Proton-Transfer Pathways in Photosynthetic Reaction Centers Analyzed by Profile Hidden Markov Models and Network Calculations

In the bacterial reaction center (bRC) of Rhodobacter sphaeroides, the key residues of proton transfer to the secondary quinone (QB) are known. Also, several possible proton entry points and proton-transfer pathways have been proposed. However, the mechanism of the proton transfer to QB remains unclear. The proton transfer to QB in the bRC of Blastochloris viridis is less explored. To analyze whether the bRCs of different species use the same key residues for proton transfer to QB, we determined the conservation of these residues. We performed a multiple-sequence alignment based on profile hidden Markov models. Residues involved in proton transfer but not located at the protein surface are conserved or are only exchanged to functionally similar amino acids, whereas potential proton entry points are not conserved to the same extent. The analysis of the hydrogen-bond network of the bRC from R. sphaeroides and that from B. viridis showed that a large network connects QB with the cytoplasmic region in both bRCs. For both species, all non-surface key residues are part of the network. However, not all proton entry points proposed for the bRC of R. sphaeroides are included in the network in the bRC of B. viridis. From our analysis, we could identify possible proton entry points. These proton entry points differ between the two bRCs. Together, the results of the conservation analysis and the hydrogen-bond network analysis make it likely that the proton transfer to QB is not mediated by distinct pathways but by a large hydrogen-bond network.