Interactions between lipids and bacterial reaction centers determined by protein crystallography

Abstract

The structure of the reaction center from Rhodobacter sphaeroides has been solved by using x-ray diffraction at a 2.55-Å resolution limit. Three lipid molecules that lie on the surface of the protein are resolved in the electron density maps. In addition to a cardiolipin that has previously been reported [McAuley, K. E., Fyfe, P. K., Ridge, J. P., Isaacs, N. W., Cogdell, R. J. & Jones, M. R. (1999) Proc. Natl. Acad. Sci. USA 96, 14706 -14711], two other major lipids of the cell membrane are found, a phosphatidylcholine and a glucosylgalactosyl diacylglycerol. The presence of these three lipids has been confirmed by laser mass spectroscopy. The lipids are located in the hydrophobic region of the protein surface and interact predominately with hydrophobic amino acids, in particular aromatic residues. Although the cardiolipin is over 15 Å from the cofactors, the other two lipids are in close contact with the cofactors and may contribute to the difference in energetics for the two branches of cofactors that is primarily responsible for the asymmetry of electron transfer. The glycolipid is 3.5 Å from the active bacteriochlorophyll monomer and shields this cofactor from the solvent in contrast to a much greater exposed surface evident for the inactive bacteriochlorophyll monomer. The phosphate atom of phosphatidylcholine is 6.5 Å from the inactive bacteriopheophytin, and the associated electrostatic interactions may contribute to electron transfer rates involving this cofactor. Overall, the lipids span a distance of Ϸ30 Å, which is consistent with a bilayer-like arrangement suggesting the presence of an ''inner shell'' of lipids around membrane proteins that is critical for membrane function.I ntegral membrane proteins are found in the cell membrane surrounded by lipids. The biophysical and biochemical properties of these proteins are critically determined by the lipid environment. The lipid composition of cell membranes is complex, containing a variety of phospholipids, glycolipids, and other small molecules. In the purple nonsulfur bacterium Rhodobacter sphaeroides, the major lipids are the phospholipids, phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, and cardiolipin, or diphosphatidyl glycerol, and two glycolipids, sulfoquinovosyl diacylglycerol and glucosylgalactosyl diacylglycerol (1). The relative amounts of these lipids vary significantly depending on specific growth conditions, in particular the oxygen level, although the fatty acid chains are always predominately 18:1 with only minor contributions from 18:0, 16:0, and 16:1 and trace amounts of smaller chains. The lipid composition is known to critically determine the morphology of the cell membrane, but whether the lipids can affect the photosynthetic energy conversion processes remains an outstanding question.Understanding the influence of the lipid properties on the photosynthetic complexes, or any other integral membrane proteins, is in most cases limited by the lack of detailed structural information concerning membrane proteins. To ...

Electron flow in multiheme bacterial cytochromes is a balancing act between heme electronic interaction and redox potentials

“…1C), with coplanar pairs within the tetra-heme chain, stacked pairs within the two heme-triples, and T-shaped connections between tetra-heme chain and heme-triples. These motifs as such are not uncommon in biological ET: the stacked heme arrangement bears some similarity with the tightly packed chlorophylls in reaction center proteins (13), and a similar T-shaped connection is found for the heme a-a 3 pair in cytochrome c oxidase (14) (Fig. S1).…”

Peptide—Lipid Interactions: Insights and Perspectives

“…Interestingly, the shorter peptides such as WALP16 were able to induce the formation of non-lamellar H II phases, and it was noted that the inter-tryptophan distance was the crucial factor for determining the activity of W 2 in ALP16 and W 2 out ALP16, rather than the length of the hydrophobic segment. 2 H NMR experiments also provided evidence that WALP16 produced an increase in acyl chain order, with concomitant increase in bilayer diameter of 0.4 Å. Solid-state 1 H NMR analysis of WALP peptide/DMPC mixtures produced upfield chemical shift changes (relative to pure DMPC) for several of the key lipid signals, with the magnitude of the chemical shifts increasing with increasing peptide length. In particular, the changes observed for the choline group of the lipid were attributed to ring current effects of tryptophan.…”

“…This not only reflects improved methods for their crystallisation using detergent depletion methods, which are sufficiently mild to allow lipid molecules to remain bound to the protein, but also the fact that membrane proteins do have specific interactions with lipids in the membrane around them. 2,4 There are some striking examples in the PDB of multiple entries for a single protein that has been crystallised in different laboratories. A cursory glance at these reveals that some lipids are invariably bound at the same site on the protein, regardless of its origin, whereas other lipids are more variant ( Figure 3).…”

EPR, ENDOR, and Special TRIPLE measurements of P•+ in wild type and modified reaction centers from Rb. sphaeroides

“…In purple anoxygenic bacteria, reaction centers (RCs) embedded in the membrane perform the primary photochemistry (Blankenship et al 1995). The RC from Rhodobacter sphaeroides consists of three protein subunits and several cofactors (see e.g., Allen et al 1987;Yeates et al 1988;Ermler et al 1994;Stowell et al 1997;Camara-Artigas et al 2002). The core L and M subunits surround the cofactors that are divided into two distinct branches related by an approximate two-fold symmetry axis that runs from the center of P to the non-heme iron (Fig.…”

“…In a series of ENDOR and TRIPLE (b) Structure of the primary donor of the RC from Rb. sphaeroides with the two BChl a molecules P L and P M (phytyl chain truncated), and the three mutated residues His L168, Asn L170, and Asn M199 (PDB entry 1M3X;Camara-Artigas et al 2002). (c) Molecular structure of bacteriochlorophyll a (BChl a) with IUPAC numbering; the two methyl groups 2 1 and 12 1 and the b-protons 7, 8, 17, and 18 are indicated studies of P •?…”