Energetics of Proton Transfer Pathways in Reaction Centers from Rhodobacter sphaeroides

Abstract

Electron transfer between the primary and secondary quinones (Q A , Q B ) in the bacterial photosynthetic reaction center (bRC) is coupled with proton uptake at Q B . The protons are conducted from the cytoplasmic side, probably with the participation of two water channels. Mutations of titratable residues like Asp-L213 to Asn (inhibited mutant) or the double mutant Glu-L212 to Ala/ Asp-L213 to Ala inhibit these electron transfer-coupled proton uptake events. The inhibition of the proton transfer (PT) process in the single mutant can be restored by a second mutation of Arg-M233 to Cys or Arg-H177 to His (revertant mutant). These revertant mutants shed light on the location of the main proton transfer pathway of wild type bRC. In contrast to the wild type and inhibited mutant bRC, the revertant mutant bRC showed notable proton uptake at Glu-H173 upon formation of the Q B ؊ state. In all of these mutants, the pK a of Asp-M17 decreased by 1.4 -2.4 units with respect to the wild type bRC, whereas a significant pK a upshift of up to 5.8 units was observed at Glu-H122, Asp-H170, Glu-H173, and Glu-H230 in the revertant mutants. These residues belonging to the main PT pathway are arranged along water channel P1 localized mainly in subunit H. bRC possesses subunit H, which has no counterpart in photosystem II. Thus, bRC may possess alternative PT pathways involving water channels in subunit H, which becomes active in case the main PT pathway is blocked.

Induced conformational changes upon Cd 2+ binding at photosynthetic reaction centers

“…First, most of these residues are those that experience pK a increases upon revertant mutations in bRC (9). The recovery of PT in revertant mutant bRC suggests that an alternative PT pathway involving Glu-H173 is operative in this case (2,8,9,32).…”

“…First, most of these residues are those that experience pK a increases upon revertant mutations in bRC (9). The recovery of PT in revertant mutant bRC suggests that an alternative PT pathway involving Glu-H173 is operative in this case (2,8,9,32). In comparison with WT-bRC (20), both Cd-bRC (15) and revertant mutant bRC (8) revealed significant structural changes at Glu-H173 and the near surroundings, which seemingly relates to variation in the activity of an additional PT pathway (i.e., being inhibited in WT-bRC and operative in both Cd-bRC and revertant mutant bRC).…”

“…These double mutants D(L213)N͞R(M233)C or D(L213)N͞R(H177)H are therefore called revertant mutants (2,6). Based on the structural changes near Glu-H173 observed in the revertant mutant bRC, the proposed mechanism to recover PT involves participation of Glu-H173 in the PT pathway (2,8,9).…”

“…Because the present study was done under the same conditions as our previous pK a computation for bRC, further details on error estimates and comparisons with the previous results can be obtained in ref. 9. With the electrostatic method used in the present study we can evaluate energetic differences in protonation states between initial states before PT and the corresponding final states, while ignoring activation barriers.…”

“…In the present study, we calculated the protonation pattern of titratable residues in Cd-bRC based on the crystal structure (15) (8). Atomic coordinates were obtained in the same way as in previous applications (9,(21)(22)(23). The positions of hydrogen atoms were energetically optimized with CHARMM (24) by using the CHARMM22 force field.…”