Ligand-protein interaction in plant seed thiamine-binding proteins. Binding of various thiamine analogues to the sepharose-immobilized buckwheat-seed protein

Soluble thiamine-binding proteins occur in microorganisms, some animal tissues, and plant seeds. Their representative, the buckwheat-seed protein, was chosen as a model for chemical studies on the mechanism of ligand-protein interaction in these systems. In this work, in order to refine a concept of the chemical topography of the thiamine-binding center, the buckwheat seed protein was immobilized in Sepharose gel and probed with a new set of thiamine-related compounds. In terms of the standard change of Gibbs free energy on the complex formation, the following energetic contributions were specifically assigned to major structural features of the thiamine molecule: (i) 35-45% to the specific electronic structure of planar, unsaturated thiazolium ring with positive charge asymmetrically delocalized, one half of that contribution being attributable to the S(1) atom, (ii) 11-18% to nitrogen atoms and their electronic coupling within the pyrimidine ring, (iii) 15% to the 4'-amino group, and (iv) less than 10% to the hydroxyethyl chain.