The application of Marcus theory to the understanding of biological electron transfer (ET) has given great fundamental insight into the governing role that the relative arrangement of amino acids and cofactors can play in intra-protein ET. A remaining fundamental question is how the environment surrounding the electron transfer site can modulate its reorganization energy, a key parameter in Marcus theory which is difficult to measure and calculate for proteins. To address this question, we performed ultrafast spectroscopy on a small subset of homologous proteins within the Old Yellow Enzyme family, which are oxidoreductases that utilize a non-covalently bound flavin mononucleotide (FMN) cofactor to perform alkene reductions. We find that 12-oxophytodienoate reductase 1 (OPR1) displays inverted electron transfer dynamics, such that the forward photoinduced ET rate  upon photoexcitation of the FMN cofactor is slower than the subsequent back-ET to reform the ground electronic state. We find that despite the large degree of overall structural homology between OPR1 and related proteins morphinone reductase (MR) and old yellow enzyme 1 (OYE1), their photoinduced ET pathways are distinct, where the latter two exhibit normal kinetics. We can rationalize their differences in photophysical dynamics within the Marcus framework if the reorganization energies for each protein ET is assumed to differ, such that OPR1 has a large enough reorganization energy for the back-ET, which is in the Marcus inverted region, to outpace the photoinduced ET. We support our findings with site-directed mutagenesis on OPR1, finding that replacing tyrosine residues greater than 7Å away from the FMN cofactor with phenylalanine decreases the reorganization energy enough to open the ET pathways to those seen in MR and OYE1. Our work implies that the programmed amino acid heterogeneity within a protein active site can control its electron transfer pathways.

High reorganization energies direct ultrafast photoinduced electron transfer pathways in homologous old yellow enzymes

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.