Tryptophan residues and the sugar binding site of potato lectin

Potato lectin (Solanum tuberosum agglutinin, STA) was found to contain fluorescent tryptophan residues highly exposed to solvent. The binding of chitin oligosaccharides to STA induced fluorescence quenching, a shift of the fluorescence maximum to shorter wavelength, a decrease in the quenching constant of iodide ion and a decrease of the number of tryptophan residues modifiable by N-bromosuccinimide. The results suggested that one tryptophan residues is located at or near a sugar binding site of STA, and that its environment is altered from hydrophilic to relatively more hydrophobic upon interaction with specific sugars. The binding constants of STA with chitin oligosaccharides were determined by measuring the peak-trough heights in the fluorescence difference spectra induced by various concentrations of sugars. The inhibition constants of chitin oligosaccharides for the hemagglutinating activity of STA were obtained by the method of Pitts and Yang [(1981) Biochem. J. 195, 435-439] and the results were in good agreement with those obtained by the fluorescence spectral method. Standard and unitary free energy changes (delta G0 and delta Gu) and standard enthalpy changes (delta H0) were also obtained. These values decreased with sugar chain length up to at least the tetramer. Thus, it was assumed that there are at least 4 subsites, A, B, C, and D, in the sugar binding site of STA. The contributions to the binding energy (delta Gu) were -17.0, -12.6, -7.3, and -4.4 kJ/mol at subsites A, B, C, and D, respectively, and the bindings of chitin monomer (GlcNAc), dimer, trimer, and tetramer were assumed to occur at subsite A, AB, ABC, and ABCD, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)