Mechanism of Reverse Photoconversion in Canonical Bacteriophytochrome

The isomerization of the chromophore is a central photoactivated process in the functioning of a class of proteins called the bacteriophytochrome. This isomerization is affected by the protein environment and leads to the specific activity of the proteins. We have studied the reverse direction (conversion of the far-red light absorbing form to the red light absorbing form) of the photoisomerization process for phytochrome. Our study reveals two different pathways that may lead to this photoisomerization, and we have identified one of them as the preferred one on the basis of energy criteria. We compared this pathway with that of the forward direction of the photoisomerization. Furthermore, we explored that the electrostatic interaction of the protein environment has a pivotal role in favoring one of the pathways for this photoconversion.