Interaction of concanavalin A with model substrates.

During the last few years concanavalin A has become the most widely known and intensively investigated phytohemagglutinin. Much of this interest stems from the discovery that this lectin possesses several remarkable biological properties. In addition to its role as a hemagglutinin,' a structural probe,' and a mitogen,"-" concanavalin A has been shown to restore normal growth patterns to transformed fibroblasts,O to serve as an anticarcinogenic agent,' and to distinguish between malignant and normal cells.'~ concanavalin A has been sequenced l1-I1 and its three-dimensional x-ray structure has been determined at the 2.0, 2.4, and 4 A levels.11~17 The solution properties of this protein have also been studied in some The results of the x-ray analysis are in general accord with analytical ultracentrifugation 1 6 w 2? . ?fi and equilibrium dialysis studies."'. The picture that has emerged is of a metalloprotein composed of subunits with a molecular weight of 27,000; each subunit possesses one sugar-binding site. At pH 5.5 and below, the protein exists as a dimer, molecular weight 54,000. Above p H 5.5 the protein dimers begin to aggregate, forming a tetramer with four binding sites. The analogy between concanavalin A-macromolecule interaction and the antibody-antigen system, first noted by Sumner and Howell,l has been commented upon by several i n v e ~ t i g a t o r s . ~ l ~ ~ In fact, the concanavalin A-carbohydrate interaction has been studied by means of a variety of immunochemical techniques: the quantitative precipitin the Landsteiner haptene . I 1 and equilibrium dialysis measurements.'::. Lit A chain-end mechanism was postulated to account for the precipitation reaction between concanavalin A and certain specific, branched macromolecules,33~ 37 . .m. -(* ( for example, a-D-glucans. a-D-mannans and 8-D-fructans) . As indicated below, this generalization requires modification. A model of the chain-end mechanism is shown in FIGURE 1. Concanavalin A is depicted as a tetrameric protein with four combining sites, each capable of interacting with specific, terminal glycosyl residues of polysaccharide or glycoprotein chain ends. Since only neutral saccharides react with concanavalin A, it has been postulated that hydrogen bonding probably plays an important role in the binding phenome-,on.:i:. .&I. 15. . I I : Through the use of simple sugars and their derivatives as inhibitors of the concanavalin A-dextran interaction, it has been possible to ascertain the configurational features that a molecule must possess in order to bind to concanavalin A. In some instances it has been possible to First purified and crystallized by James Sumner,'.