Electrostatic properties of cytochrome f: implications for docking with plastocyanin.

The electrostatic properties of cytochrome f (cyt f), a member of the cytochrome b6f complex and reaction partner with plastocyanin (PC) in photosynthetic electron transport, are qualitatively studied with the goal of determining the mechanism of electron transfer between cyt f and PC. A crystal structure for cyt f was analyzed with the software package GRASP, revealing a large region of positive potential generated by a patch of positively charged residues (including K58, K65, K66, K122, K185, K187, and R209) and reinforced by the iron center of the heme. This positive field attracts the negative charges of the two acidic patches on the mobile electron carrier PC. Three docked complexes are obtained for the two proteins, based on electrostatic or hydrophobic interactions or both and on steric fits by manual docking methods. The first of these three complexes shows strong electrostatic interactions between K187 on cyt f and D44 on PC and between E59 on PC and K58 on cyt f. Two other manually docked complexes are proposed, implicating H87 on PC as the electron-accepting site from the iron center of cyt f through Y1. The second complex maintains the D44/K187 cross-link (but not the E59/K58 link) while increasing hydrophobic interactions between PC and cyt f. Hydrophobic interactions are increased still further in the third complex, whereas the link between K187 on cyt f and D44 on PC is broken. The proposed reaction mechanism, therefore, involves an initial electrostatic docking complex that gives rise to a nonpolar attraction between the regions surrounding H87 on PC and Y1 on cyt f, providing for an electron-transfer active complex.