Figure 4: Photocycles of KR2 for sodium ion pump activity.

(a) Transient absorption spectra of KR2 reconstituted in DOPC in 100 mM NaCl at pH 8.0. (b) Time traces of absorption changes of KR2 in NaCl at 420 (blue), 472 (light green), 525 (green) and 605 nm (orange) probe wavelengths. Red lines indicate fitting lines based on the sequential kinetic model shown in d. (c) Absorption spectra of photointermediates in NaCl, which are calculated from decay-associated spectra. (d) Photocycle scheme of KR2 in NaCl determined by flash photolysis. Sodium ion uptake occurs upon formation of the O intermediate, while sodium ion release is expected to occur upon decay of the K intermediate or decay of the O intermediate (dashed arrows). The O decay is described by two time constants (τ1/e) of 7.9 ms (90.6%) and 112 ms (9.4%). (e) Transient absorption spectra of KR2 reconstituted in DOPC in 100 mM KCl at pH 8.0. (f) Time traces of absorption changes of KR2 in KCl at 420 (blue), 472 (light green), 525 (green) and 605 nm (orange) probe wavelengths. Red lines indicate the fitting lines based on the sequential kinetic model shown in h. (g) Absorption spectra of photointermediates in KCl calculated from decay-associated spectra. (h) Photocycle scheme of KR2 in KCl determined by flash photolysis. The time constant of the K decay is not determined as the decay signal is noisy. The recovery of KR2 is described by three time constants (τ1/e) of 107 ms (67.1%), 580 ms (24.2%) and 7.0 s (8.7%).