Structure of the reaction center from Rhodobacter sphaeroides R-26 and 2.4.1: protein-cofactor (bacteriochlorophyll, bacteriopheophytin, and carotenoid) interactions.

Abstract

The three-dimensional structures of the cofactors and protein subunits of the reaction center (RC) from the carotenoidless mutant strain of Rhodobacter sphaeroides R-26 and the wild-type strain 2.4. The reaction center (RC) is an integral membrane proteinpigment complex that mediates the primary processes of photosynthesis-i.e., the light-induced electron transfer from a donor to a series of acceptors. reported the structure of the cofactors (3), protein subunits (5), and the membrane-protein interactions (6) ofthe RC from Rb. sphaeroides R-26. In this paper, we discuss the interactions of the proteins with the cofactors D, B, and 4, the carotenoid, C, and the detergent f3-octyl glucoside (J30G).The protein interactions involving the quinones and iron will be reported in a subsequent publication (7). The structure of the RC from Rb. sphaeroides is compared with that of Rhodopseudomonas viridis (8), with special emphasis on the structural features that are believed to be important in the electron transfer process. It should be noted that only the structure of the unexcited state of the RC has been reported (2, 8, 9); conformation changes accompanying the absorption of light and subsequent charge separation have not yet been reported. EXPERIMENTAL PROCEDURESRC from Rb. sphaeroides R-26. Data collection, processing, and initial refinement of the structure of the RC from Rb. sphaeroides R-26 at 2.8 A resolution has been described (3).Subsequent rebuilding of the atomic structure was facilitated by the use of modified "omit-maps." These are typically calculated by removing all atoms from a specified volume in the unit cell and then using the remaining atoms to phase the Fourier calculation for electron density in that volume (10). This approach tends to minimize bias in the electron density map toward the current atomic model. In the present work, a least-squares refinement of an initial omit-map was implemented. The electron density map was sampled at 144, 96, and 144 grid points along the crystallographic a, b, and c axes, respectively. Electron density values within a specific block of 12 x 12 x 12 grid points were varied, while grid points outside this block were fixed at the electron density of the current model. Values of the electron density within the block were refined by a least-squares calculation to minimize the difference between the magnitudes of observed and calculated structure factors. This procedure was repeated for all 288 blocks in the crystallographic asymmetric unit. An asymmetric unit was reconstructed from the blocks, followed by a least-squares version of solvent flattening. The quality of the resulting electron density maps was significantly improved relative to (IF01 -IFJI) or (21F01 -IFJI) maps, which are often used in model building. After adjustments of the cofactors, the residues in the L and M subunits, and the addition of two POG molecules, the atomic structure was refined to an R factor of 24.0%, with the restrained least-

Structural Analysis of the Reaction Center Light-harvesting Complex I Photosynthetic Core Complex of Rhodospirillum rubrum Using Atomic Force Microscopy

“…Once fixed, this parameter allows the positions of aromatic residues of the RC to be examined, because the vertical distance between these residues and the extremity of the RC-H subunit is known from the Rb. sphaeroides RC structure, which is highly homologous to the RC of R. rubrum (31). The vertical positions of these residues are of interest because they have been proposed to lie at the interface between hydrophobic and head group regions of a membrane bilayer, acting as membrane "anchors".…”

Structure of the photochemical reaction centre of a spheroidene-containing purple-bacterium,Rhodobacter sphaeroidesY, at 3 Å resolution

“…viridis (Deisenhofer & Michel, 1989). For a detailed comparison of Y and R26 structures, we used the refined coordinates published recently by Yeates et al (1988) (PDB entry 4RCR), for which completeness of X-ray data is close to our set. The best fit between the Co~ atoms of the 11 transmembrane helices of the two structures indicates an r.m.s, deviation of 0.87 ,A,.…”

Two Different Charge Separation Pathways in Photosystem II

“…The central excitonically coupled special pair, P, of BChl molecules [P A and P B , ∼8 Å apart, center to center (6)], absorbing at ∼875 nm in Rhodobacter sphaeroides, are electronically excited after the energy transfer from the LH1 core antenna (absorbing at approximately equal energy). Then, an electron is transferred to the BChl molecule on the active branch (B A ) in 3 ps, and from B A to the BPhe (H A ) in 1 ps, resulting in the final charge-separated state, P þ H A -.…”