Light-induced enzyme activity changes associated with the photoisomerization of bound spiropyran

A spiropyran compound which undergoes photoisomerization was covalently attached to several enzymes in order to alter their activity by imposing light on them. The enzyme-bound spiropyran exhibited normal photochromism when the matrix enzyme was hydrophobic in nature, whereas reverse photochromism was evident when the spiropyran was attached to hydrophilic enzymes. The activity of the modified enzymes was found to change because of photoisomerization of the bound spiropyran. A fluorescent probe for hydrophobic sites revealed that the modified enzymes remained hydrophobic when the bound spiropyran closed and turned hydrophilic when the spiropyran opened. A mechanism for the photoinduced activity changes is postulated in relation to the enzyme-substrate affinity in the open- and closed-ring states of spiropyran.