Species differences in unlocking B‐side electron transfer in bacterial reaction centers

Abstract

The structure of the bacterial photosynthetic reaction center (RC) reveals symmetry-related electron transfer (ET) pathways, but only one path is used in native RCs. Analogous mutations have been made in two Rhodobacter (R.) species. A glutamic acid at position 133 in the M subunit increases transmembrane charge separation via the naturally inactive (B-side) path through impacts on primary ET in mutant R. sphaeroides RCs. Prior work showed that the analogous substitution in the R. capsulatus RC also increases B-side activity, but mainly affects secondary ET. The overall yields of transmembrane ET are similar, but enabled in fundamentally different ways.

Switching sides—Reengineered primary charge separation in the bacterial photosynthetic reaction center

“…2A) and close to the position of P + B A − in WT (18,19). We have previously shown that these 3 mutations in R. sphaeroides RCs result in ∼20% yield of P* → P + H B − and that additionally replacing ThrM133 with Glu increases the P + H B − yield to ∼60% (17). These 4 mutations are the core of 2 RC backgrounds used here, denoted bg1 and bg2.…”

“…On the B side, the residue that is related by C 2 symmetry to GluL104 is ThrM133, which is changed to Glu here in bg1, bg2, Arg-bg1, and Arg-bg2 RCs. Glu substituted at M133 is also thought to be neutral and make a hydrogen bond to H B because this substitution is shown in prior work (17,(31)(32)(33)(34) to red shift the Q x bands of H B by ∼7 nm (SI Appendix, Table S2). We find here that the presence of ArgL185 results in a blue shift of the Q x bands of H B by ∼7 nm in Arg-bg1 and Arg-bg2 RCs compared to bg1 and bg2 RCs (SI Appendix, Figs.…”

“…3A shows that the positions of the Q x (0,0) and Q x (1,0) bands of H B in the ArgL185 mutants are blue shifted compared to those of H A and do not vary as a function of P + H B − yield (or culture density). The inherent (native) differences between the positions of the Q x absorption bands of H A and H B have been described (17,(30)(31)(32)(33)(34). In brief, the A-side native GluL104 (thought to be neutral) forms a hydrogen bond to the ring-V keto group of H A .…”

“…1B. The template mutations give a "starting point" advantage to B-side ET over A-side ET (15)(16)(17). TyrM210 near B A is changed to Phe, and PheL181 near B B is changed to Tyr.…”

Photosynthetic reaction center variants made via genetic code expansion show Tyr at M210 tunes the initial electron transfer mechanism

“…1B) (20)(21)(22) but it is difficult to determine these energetics either experimentally or theoretically (1,23). Contributions of individual symmetry-breaking amino acid have been thoroughly studied (1,15,(24)(25)(26)(27)(28) and while the importance of certain amino acids has been ascertained, the roles of local protein-chromophore interactions are not always fully understood (8,24,25). One highly examined residue has been the tyrosine at site M210 (RC residue numbers are preceded by the protein subunit designation: H, L, or M) because it is a clear deviation in symmetry between A-and B-branches (Fig.…”