Carbohydrate-Protein Recognition Model

The interaction of vesicles with molecules in the surrounding solution is a particularly fascinating topic. Small and large molecules can bind to the bilayer membrane and mediate the interactions between vesicles. In this respect, synthetic vesicles are versatile model systems for the protein- and carbohydrate-mediated recognition, adhesion, and fusion of membranes that occur during endocytosis, viral infection, cell adhesion, and the growth of tissue from individual cells. 

The molecular recognition of septanose carbohydrates has been investigated in depth by using concanavalin A68 as a model lectin. Complex formation was analysed by STD experiments and showed the first direct evidence of binding, by a natural protein, for this class of ring-expanded carbohydrate molecules. 

The small size of hevein (43 residues), and the ease of its availability by biochemical purification or methods of peptide synthesis make this domain an excellent model system for the study of carbohydrate recognition by proteins. Herein, and taking the hevein domain as a model, we focus on the study of those molecular-recognition features relevant for the interactions between carbohydrates and proteins. We detail all of the techniques that are instrumental for tackling this problem, and how these can strategically be combined in an efficient manner. Particular emphasis is placed on the acquisition and analysis of data at atomic resolution (by NMR and/or X-ray ), and how these structural data relate with thermodynamic and kinetic information in reaching an understanding of the forces and interactions that play decisive roles in the interactions between carbohydrates and proteins. 

N. Aboitiz, M. Vila-Perello, P. Groves, J. L. Asensio, D. Andreu, F. J. Canada, and J. Jimenez-Barbero, NMR and modeling studies of protein-carbohydrate interactions Synthesis, three-dimensional structure, and recognition properties of a minimum hevein domain with binding affinity for chitooligosaccharides, ChemBioChem, 5 (2004) 1245-1255. 

We are now preparing and studying membrane models formed by ternary systems amphipatic block copolymer/lipids/water. From the interaction with our polymeric models of lectins (lectins are proteins or glycoproteins specific of different sugar residues] we hope to obtain informations about the respective parts played by the different carbohydrate chains and the polypeptide skeleton of glycoproteins and perhaps help to throw some light on problems as important as cell recognition and cell contact inhibition. 

The currently accepted structure of B. is the fluid mosaic model. Lipid molecules and membrane proteins are free to diffuse laterally and to spin within the bilayer in which they are located. However, a flip-flop motion from the inner to the outer surface, or vice versa, is energetically unfavorable, because it would require movement of hydrophilic substituents through the hydrophobic phase. Hence this type of motion is almost never displayed by proteins, and it occurs much less readily than translational motion in the case of lipids. Since there is little movement of material between the inner and outer layers of the bilayer, the two faces of the B. can have different compositions. For membrane proteins, this asymmetry is absolute, and, at least in the plasma membrane, different proportions of lipid classes exist in the two monolayers. Attached carbohydrate residues appear to be located only on the noncytosolic surface. Carbohydrate groups extending from the B. participate in cell recognition, cell adhesion, possibly in intercellular communication, and they also contribute to the distinct immunological character of the cell. 

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