Marion C. Wakeham

Is There a Conserved Interaction between Cardiolipin and the Type II Bacterial Reaction Center

In a recent publication, the structural details of an interaction between the Rhodobacter sphaeroides reaction center and the anionic phospholipid diphosphatidyl glycerol (cardiolipin) were described (K. E. McAuley, P. K. Fyfe, J. P. Ridge, N. W. Isaacs, R. J. Cogdell, and M. R. Jones, 1999, Proc. Natl. Acad. Sci. U.S.A. 96:14706-14711). This was the first crystallographic description of an interaction between this biologically important lipid and an integral membrane protein and was also the first piece of evidence that the reaction center has a specific interaction with cardiolipin. We have examined the extent to which the residues that interact with the cardiolipin are conserved in other species of photosynthetic bacteria with this type of reaction center and discuss the possibility that this cardiolipin binding site is a conserved feature of these reaction centers. We look at how sequence variations that would affect the shape of the cardiolipin binding site might affect the protein-cardiolipin interaction, by modeling the binding of cardiolipin to the reaction center from Rhodopseudomonas viridis.

Photo-accumulation of the P+QB− radical pair state in purple bacterial reaction centres that lack the QA ubiquinone

Photo‐excitation of membrane‐bound Rhodobacter sphaeroides reaction centres containing the mutation Ala M260 to Trp (AM260W) resulted in the accumulation of a radical pair state involving the photo‐oxidised primary electron donor (P). This state had a lifetime of hundreds of milliseconds and its formation was inhibited by stigmatellin. The absence of the QA ubiquinone in the AM260W reaction centre suggests that this long‐lived radical pair state is P+QB −, although the exact reduction/protonation state of the QB quinone remains to be confirmed. The blockage of active branch (A‐branch) electron transfer by the AM260W mutation implies that this P+QB − state is formed by electron transfer along the so‐called inactive branch (B‐branch) of reaction centre cofactors. We discuss how further mutations may affect the yield of the P+QB − state, including a double alanine mutation (EL212A/DL213A) that probably has a direct effect on the efficiency of the low yield electron transfer step from the anion of the B‐branch bacteriopheophytin (HB −) to the QB ubiquinone.