Hydrophobic mismatch model for cytochrome b6f driven state transitions is attractive for computer simulations

Abstract

Keywords: cytochrome b₆f, chlorophyll a, hydrophobic thickness, hydrophobic mismatch, membrane reorganizations

In contrast to the bacterial anoxygenic photosynthetic membranes, which have one photosystem (PS) and cytochrome bc₁ complex (cyt bc₁), the oxygenic photosynthetic membranes of cyanobacteria, algae and plants have two PSs (PSII and PSI) and cytochrome b₆f complex (cyt b₆f) between them. Both cytochromes belong to the cytochrome bc-complexes, which perform proton-coupled electron transfer in biological membranes. Cyt b₆f is unique because it also plays the major role in regulation of photosynthetic electron transport via triggering the short-term adaptive mechanism state transitions. State transitions balance the distribution of the excitation light energy between PSII and PSI, in response to changes in the redox state of the plastoquinone (PQ) pool – intramembrane electron carrier from PSII to cyt b₆f. However, the molecular mechanism for induction of state transitions by cyt b₆f is not yet evidenced. Recently, by analyzing the available X-ray crystal structures of cyt bc complexes we found [1] that the single chlorophyll a (Chla) molecule in cyt b₆f plays the dual role of redox sensor and signal transmitter for the PQ pool redox state changes. Chla performs this role via molecular volume changes, which are in correlation with (i) conformational changes at both cyt b₆f membrane sides, (ii) rotation of Phe/Tyr124 in petD, and (iii) cyt b₆f hydrophobic thickness. These results suggested that the driving force for membrane reorganization during the induction and progression of state transitions is the hydrophobic mismatch induced by the changed hydrophobic thickness of the cyt b₆f upon the change in the redox state of the PQ pool. On the other side, our investigations on the kinetics of state transitions in a great variety of field plants pointed out to possible evolution-assisted changes in the amino acid sequence of cyt b₆f.В В В  В В 

With this presentation we would like to attract the interest of the biomathematical community to the uniqueness of the hydrophobic mismatch model for induction of structural reorganizations in the oxygenic photosynthetic membranes, which has no analog in the other biological membranes. We will show why the molecular dynamic simulations would be beneficial to evidence the hydrophobic mismatch model for state transitions in oxygenic photosynthetic membranes.

Acknowledgements: This work was supported by the Bulgarian Academy of Sciences, by a grant-in-aid for scientific research from JSPS (No. 26450200) and by the JSPS Invitation Fellowships in Japan (to R.V.).