Binding of 4-methylumbelliferyl alpha-D-mannopyranoside to tetrameric concanavalin A Fluorescence temperature-jump relaxation study

Immuno-receptors: from recognition to signaling and function

The vertebrate adaptive immune response is initiated by specific recognition of antigens. This is carried out by molecules, soluble or cell surface receptors that are members of the Multichain Immune Recognition Receptors (MIRR) group of proteins. The soluble arm of the response is based on antibodies. Kinetic analysis of antibody-antigenic epitope interactions pioneered insights into the complexity underlying the capacity of relatively limited repertoires of antibodies to recognize an essentially unlimited range of epitopes by employing conformational diversity of a given single sequence. The arm responsible for recognition of cellular targets involves a considerably more elaborate process, predominantly of antigen-derived peptides presented bound to molecules encoded by the major histocompatibility complex (MHC). This remarkable cellular recognition process performed by T-cell receptors requires earlier steps of peptide presentation and involves interactions of the receptor sites with the array of its MHC-peptide composite ligand. In both cases, antigen recognition needs to be followed by its coupling, by biochemical cascades, to different specific responses, namely activation of effector functions. The parameters required for coupling to functional responses are still a focus of intense research. In solution, antigen-antibody aggregation is one established activation process. Those required for coupling antigen recognition to cell activation, whether by Fc receptor bound antibodies or by the B-cell antigen receptor, are also still subject to active research efforts. Though activation by immune-receptors requires antigen recognition, considerable differences could exist among the requirements set by distinct cell types. Moreover, antigen binding requiring intercellular interactions introduces additional complexity.

Effect of pH on oligomeric equilibrium and saccharide-binding properties of peanut agglutinin

The conformation and saccharide-binding properties of peanut agglutinin (PNA) depend on pH as studied by analytical ultracentrifugation, fluorescence, circular dichroism, equilibrium dialysis, and absorption spectroscopy. PNA is tetrameric in neutral solution and dissociates reversibly into dimers below pH 5.1. Below pH 3.4, the lectin is totally dimeric. Lowering of the pH induces reversible changes in the tertiary and secondary structures of PNA. Binding of saturating amounts of lactose to tetrameric (pH 6.9) or dimeric (pH 3.2) PNA resulted in identical ultraviolet difference spectra. Fluorescence studies of PNA as a function of pH in the presence of lactose indicated that tryptophanyl residues, present at or near the saccharide binding site, are more accessible to the ligand in dimeric than in tetrameric PNA. For solutions of dimeric PNA, containing only minor amounts of tetramers (pH 3.6), equilibrium dialysis with MeUmb-beta Gal beta(1----3)GalNac showed that the binding capacity of PNA was the same as for tetrameric PNA (one binding site per protomer) but the apparent association constant was one order of magnitude lower than for tetrameric PNA. The enhancement of MeUmb-beta Gal beta(1----3)GalNac fluorescence upon binding to PNA was pH dependent: 50% at neutrality, 16% at pH 3.7, and unobservable at pH 3.0, suggesting that the microenvironment of this PNA-bound chromophore changed progressively with pH and was dependent on ionization of an acidic amino acid residue.

Kinetics of interaction of some alpha- and beta-D-monosaccharides with concanavalin A

The rates of formation and dissociation of concanavalin A with some 4-methylumbelliferyl and p-nitrophenyl derivatives of alpha- and beta-D-mannopyranosides and glucopyranosides were measured by fluorescence and spectral stopped-flow methods. All processes examined were uniphasic. The second-order formation rate constants varied only from 6.8 x 10(4) to 12.8 x 10(4) M-1 x s-1, whereas the first-order dissociation rate constants ranged from 4.1 to 220 s-1, all at pH 5.0, I=0.3 M, and 25 degrees C. Dissociation rates thus controlled the value of the binding constant. The effect of temperature on these reactions was examined, from which enthalpies and entropies of activation and of reaction could be calculated. The effects of pH at 25 degrees C on the reaction rates of 4-methylumbelliferyl alpha-D-mannopyranoside and 4-methylumbelliferyl alpha-D-glucopyranoside with concanavalin A were examined. The value of the binding constant Kap (derived from the kinetics) at any pH could be related to the intrinsic binding constant K by the expression Kap = KaK(Ka + [H+])-1. The values of Ka, the ionization constant of the protein segment responsive to sugar binding, were 3 x 10(-4) M and 1 x 10(-4) M for 4-methylumbelliferyl alpha-D-mannopyranoside and 4-methylumbelliferyl alpha-D-glucopyranoside, respectively. The binding constant of p-nitrophenyl alpha-D-mannopyranoside is surprisingly much less sensitive to a pH change from 5.0 to 2.7. Ionic strength had little effect on the binding characteristics of 4-methylumbelliferyl alpha-D-mannopyranoside to concanavalin A at pH 5.2 and 25 degrees C.

Binding of manno-oligosaccharides to concanavalin A. Substitution titration with a fluorescent-indicator ligand

The association constants for binding of methyl alpha-D-mannopyranoside (I), mannobiose (II) and mannotriose (III) to concanavalin A were determined in the temperature range 285-313 K by a substitution titration, using 4-methylumbelliferyl alpha-D-mannopyranoside as a carbohydrate-specific and fluorescent indicator. All binding equilibria are simple, but establish extremely slowly with II and III. At 298.3 K, K increases moderately from I to III: (6.4 +/- 0.5) x 10(3), (1.2 +/- 0.1) x 10(4) and (1.10 +/- 0.05) x 10(5) M-1. For binding of I, II and III, the - delta H degree values are constant (36 +/- 2 kJ mol-1) and equal to the average value (36.1 +/- 0.6 kJ mol-1) obtained for the three corresponding 4-methylumbelliferyl alpha-D-manno-oligosaccharides [Van Landschoot, A., Loontiens, F. G., and De Bruyne, C. K (1978) Eur. J. Biochem. 83, 277-285]. The data are interpreted as arising from specific binding to a single mannopyranosyl residue in (alpha 1 leads to 2)-linked manno-oligosaccharides.