Blue light using flavin (BLUF) photoreceptor proteins are critical for the light regulation of many physiologically important processes and serve as a prototype for photoinduced proton-coupled electron transfer (PCET) in proteins.  Photoexcitation of the flavin chromophore induces PCET, as well as local conformational changes that subsequently propagate to distal parts of the protein and drive other chemical and physical changes.  In the Slr1694 BLUF photoreceptor, experiments indicate that photoexcitation to a locally excited state within the flavin instigates electron transfer from a tyrosine to the flavin, followed by proton transfer from this tyrosine to the flavin via an intervening glutamine residue.  Free energy simulations of the entire solvated system show that photoexcitation to the locally excited state of the flavin induces formation of the proton relay from tyrosine to glutamine to the flavin if it is not already formed.  Electrostatically embedded time-dependent density functional theory (TDDFT) and complete active space self-consistent-field (CASSCF) calculations indicate that formation of the proton relay reduces the energy gap between the locally excited state and the charge transfer state associated with electron transfer from the tyrosine to the flavin.  Although the charge transfer state is higher in energy than the locally excited state prior to photoexcitation, the protein environment can reorganize in a manner that stabilizes the charge transfer state, enabling a transition from the locally excited state to the charge transfer state.  An electrostatic analysis identified motions of individual residues, such as a nearby arginine, that stabilize electron transfer from the tyrosine to the flavin.  The conformational changes associated with rearrangements of hydrogen-bonding networks within the active site and movement of charged residues surrounding the active site alter the electrostatic environment in a manner that facilitates photoinduced PCET within the BLUF photocycle.<br/>

Conformational motions and electrostatics facilitate proton-coupled electron transfer in BLUF photoreceptor proteins

2019 Spring National Meeting

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