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

**Distribution**

24 H-type Six-stranded

20 Insecticides - Agriculture and Toxicology

27 TgMIC1 Sialic acid binding

32 CV-N Three-stranded

33 PVL-like Seven-bladed

34 AAL-like Six-bladed

antiparallel β-sandwich

25 Cystine-knot Cystine-knot motif X 26 TgMIC4 α/β-fold Tandem repeat x

protein

30 Monocot β-Prism II Monomer, dimer,

β-sheet and β-hairpins

β-propeller

β-propeller

42 PapG β-Sandwich Linked to

43 FimH β-Sandwich Linked to

44 F17-G β-Sandwich Linked to

Folding, assembly, and source of each family is shown.

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

**No. Family Fold Assembly Animal Plant Fungi Bacteria Virus**

Linked to different domains

tetramer

28 LysM βααβ-Motif Triple repeat x x x x 29 LNP-type α/β-fold Monomer x x x

35 Flocculins β-Sandwich Monomer x x

 BC2LCN Jellyroll Trimer x Staphylococcal toxin β-Barrel Monomer x AB5 toxin α/β-fold AB5 x PA-IIL-like β-Sandwich Dimer x MVL α/β-fold Dimer x

> different domains

> different domains

> different domains

 Hemagglutinin Jelly roll Trimer x RotavirusVP4 Jelly roll Virus capsid x Viral proteins β-Sandwich Virus capsid x Knob domain Jelly roll Virus capsid x

36 PCL-like Jelly roll Tandem repeat x

31 ABL-like α/β-sandwich Dimer, tetramer x x

Hexamer x x

x

Monomer x x x

Monomer x x

Monomer x

x x

x

x

x

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].

As previously mentioned, based on their number domains and their characteristics, plant lectins can be divided into four classes [9]:


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

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 carbohydrate-binding site [12–14]:


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 domain (**Figure 1B**).

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

applications [61]. In general, plant lectins have been widely used for studying carbohydrates on cell surface, for typing blood groups, isolating glycoconjugates, and detecting changes in normal oligosaccharide synthesis in tumoral disorders and other pathologies [62–66].

Plant Lectins with Insecticidal and Insectistatic Activities http://dx.doi.org/10.5772/intechopen.74962 23

Lectins from Fabaceae have been extensively studied and have a broad specificity for any carbohydrate moieties regardless of having highly conserved amino acid sequences between different species. These proteins have been for a long time a paradigm in the research of interaction protein-carbohydrate and their relationship structure-function [67, 68]. Available sequences (RCSB PDB, UniProtKB/Swiss-Prot) show 20% similarity and 20% of identical amino acids, and conserved amino acids are in the "binding site" and coordinate metal ions [9]. These proteins generally have two or four identical subunits with a molecular weight around 25 kDa; each one contains a binding site for metal ions. A typical example of dimeric lectins belongs to the Viceae tribe. The tetrameric lectins are present in species of the tribe Diocleae, specific by glucose/mannose. In these tribes, many lectins have been isolated and characterized with some biochemical differences and molecular similarities [47]. Recently, subtribe Diocleinae in the Millettioid legumes have been taxonomically tangled together with the large heterogeneous tribe Phaseoleae; however, a comprehensive molecular phylogenetic analysis based on nuclear and chloroplast markers includes all genera ever referred to Diocleae except for the monospecific Philippine *Luzonia*, resolving several key generic relationships within the Millettioid legumes and considered classification of Diocleinae subtribe as a tribe with three main clades: *Canavalia*, *Dioclea*, and *Galactia*. *Canavalia* clade has species gender *Canavalia*; *Dioclea* clade includes *Dioclea*, *Cymbosema*, *Cleobulia* and *Macropsychanthus*; and *Galactia* clade gender has *Galactia*, *Neorudolphia*, *Rhodopsis*, *Bionia*, *Cratylia*, *Lackeya*, *Camptosema*, and *Collaea* [69].

This tribe is widely distributed throughout the neotropics, and several species from the genus *Dioclea* have been shown to possess a lectin closely related to ConA (lectin type I). The better characterized lectins have been those from *D. grandiflora* [70, 71], *D. lehmanni* Diels [72], and *D. sericea* Kunth [73], among others, all of them belong to the Man/Glc group; their physicochemical properties and structural features are very similar [74].

Studies carried out in the PRG have allowed us to find other lectins having distinct structural and functional properties (named lectin type II) from *Diocleae lehmanni* (DLL), *Dioclea sericea* (DSL), *Dioclea grandiflora* (DGL), *Canavalia ensiformis* (CEL), and *Galactia lindenii* (GLL) [73, 75–77]. These lectins are localized in the same cellular compartment as happens in *D. lehmanni* seeds [78] and have different physicochemical properties; this allow us to question about the physiological role of these proteins. Lectin type II has high affinity toward H type 2 blood group (α-L-Fuc (1–2)-β-D-Gal (1–4)-β-D-GlcNAc-O-R), and the N-terminal region presents a unique sequence hitherto found in some Diocleinae lectins and suggests a functional similarity among this type of lectin which possesses distinctive characteristics differentiating them from "classical" mannose/glucose (Man/Glc) lectins. Taking subunit MW into account, it has been demonstrated that tetrameric forms prevailed in type I lectins, being in fast equilibrium with dimers and monomers whose amount depended upon pH or solution ionic strength [79], while some lectins from type II prevalence dimeric forms (**Table 2**). Despite their high similarity, these ConA-like (type II) lectins could induce different responses in biological assays; for example, when tested for stimulation of human lymphocyte proliferation in vitro, ConBr had a higher proliferation index

than ConA, possibly due to minor changes in binding specificities [80].

**Figure 1.** Structural conformation of plant lectins. (A) *Pterocarpus angolensis* homodimer lectin (PDB code (2PHF)). The β-sheet conformation is the most usual in plant lectins (β-sandwich). (B) The carbohydrate recognition domain (CRD) is highly conserved in plant lectins, according to its specificity.

they have been shown to play a role in plant defense [15]. But, plants also express minute amounts of specific lectins as particular responses toward environmental stresses and pathogen attack. In the absence of plant stress, the inducible lectins are not expressed at detectable levels [16]. According that, a central question which has often been asked but up to now not yet been answered definitively is that on the biological function(s) of plant lectins. Several functions have been mentioned, but there is not a final decision about that. However, because of its carbohydrate interactions, lectins have been tested for several biological functions, getting interesting results in some of them. Biological activities are related to immunomodulatory and antitumor [17–19], antifungal [20–23], antiparasitic [24–26], antiproliferative [27–30], healing process [31–33], drug delivery [34–36], as histochemical markers [37–39], biosensors [40, 41], insecticide [42–46], etc.
