**7.1 Mannose/glucose**

152 Antimicrobial Agents

Lectins have been found in a wide variety of species almost every major taxonomical classification of flowering plants (Allen and Brilliantine 1969; and Mialonier et al., 1973). Many plants and their individual tissues have been routinely screened for lectins by measuring the ability of their extracts to agglutinate erythrocytes. Although this hemagglutination assay has been of great value in detecting lectins, it is at best semiquantitative; it will not detect inactive or monovalent lectin, nor will it provide accurate estimates of lectin if an endogenous receptor for that lectin is present in the extract. The assay can at times yield false positive results because of nonspecific hemagglutination caused by lipids or by polyphenols such as tannins that are often abundant in plant tissues. It is therefore advisable to verify positive hemagglutination data by inhibiting the activity

The carbohydrate specificities and structures of lectins from a large variety of plants have been studied in considerable detail. In general, lectins from plants within particular taxonomical groups have distinctive properties that distinguish them from lectins of less closely related plants. It is important to note that the lectins used in these comparisons represent the most abundant and therefore most intensively studied lectins in the plants of these families. These lectins are not all derived from homologous tissues. These differences in origin must be remembered in interpreting these comparisons since, as is discussed below, it is possible that different tissues within the same plant may contain different lectins. This reservation does not apply to comparisons of lectins obtained from homologous tissues of plants within the same family. Homologies within two of these families, the Graminaceae

Graminaceae: The lectin from monocotyledonous plants is the wheat germ agglutinin, which is a 36,000 molecular weight dimer of identical protein subunits linked by interchain disulfide bonds (Nagata and Burger, 1974; and Rice and Etzler, 1974). The complete amino acid sequence of this lectin has recently been determined (Wright et al., 1984). This lectin has a specificity for oligomers of β (1-4)-*N*-acetyl-o-glucosamine (Allen et al., 1973). Lectins with similar specificities and molecular properties have been isolated from rye (Peumans et al., 1982b) and barley embryos (Mishkind et al.,1983; Peumans et al., 1982b). Indeed, these lectins are so similar that they can undergo subunit exchange to form heterodimers

Leguminoseae: The seeds of legumes are particularly rich in lectins, and many of these lectins have been characterized extensively (Goldstein and Hayes 1978; Lis and Sharon 1986). As this review was prepared, the complete amino acid sequences of Concanavalin A (Edelman et al. 1972), favin (Cunningham et al., 1979), and lectins from lentil (Foriers et al., 1981), sainfoin (Kouchalakos et al., 1984), *Phaseolus vulgaris* (Hoffman et al., 1982), soybean (Hemperly et al., 1983), and pea (Higgins et al., 1983) have been determined. In addition, the NH2 terminal amino acid sequences of at least 15 other legume lectins are available. Comparisons of these sequences have shown extensive homologies, particularly among those lectins from plants within the same tribes. It is clear that these lectins have been conserved during evolution of the legumes and that the homologies in their NH2 terminal amino acid sequences reflect the taxonomical relationships of the plants in this family

with specific sugars or by isolating the lectin (Tsivion and Sharon, 1981).

and Leguminoseae, are discussed in further detail below.

(Peumans et al., 1982a).

(Foriers et al., 1977; and Foriers et al., 1979).

**6. Plant lectin** 

A lectin with specificity for mannose and glucose residues has been isolated in crystalline form the fava bean (*Vicia laba*) by a procedure which included absorption to Sephadex. It has a molecular weight of 50,000 Da and appears to be a tetramer made of two subunits of 18,000 Da and two subunits of 9,000 Da. These studies determine amino acid sequence and three-dimensional structure of lectin were similar with structural features of Concanavalin A (Irvin, 1976).

Fig. 2. Common structural features of *N*-acetylneuraminic acid and *N*-acetylglucosamine (A) and of mannose and fucose (B). Similarity of *N*-acetylglucosamine and *N*-acetylneuraminic acid at positions C-2 (acetamido) and C-3 (hydroxyl) of the pyranose ring is observed when the sialic acid molecule is suitably rotated. Rotation of the fucose molecule by 180 Å allows superimposition of its ring oxygen, 4-OH, 3-OH and 2-OH with the ring oxygen, 2-OH, 3- OH and 4-OH of mannose, respectively. Groups that thus occupy the same positions in space are underlined. (Sharon, 1993).

Antimicrobial Activity of Lectins from Plants 155

Different lectin families are in general structurally unrelated. And even in those cases where a common fold is recruited, convergent evolution is the most likely explanation. Some lectin families such as the galectins recognize only one specific oligosaccharide, and consequently have a very conserved recognition site. On the other extreme, members of the C type lectin family span a wide variety of specificities. Consequently, their recognition sites are highly variable, and different specificities can easily be engineered by site directed mutagenesis (Iobst and Drickamer, 1994; and Kolatkar and Weiss, 2009). A general feature of binding sites of all lectins seems to be that they consist of a primary binding site that is capable of recognizing in a specific way a single monosaccharide residue, usually with a low affinity (in the millimolar range). Very often, but not always, there are further subsites that can be occupied by sugar residues connected to the one bound in the primary site. This allows for a modest increase in affinity. Folding in common between plant and animal lectins are βsandwich fold, β-Trefoil folds and Hevein domains. The legume lectin-like β-sandwich fold found in Galectins that conserved family of β-galactosyl binding lectins that occur in both vertebrates and invertebrates (Hirabayashi et al., 2009). Except for the legume lectins, galectins and pentraxins, it is observed in a number of carbohydrate processing and other enzymes such

as β-glucanase and asparagine amidase (Keitel et al., 1993; and Kuhn et al., 1994).

Fig. 3. The legume lectin-like β-sandwich folds illustrated by a member of the legume lectins (Concanavalin A in complex with the trisaccharide [Man(α-1, 3)]Man(α-1, 6)Man). In each case, only a single monomer of the multimeric protein is shown. Bound carbohydrate is shown in ball-and-stick. Metal ions are shown as grey spheres, bound ligands are shown in

The β-trefoil fold was first identified as a carbohydrate recognition domain in ricin (Montfort et al., 1987). Later, it was also found to be the fold of amaranthin. The β-trefoil fold is another fairly common fold, first identified in soybean trypsin inhibitor (Sweet et al. 1974). It consists of a repeat of three subdomains, each consisting of a fourstranded

green ball-and-stick representation (Loris, 2002).

antiparallel β-sheet.

**8. Structure of plant lectin** 

#### **7.2 Galactose/***N***-acetylgalactosamine**

As mentioned, lectin interacts with galactose or *N*-acetylgalactosamine such as the lectin from the corn coleoptyle. It is a glycoprotein had molecular mass under non-denaturing conditions was 88.7 kDa And had carbohydrates that constituted 12% of the total weight comprised galactose, mannose, and *N*-acetyl-D-glucosamine (Martinez-Cruz et al., 2001). In 2003 Konozy et al. were found *Erythrina speciosa* seeds can be specific with D-galactose and had two identical subunits of molecular mass was 27.6 kDa include the lectin was a neutral carbohydrate content of 5.5% (Konozy et al., 2003). *N*-acetyl-D-galactosamine-specific lectin isolation from *Glycine max* L. Merrill SA88 them were found the soybean lectin consists of four subunits it had molecular weight of each 30,000 Da in one-step purification with high purity and high yield (about 90% recovery from the crude extract) by use Poly (hydroxypropyl methacrylate-glycidyl methacrylate) beads were as an affinity matrix and *N*-acetyl-D-galactosamine (GalNAc) was as an affinity ligand (Percin et al., 2009).

### **7.3 Fucose**

*Aleuria aurantia* lectin (AAL) is a commercially available lectin that is known for its high affinity for α-1, 6-fucosylated oligosaccharides and it is widely used to estimate the extent of α-1,6-fucosylation on glycoproteins and to fractionate glycoproteins. For research a novel probe for core fucose from *Aspergillus oryzae* L-fucose-specific lectin (AOL) has strongest preference for the alpha 1,6-fucosylated chain among α-1,2-, α-1,3-, α-1,4-, and α-1,6 fucosylated pyridylaminated (PA)-sugar chains. These results suggest that AOL is a novel probe for detecting core fucose in glycoproteins on the surface of animal cells (Matsumura et al., 2007). Furthermore, *Lotus tetragonolobus* lectin is a fucose-specific legume lectin. It is a homotetramer composed of four legume lectin domains was 27,800 Da (Moreno et al., 2008).
