Comparison of proton transfer paths to the QA and QB sites of the Rb. sphaeroides photosynthetic reaction centers

The photosynthetic bacterial reaction centers from purple non-sulfur bacteria use light energy to drive the transfer of electrons from cytochrome c to ubiquinone. Ubiquinone bound in the Q(A) site cycles between quinone, Q(A), and anionic semiquinone, Q(A)(center dot-), being reduced once and never binding protons. In the Q(B) site, ubiquinone is reduced twice by Q(A)(center dot-), binds two protons and is released into the membrane as the quinol, QH(2). The network of hydrogen bonds formed in a molecular dynamics trajectory was drawn to investigate proton transfer pathways from the cytoplasm to each quinone binding site. Q(A) is isolated with no path for protons to enter from the surface. In contrast, there is a complex and tangled network requiring residues and waters that can bring protons to Q(B). There are three entries from clusters of surface residues centered around HisH126, GluH224, and HisH68. The network is in good agreement with earlier studies, Mutation of key nodes in the network, such as SerL223, were previously shown to slow proton delivery. Mutational studies had also shown that double mutations of residues such as AspM17 and AspL210 along multiple paths in the network presented here slow the reaction, while single mutations do not. Likewise, mutation of both HisH126 and HisH128, which are at the entry to two paths reduce the rate of proton uptake.