Arginine52 controls the photoisomerization process in photoactive yellow protein

We have performed excited-state dynamics simulations of the Arg52Gln (R52Q) mutant of photoactive yellow protein (PYP). The results of these simulations demonstrate that in the mutant the primary events after photoexcitation are different from those in the wild-type. In the mutant, the chromophore predominantly undergoes single bond photoisomerization, whereas in the wildtype, photoisomerization of the double bond occurs. Furthermore, the excited-state lifetime is around three times longer than in wild-type PYP, which agrees well with recent transient absorption measurements. In 20% of the trajectories, we observe the formation of a photoproduct that has the carbonyl oxygen atom of the chromophore flipped by almost 180 disrupting the hydrogen bond between the chromophore and the backbone amino group of Cys69. This observation, in combination with the fact that the mutant is pholoactive, suggests that the break of the hydrogen bond is the key step in the photoactivation process rather than the double bond trans-to-cis isomerization.