Light-activated biological systems are vital to life and often highly complex, involving multiple proteins, chromophores, and complex structural and electronic relationships. Photoenzymes are a subset of these systems that are relatively small (&lt;1000 protein residues) and use light for biological catalysis. Fatty acid photodecarboxylase (FAP) is a newly discovered photoenzyme that converts fatty acids to hydrocarbon chains and CO<sub>2</sub>, driven by blue light. However, the reaction mechanism is not yet known. We have examined the excited state dynamics and charge transfer steps of the catalytic cycle using a combination of molecular dynamics (MD) simulations, QM/MM, and excited state methods. We found that the protein environment and solvent play a critical role in sampling favorable substrate and chromophore orientations. Once a favorable geometry is adopted, we observe that the oscillator strength of S2 increases slightly by 0.06 eV while the energy gap between S1 and S2 decreases by 0.3 eV. Taken together, these results suggest that the charge transfer step of the reaction requires population transfer between the first two excited states. Our results provide a mechanistic basis for the first step of the reaction mechanism of FAP and identify important relationships between the dynamic protein environment and the reaction mechanism, presenting a unique opportunity for enzymatic re-design and clean production of biofuels.

Mechanistic insights into photodecarboxylation of fatty acids from classical and QM/MM simulations

2019 Fall National Meeting

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