Energetics of the Electron Transfer Pathways in the Homodimeric Photosynthetic Reaction Center

Abstract

Photosynthetic reaction centers from a green sulfur bacterium (GsbRC), the PscA/PscA proteins, and photosystem I (PSI), PsaA/PsaB proteins, share structural similarities. Here, we report the redox potential (E m ) values of GsbRC by solving the linear Poisson−Boltzmann equation and considering the protonation states of all titratable sites in the entire GsbRC protein and identify the factors that shift the E m values with respect to PSI. The E m values for oneelectron reduction of the accessory (A −1 ) and adjacent (A 0 ) chlorophylls in GsbRC are 100−250 mV higher than those in PSI, whereas the E m values for the Fe 4 S 4 cluster (F X ) are at the same level. The PsaA-Trp697/PsaB-Trp677 pair in PSI, which forms the A 1 -quinone binding site, is replaced with PscA-Arg638 in GsbRC. PsaB-Asp575 in PSI, which is responsible for the E m difference between A 1A and A 1B quinones in PSI, is absent in GsbRC. These discrepancies also contribute to the upshift in E m (A −1 ) and E m (A 0 ) in GsbRC with respect to PSI. It seems likely that the upshifted E m for chlorophylls in GsbRC ultimately originates from the characteristics of the electrostatic environment that corresponds to the A 1 site of PSI.

Energetic Diversity in the Electron-Transfer Pathways of Type I Photosynthetic Reaction Centers

“…1,33 The uptake of 0.4 H + by PscA-Asp634 was also observed in response to F X − formation in GsbRC, which suggests that its protonation state is coupled with the redox state of F X in GsbRC. 15 However, the present study shows that no proton uptake is observed in response to the formation of A 0…”

“…They are conserved as PscA-Arg502 and PscA-Arg638 in GsbRC (Figure ). In GsbRC, PscA-Arg502 also increases E m (A 0 /GsbRC) most significantly . PscA-Arg638 not only serves as a binding site of a phosphatidylglycerol molecule but also increases E m (A 0 /GsbRC) with respect to E m (A 0 /PSI) .…”

“…■ RESULTS AND DISCUSSION E m (A 0 /HbRC) was experimentally estimated to be −854 mV. 11 The E m value is as high as E m (A 0 /GsbRC) (−800 mV 15 ) but ∼200 mV higher than E m (A 0 /PSI) (−1050 mV 36,37,38 ), which suggests that the high E m (A 0 ) values are a feature of homodimeric reaction centers (Figure 2). Nevertheless, the influence of the HbRC protein environment on E m (A 0 /HbRC) differs significantly from the influence of the GsbRC protein environment on E m (A 0 /GsbRC).…”

“… E m for one-electron reduction in the electron-transfer chains in type I reaction centers. (a) HbRC, (b) GsbRC, and (c) PSI. , The experimentally estimated values are shown for E m (A 0 /HbRC) (−854 mV) and E m (F X /PSI) (−705 mV).…”

“…However, the corresponding membrane-extrinsic protein subunit is not observed in the HbRC structure. 9 The absence of a tightly bound membrane-extrinsic protein subunit may be associated with the E m (F X /HbRC) value of −440 to −510 mV, 11−13 which is ∼200 mV higher than those of E m (F X /GsbRC) 14,15 and E m (F X /PSI). 16−19 However, the loss of the PsaC protein subunit leads to an increase of only 40 mV in E m (F X /PSI), 20 which does not sufficiently account for the high E m (F X /HbRC) value.…”