*4.10. NADPH Assay*

The rate of NADPH (<sup>ε</sup> = 6.2 108 mM−1·cm−<sup>1</sup> at A340nm) consumption by each CYP2C8 variant (0.2 μM) incubated with CPR (0.6 μM) and PAC (70 μM) in 0.1 M potassium phosphate buffer and 200 μM NADPH was determined via UV–vis spectroscopy using a Cary 300 UV–vis spectrometer in kinetics mode (Agilent Technologies), as previously described [41,44].

#### *4.11. Stopped-flow Kinetics of Electron Transfer*

An Applied Photophysics SX-17 MV Spectrophotometer (Leatherhead UK) was used to monitor the reduction of CYP2C8\*1/\*2/\*3/R139K/K399R, as previously described with the following modifications [41]. Reaction cell 1 containing CYP2C8 (2 μM) in 0.1% cholate, CPR (2 or 6 μM), paclitaxel (70 μM), glucose oxidase (1 U/mL), and glucose (10 mM) dissolved in 100 mM potassium phosphate buffer was kept anaerobic and CO-saturated. Reaction cell 2 containing excess NADPH (1 mM), paclitaxel (70 μM), glucose oxidase (1 U/mL), and glucose (10 mM) dissolved in 100 mM potassium phosphate buffer was also kept anaerobic. The reaction cells were kept at 4 ◦C until rapid mixing followed by absorbance readings at 37 ◦C.

#### *4.12. Data Analysis of Stopped-Flow Experiments*

The reduction of ferric CYP2C8 to a ferrous–CO complex was monitored near A450nm upon mixing the two separate reaction cells in logarithmic mode and analyzed as described previously, with the following changes [41]. All data indicate the average of 3–6 individual reactions fitted using either a monophasic or biphasic exponential equation using OriginPro 2017. The initial decrease in absorbance at 450 nm (A450nm) corresponding to the rapid reduction of CPR were not included in these analyses due to spectroscopic noise. *R<sup>2</sup>* values for fits exceeded 0.99 in most cases. The errors reported are SEM.
