Structural Studies of a Functional Cytochrome c—Photosynthetic Reaction Center Membrane

Abstract

Structural information on cytochrome c binding to photosynthetic reaction center/phospholipid membranes has been derived by using neutron diffraction. A reconstituted membrane system was formed from either deuterated or protonated reaction center (RC) protein (isolated from the photosynthetic bacterium Rps. sphaeroides R26 mutant) and pure egg phosphatidylcholine (EPC) membrane bilayers. Partially dehydrated, oriented membrane multilayers were formed from a dispersion of EPC/RC membrane vesicles and horse heart cytochrome c (with a molar ratio of EPC/RC = 100 and cytochrome c/RC = 0.8 in the multilayers). Neutron scattering membrane profiles were determined from the lamellar neutron diffraction of oriented membrane multilayers containing either 1) cytochrome c/EPC/deuterated RC or 2) cytochrome c/EPC/protonated RC. The difference between these two scaled profile structures directly determined the reaction center profile within the cytochrome c/EPC/RC membrane. This protein profile was compared to the reaction center profile within a membrane without cytochrome c (previously derived); differences between the two RC profiles were significant. A cytochrome c/EPC profile was also derived, showing that the distribution of lipid on opposite sides of the membrane was shifted with the addition of cytochrome c. Finally, model profiles calculated by using the neutron diffraction information were compared with x-ray diffraction profiles of the cytochrome c/EPC/RC membranes to determine the position of cytochrome c in the membrane profile to 10-Å resolution. (Supported by NIH grant GM 27309.)