Modelling of the [cytochrome f:acceptor] electron transfer complexes of diatoms Phaeodactylum tricornutum (containing cytochrome c6) and Thalassiosira oceanica (containing plastocyanin)

Abstract

In the red lineage of photosynthetic organisms, the coastal diatom Phaeodactylum tricornutum contains cytochrome c6 as the only electron carrier between cytochrome b6f and photosystem I. However, in the oceanic diatom Thalassiosira oceanica, a functional plastocyanin has been described to act as the main electron carrier, replacing cytochrome c6 and thus decreasing the cell¿s iron requirements. T. oceanica plastocyanin seems to have been acquired by lateral gene transfer from a green alga and, consequently, shows the typical strong negative electrostatics of a ¿green-type¿ plastocyanin. We have carried out docking simulations to study the [cytochrome f:acceptor] complex in P. tricornutum and T. oceanica by using the pyDock docking protocol. The best docking model of the [cytochrome f:cytochrome c6] complex of P. tricornutum has a different orientation compared with the equivalent ¿side-on¿ complex of green algae, that involves both the large and small domains of cytochrome f. Thus, the P. tricornutum complex shows a ¿head-on¿ orientation, more similar to that described previously in cyanobacteria, in which the small domain of cytochrome f does not play a relevant role. In addition, docking models indicate that the [cytochrome f:plastocyanin] complex of T. oceanica shows a much larger population of low-energy docking orientations compared to the P. tricornutum complex, suggesting that the acquired green-type plastocyanin of T. oceanica would not have yet achieved a prevalent configuration optimized for electron transfer.