Conserved lipids in photochemical reaction centers

Back in 2001, Wakeham et al. (2001) wondered whether a lipid molecule found in the Type II reaction center from Rhodobacter sphaeroides (Fig. 1) was conserved in all anoxygenic Type II reaction centers.

Fig. 1. Cardiolipin binding in the Type II RC from Rhodobacter sphaeroides, 1qov. The M subunit is shown in orange and the L subunit in gray. Cardiolipin is displayed as spheres.

Similar lipid binding sites in Photosystem II (Fig. 2 and 3) and in Photosystem I (Fig. 4 and 5) can be seen in the crystal structures. However, unlike the reaction center from R. sphaeroides, which appears to bind only one cardiolipin, Photosystem II and Photosystem I bind lipids symmetrically on both sides of the reaction center. In the crystal structure from plant Photosystem I only one 1,2-distearoyl-monogalactosyl-diglyceride (3lw5) was found in a similar position to the one in Synechococcys elongatus. I suspect the symmetrical counterpart was not seen because of the low resolution.

Fig. 2. 1,2-dipalmitoyl-phosphatidyl-glycerol and sulfoquinovosyldiacylglycerol binding in Photosystem II from Thermosynechococcus vulcanus, 3wu2. The D1 subunit is shown in orange and D2 in gray. The lipids are displayed as spheres.

Fig. 3. Symmetrical binding of lipids in Photosystem II.

Fig. 4. Binding of 1,2-distearoyl-monogalactosyl-diglyceride in Photosystem I from Synechococcus elongatus, 1jb0. The PsaB subunit is shown in orange and the PsaA in gray. The lipid is shown as spheres.  The antenna domain has been omitted for clarity.

Fig. 5. Like in Photosystem II, the lipids bind symmetrically in Photosystem I.

It appears then that the binding of lipids in that position is a conserved feature of all reaction centers and might have existed in the primordial reaction center at the dawn of photosynthesis. It is possible that in each type of reaction center the role of these lipids have changed. I found a paper that suggested that cardiolipin affects charge recombination in R. sphaeroides (Giustini et al. 2005).  But in Photosystem II the role of this lipids might be more related to assembly and repair, besides specific structural roles (Mizusawa and Wada 2012). The role of these lipids found in Photosystem I is less clear but it has been suggested that they might influence the phylloquinones in some manner (Fromme et al. 2001).

References
Fromme, P., Jordan, P. & Krauss, N. Structure of Photosystem I. BBA-Bioenergetics 1507, 5-31 (2001).

Giustini, M. et al. Influence of cardiolipin on the functionality of the QA site of the photosynthetic bacterial reaction center. J Phys Chem B 109, 21187-21196 (2005).

 Mizusawa, N. & Wada, H. The role of lipids in photosystem II. BBA-Bioenergetics 1817, 194-208 (2012).

Wakeham, M. C., Sessions, R. B., Jones, M. R. & Fyfe, P. K. Is there a conserved interaction between cardiolipin and the type II bacterial reaction center? Biophys J 80, 1395-1405 (2001).