Electron-transfer rates govern product distribution in electrochemically- driven P450-catalyzed dioxygen reduction

Developing electrode-driven biocatalytic systems utilizing the P450 cytochromes for selective oxidations depends not only on achieving electron transfer (ET) but also doing so at rates that favor native-like turnover. Herein we report studies that correlate rates of heme reduction with ET pathways and resulting product distributions. We utilized single-surface cysteine mutants of the heme domain of P450 from Bacillus megaterium and modified the thiols with N-(1-pyrene)-iodoacetamide, affording proteins that could bond to basal-plane graphite. Of the proteins examined, Cys mutants at position 62, 383, and 387 were able to form electroactive monolayers with similar E _1/2 values (- 335 to - 340 mV vs AgCl/Ag). Respective ET rates (k _s ^o) and heme-cysteine distances for 62, 383, and 387 are 50 s ^-1 and 16 Å, 0.8 s ^{- 1} and 25 Å, and 650 s ^{- 1} and 19 Å. Experiments utilizing rotated-disk electrodes were conducted to determine the products of P450-catalyzed dioxygen reduction. We found good agreement between ET rates and product distributions for the various mutants, with larger k _s ^o values correlating with more electrons transferred per dioxygen during catalysis.