Structural studies on two hemagglutinins from cicer arietinum and moringa oleifera, and a study of polymorphism in the crystals of plant lectins

Abstract

Many biological processes, such as viral, bacterial, mycoplasmal and parasitic infections, targeting of cells and soluble components, fertilization, cancer metastasis and growth and differentiation involve interaction between carbohydrate molecules and proteins. Lectins or hemagglutinins are proteins that are involved in the carbohydrate recognition and the crystal structures of lectin-carbohydrate complexes turn out to be excellent model systems to study protein-carbohydrate interactions. About 42% of the structures reported in the 3-D lectin database (http://www.cermav.cnrs.fr/lectines/) belong to lectins from plants; the remaining ones are from animals, bacteria, fungi, algae and viruses. Out of the several distinct families of plant lectins identified, the largest and the best-characterized family is from seeds of leguminous plants. One of the hemagglutinin studied in this thesis is from Cicer arietinum (chickpea), a member of the family leguminosae. This hemagglutinin shows specificity towards glycoproteins, like fetuin and desialated fetuin, but not towards any of the simple mono- or oligo-saccharides. It possesses an unusual combination of secondary structure elements for a legume lectin such as and#945; helix, 34%, and#946; sheet, 28% and random coil, 38%. X-ray crystallographic studies have been undertaken to determine the three-dimensional structure of this lectin. A second hemaglutinin included in this study is from Moringa oleifera (drumstick) a plant cultivated in India to use its leaves, fruits, and roots as food and medicine. The hemagglutinin is isolated from the seeds of this plant. This also showed specificity towards glycoproteins like fetuin, desialated fetuin, holotransferin and thyroglobulin but not towards any of the simple mono- or oligo-saccharides. Purification and biochemical and biophysical characterization of this lectin have been carried out to reveal its structure-function relationship.