived from the Latin word *legere* meaning "to choose" or "select" and has been generalized to encompass all nonimmune carbohydrate-specific agglutinins regardless of blood type specificity or source. Lectins were initially found and described in plants, but in subsequent years, multiple lectins were isolated from microorganisms and also from animals [4]. Interestingly, plant and animal lectins show no primary structural homology, but they demonstrate similar preferential binding to carbohydrates [5]. This suggests that animal and plant lectin genes may have coevolved, thus highlighting the

Based on the amino acid sequences of available lectins, it is deduced that the carbohydratebinding property of most lectins resides in a polypeptide sequence, which is termed as "carbohydrate-recognition domain" [7]. The binding with simple or complex carbohydrate conjugates is reversible and non-covalent. The specificity of lectins toward carbohydrates can be defined on the basis of "hapten inhibition test," in which various sugars or saccharides are tested for their capacity to inhibit the property of hemagglutination of erythrocytes [8].

Lectins have been classified according to different features such as source (animal, vegetal, fungal, viral), carbohydrate affinity (mannose, glucose, galactose, fucose, sialic acid), number, and specificity of carbohydrate recognition domains (merolectins, hololectins, chimerolectins, and superlectins) [9]. However, current classification is based on 3D structure and

**2. Definition, classification, and general features of lectins**

importance of lectin-carbohydrate interactions in living systems [6].

decrease in intestinal absorption capacity.

18 Insecticides - Agriculture and Toxicology

are commonly called hemagglutinins [3].

is related to 48 families (**Table 1**) [10].


**3. Structure and biological activities of plant lectins**

the merolectins do not present agglutinating activity.

*Nicotiana tabacum* agglutinin family, and the ricin B family [12].

interactions with larger oligosaccharide ligands.

• **Hololectins** contain two or multivalent carbohydrate-binding sites.

lectins can be divided into four classes [9]:

confers other biological activities.

drate-binding site [12–14]:

domain (**Figure 1B**).

recognize structurally unrelated sugars.

Lectins are mainly present in seeds of plants [4, 8, 9], but they are also identified in vegetative tissues such as bulbs, tubers, rhizomes, roots, bark, stems, fruits, and leaves [11].

Plant Lectins with Insecticidal and Insectistatic Activities http://dx.doi.org/10.5772/intechopen.74962 21

As previously mentioned, based on their number domains and their characteristics, plant

• **Merolectins** are lectins that possess a single carbohydrate-binding domain. As a result,

• **Chimerolectins** possess a carbohydrate-binding domain and an additional domain that

• **Superlectins** are lectins with two or multivalent carbohydrate domains that are able to

However, since 1998, five novel lectin domains have been identified in plants. At present, plant lectins are classified into 12 different families, with distinct carbohydrate-binding domains. The families are *Agaricus bisporus* agglutinin homologs, amaranthines, class V chitinase homologs, *Euonymus europaeus* agglutinin family, *Galanthus nivalis* agglutinin family, proteins with hevein domains, jacalins, proteins with legume lectin domains, LysM domain proteins, the

In general, the three-dimensional structure of lectins is composed of a high content of β-sheets with little contribution from α-helixes. The β-sheets are connected by loops forming antiparallel chains. The stability of dimers and tetramers is conferred by hydrophobic interactions, hydrogen bonds, and salt links [13]. Three regions are formed in carbohy-

• The central region is constituted by a conserved core in which residues interact with metallic ions (Mg2+, Mn2+, and Ca2+), required for carbohydrate interactions. This core provides necessary binding energy, but it is not important to the lectin's carbohydrate specificity. • Some aromatic residues surround the core and occupy variable positions in a horseshoe

• Finally, residues with higher variability are located in the outer zone and are involved in

The structural features of plant lectins are shown in **Figure 1**, which is possible to see the high content of β-sheets (**Figure 1A**) and the structure of a typical carbohydrate recognition

However, the kind of expressed lectins can have some differences according to the specific tissue or the moment in which the plant is expressing it. A lot of plant lectins are constitutively expressed in high amounts in seeds and vegetative storage tissues where

shape. This region is fully involved in the lectin's monosaccharide specificity.

**Table 1.** Lectin families in nature.
