Reversible control of enzyme-inhibitor interactions with light illumination using a photoresponsive surfactant

The effects of a photoresponsive surfactant and light illumination on the complex formed between ribonuclease A (RNase A) and a protein ribonuclease inhibitor (RI) have been investigated to develop a light-based technique to reactivate an enzyme through surfactant-induced dissociation of the enzyme-inhibitor complex. The photoresponsive surfactant undergoes a photoisomerization from the relatively hydrophobic trans isomer under visible light to the relatively hydrophilic cis isomer upon UV illumination, providing a means to reversibly control protein-inhibitor interactions. In the absence of surfactant, RI binds tightly to RNase A through noncovalent interactions, which inhibits the enzyme activity. Upon addition of the surfactant under visible light, RNase A is reactivated, regaining ~75% of the native activity in the absence of RI. In the presence of the surfactant under UV light, however, the enzyme remains inhibited. Fluorescence spectroscopy, dynamic light scattering, and circular dichroism spectroscopy reveal that RI dramatically unfolds upon addition of the trans form of the surfactant, while RNase A does not undergo noticeable structural changes under the same conditions. This indicates that RNase A reactivation occurs through dissociation of the enzyme-inhibitor complex arising from surfactant-induced unfolding of the inhibitor. As a result, photoresponsive surfactant and light illumination can be used as a novel light-based technique to dissociate enzyme-inhibitor complexes and, thus, reactivate an inhibited enzyme.