Spiropyran modified concanavalin A, Con. A, reveals photoregulated binding towards monosaccharides. The association constants of the substrate towards the spiro-Con A and the kinetics of the association processes are photostimulated. These photostimulated properties are originated from a structural perturbation of the protein upon photoisomerization of spiro-Con. A to the zwitterionic Con. A. Time resolved light scattering reveals that the structural change of the protein involves a shrinkage.