Thermal isomerization of the chromoprotein asFP595 and its kindling mutant A143G: QM/MM molecular dynamics simulations.

Chromoprotein asFP595 and its A143G variant called kindling fluorescent protein (KFP) are among the chronologically first species for which trans-cis chromophore isomerization has been proposed as a driving force of photoswitching. In spite of long-lasting efforts to characterize the route between protein conformations referring to the trans and cis forms of the chromophore, the molecular mechanism of this transformation is still under debate. We report the results of computational studies of the trans-cis isomerization of the anionic and neutral chromophore inside the protein matrices in the ground electronic state for both variants, asFP595 and KFP. Corresponding free energy profiles (potentials of mean force) were evaluated by using molecular dynamics simulations with the quantum mechanical-molecular mechanical (QM/MM) forces. The computed free energy barrier for the cis-trans ground state (thermal) isomerization reaction is about 2 kcal/mol higher in KFP than that in asFP595. These results provide interpretation of experimental studies on thermal relaxation from the light-induced activation of fluorescence of these proteins and correctly show that the A143G mutation in asFP595 noticeably increases the lifetime of the fluorescence species.