| Cytochrome f (cyt f), a member of the membrane-bound cytochrome b6f complex, is a participant in electron transport in photosynthetic organisms. It receives an electron from the Rieske Fe-S protein, also within the cyt b6f complex, and passes that electron on to plastocyanin (PC) in higher plants and algae, a beta-barrel copper protein, or to cytochrome c6 (c6) in algae and cyanobacteria, an alpha-helical heme protein.; Despite the differences in the proteins, the orientation of negatively charged residues on the surface of PC and c6 generates a similar potential field. There is an interaction between those negative charges and positive charges on cyt f in complex formation between the two pairs of molecules.; It was postulated, based on electrostatic modeling and available biochemical data, that electrostatic interactions draw the acidic patch of PC and the basic patch of cyt f into a pre-docking configuration in which there is no interaction between any potential electron-transfer surfaces, after which rearrangement results in a electron-transfer active complex in which H87 on PC, a ligand to the copper atom, would contact Y1 on cyt f, the sixth ligand to the heme. Electrostatic interactions would not be sufficient to form such a complex; hydrophobic interactions would likely be required.; Brownian dynamics (BD), simulating Brownian motion of PC in the presence of electrostatic attraction or repulsion to cyt f, was then used to model the interaction between PC and cyt f quantitatively and qualitatively and evaluate this hypothesis. Electrostatic interactions tended to draw PC into a single dominant orientation with respect to cyt f; such a orientation was observed to be similar to the postulated electron-transfer active complex mentioned above. Experiments between poplar PC and turnip cyt f agreed with later spinach PC/turnip cyt f and Chlamydomonas PC/cyt f experiments; the tendency to form the same dominant complex remains.; BD simulations were then used to model the action of mutants of PC and cyt f residues for comparison with previous results and to provide predictions for future experiments, and to simulate the weaker electrostatic interaction between Chlamydomonas c6 and cyt f. |
