Binding of 4-methylumbelliferyl N-acetyl-chitooligosaccharides to wheat-germ agglutinin: a reinvestigation of equilibrium studies

The binding of 4-methylumbelliferyl per-N-acetyl-chitooligosaccharides, MeUmbGlcNAc (I), MeUmb(GlcNAc)2 (II) and MeUmb(GlcNAc)3 (III) to wheat germ agglutinin, was studied by equilibrium dialysis, difference absorption spectrometry and extrinsic fluorescence quenching [Privat, J. P., Delmotte, F. & Monsigny, M. (1974) FEBS Lett. 46, 229–232] titration at a fixed wavelength or above 370 nm. The change in optical properties of the MeUmb group upon binding of the glycosides to the agglutinin depends on the length of the carbohydrate chain. An intense and carbohydrate-specific difference absorption spectrum was observed with I (-Δɛ at 316 nm equal to 2770 M−1 cm−1), a weaker one with II (-Δɛ at 316 nm = 1070 M−1 cm−1), but none was observed with III; the fluorescence quenching is 100% for compounds I and II and 44% for compound III. As determined with I, using equilibrium dialysis and difference absorption spectrometry, there are two identical and independent binding sites per subunit. For I the value of the association constant is independent of the experimental method or the saturation range used. The association constants increase with the chain length of the fluorescent oligosaccharides: at 25°C the values (in M−1) are (1.98 ± 0.12)104 for I, (7.5 ± 0.3)104 for II and (1.17 ± 0.60)105 for III; the thermodynamic parameters do not differ greatly: ΔH0 and ΔS0 are -8.2 ± 0.4 kcal (–34.3 kJ) mol−1 and –7.8 ± 1.5 cal (–32.6 J) mol−1 K−1 for I, –10.0 ± 0.3 kcal (–41.8 kJ) mol−1 and –11.2 ± 1.0 cal (–46.9J) mol−1 K−1 for II, –8.4 ± 0.3 kcal (–35.1 kJ) mol−1 and –4.8 ± 1.2 cal (–20.1 J) mol−1 K−1 for III. The results, taken together with data from the literature, are interpreted as binding to a region composed of subsites with a protein – MeUmb interaction for I and II amounting to a contribution of the MeUmb group in the free energy of binding, equal to – 2 kcal (– 8.4 kJ) mol−1 for I