Accurate Determination of Human CPR Conformational Equilibrium by smFRET using Dual Orthogonal Non-Canonical Amino Acid Labeling

Abstract

Conjugation of fluorescent dyes to proteins - a prerequisite for the study of conformational dynamics by single molecule Forster resonance energy transfer (smFRET) - can lead to substantial changes of the dyes photophysical properties, ultimately biasing the quantitative determination of inter-dye distances. In particular the popular cyanine dyes and their derivatives, which are by far the most used dyes in smFRET experiments, exhibit such behavior. To overcome this, a general strategy to site-specifically equip proteins with FRET pairs by chemo-selective reactions using two distinct non-canonical amino acids simultaneously incorporated through genetic code expansion in Escherichia coli was developed. Applied to human NADPH- cytochrome P450 reductase (CPR), the importance of homogenously labeled samples for accurate determination of FRET efficiencies was demonstrated. Furthermore, the effect of NADP+ on the ionic strength dependent modulation of the conformational equilibrium of CPR was unveiled. Given its generality and accuracy, the presented methodology establishes a new benchmark to decipher complex molecular dynamics on single molecules.