Local Electric Fields Drives the Proton-Coupled Electron Transfer within Cytochrome P450 Reductase

Cytochrome P450 enzymes play a pivotal role in biosynthetic and metabolic transformations. Especially, cytochrome P450 reductase (CPR) acts as the key electron donor for the oxygen activation by P450 monoxygenases, but the electron transfer mechanism within CPR is largely elusive. Here, extensive molecular dynamics (MD) and quantum mechanics/molecular mechanics (QM/MM) calculations were performed to elucidate CPR's electron transfer mechanism. We found that electron transfer from FADH– to FMN occurs through a proton-coupled electron transfer (PCET) mechanism. Here, Glu142 transfers a proton to FMN via a two-water-molecule chain, concurrent with electron transfer from FADH– to FMN. The subsequent ET from FADH• to FMNH• involves an Asp675-mediated PCET process, where the Ser457-assisted proton transfer from FADH• to Asp675 is coupled with electron transfer from FADH• to FMNH•. Notably, the local electric field from the doubly protonated His180 significantly enhances the PCET reactions both kinetically and thermodynamically. This study highlights the vital role of the local electric field in facilitating biological electron transfer for enzymatic reactions.