Tina D. Howard

The Structure and Thermal Motion of the B800–850 LH2 Complex from Rps. acidophila at 2.0 Å Resolution and 100 K: New Structural Features and Functionally Relevant Motions

The structure at 100K of integral membrane light-harvesting complex II (LH2) from Rhodopseudomonas acidophila strain 10050 has been refined to 2.0A resolution. The electron density has been significantly improved, compared to the 2.5A resolution map, by high resolution data, cryo-cooling and translation, libration, screw (TLS) refinement. The electron density reveals a second carotenoid molecule, the last five C-terminal residues of the alpha-chain and a carboxy modified alpha-Met1 which forms the ligand of the B800 bacteriochlorophyll. TLS refinement has enabled the characterisation of displacements between molecules in the complex. B850 bacteriochlorophyll molecules are arranged in a ring of 18 pigments composed of nine approximate dimers. These pigments are strongly coupled and at their equilibrium positions the excited state dipole interaction energies, within and between dimers, are approximately 370cm(-1) and 280cm(-1), respectively. This difference in coupling energy is similar in magnitude to changes in interaction energies arising from the pigment displacements described by TLS tensors. The displacements appear to be non-random in nature and appear to be designed to optimise the modulation of pigment energy interactions. This is the first time that LH2 pigment displacements have been quantified experimentally. The calculated energy changes indicate that there may be significant contributions to inter-pigment energy interactions from molecular displacements and these may be of importance to photosynthetic energy transfer.

Structural factors which control the position of the Qy absorption band of bacteriochlorophyll a in purple bacterial antenna complexes

This paper presents a concise review of the structural factors which control the energy of the Qy absorption band of bacteriochlorophyll a in purple bacterial antenna complexes. The energy of these Qy absorption bands is important for excitation energy transfer within the bacterial photosynthetic unit.

Detergent structure in crystals of the integral membrane light-harvesting complex LH2 from Rhodopseudomonas acidophila strain 10050.

Integral membrane proteins are solubilized by their incorporation into a detergent micelle. The detergent micelle has a critical influence on the formation of a three-dimensional crystal lattice. The bulk detergent phase is not seen in X-ray crystal structures of integral membrane proteins, due to its disordered character. Here, we describe the detergent structure present in crystals of the peripheral light-harvesting complex of the purple bacteria Rhodopseudomonas acidophila strain 10050 at a maximal resolution of 12A as determined by neutron crystallography. The LH2 molecule has a toroidal shape and spans the membrane completely in vivo. A volume of 16% of the unit cell could be ascribed to detergent tails, localized on both the inner and outer hydrophobic surfaces of the molecule. The detergent tail volumes were found to be associated with individual LH2 molecules and had no direct role in the formation of the crystalline lattice.

The structural basis of light-harvesting in purple bacteria

A typical purple bacterial photosynthetic unit consists of two types of light‐harvesting complex (LH1 and LH2) together with a reaction centre. This short review presents a description of the structure of the LH2 complex from Rhodopseudomonas acidophila, which has recently been improved to a resolution of 2.0 Å [Papiz et al., J. Mol. Biol. 326 (2003) 1523–1538]. We show how this structure has helped to reveal the details of the various excitation energy transfer events in which it is involved.

Spectroscopy on individual light-harvesting 1 complexes of Rhodopseudomonas acidophila

In this paper the fluorescence-excitation spectra of individual LH1-RC complexes (Rhodopseudomonas acidophila) at 1.2 K are presented. All spectra show a limited number of broad bands with a characteristic polarization behavior, indicating that the excitations are delocalized over a large number of pigments. A significant variation in the number of bands, their bandwidths, and polarization behavior is observed. Only 30% of the spectra carry a clear signature of delocalized excited states of a circular structure of the pigments. The large spectral variety suggests that besides site heterogeneity also structural heterogeneity determines the optical spectrum of the individual LH1-RC complexes. Further research should reveal if such heterogeneity is a native property of the complex or induced during the experimental procedures.

Rhodopseudomonas acidophila strain 10050 contains photosynthetic LH2 antenna complexes that are not enriched with phosphatidylglycerol, and the phospholipids have a fatty acyl composition that is unusual for purple non-sulfur bacteria

The phospholipid composition of Rhodopseudomonas acidophila strain 10050 grown aerobically or anaerobically in the light was determined. The major phospholipids present in the aerobic cells were phosphatidylethanolamine (PE; 54%), phosphatidylglycerol (PG; 24%) and cardiolipin (diphosphatidylglycerol, DPG) (14%), together with phosphatidylcholine (PC; 5%). On moving the cells to anaerobic photosynthetic growth in the light PE remained the major phospholipid (37-49%), but there was a major change in the proportion of PC, which increased to 31-33%, and corresponding reductions in the contents of PG to 11-16% and DPG to 4-5%. The fatty acid composition of the phospholipids was unusual, compared with other purple non-sulfur photosynthetic bacteria, in that it contained 16:0 (29%), 17:1 (20%) and 19:1 (9%) plus several mainly unsaturated 2-OH fatty acids (9% total) as major components, when grown aerobically in the dark. In contrast when grown photosynthetically under anaerobic conditions there was <2% 17:1 or 19:1 present, while the amounts of 16:1 and 18:1 increased, and 16:0 decreased. The phospholipid composition of the purified light-harvesting complex 2 (LH2) complex was PE (43%), PC (42%) and DPG (15%). Unexpectedly, there was no PG associated with the purified LH2. These findings contrast with previous studies on several other photosynthetic bacteria, which had shown an increase in PG upon photosynthetic growth [Biochem. J. 181 (1979) 339]. The prior hypothesis that phosphatidylglycerol has some specific role to play in the function of light-harvesting complexes cannot be true for Rps. acidophila. It is suggested that specific integral membrane proteins may strongly influence the phospholipid content of the host membranes into which they are inserted.

The Structure and Function of the LH2 Complex from Rhodopseudomonas acidophila Strain 10050, with Special Reference to the Bound Carotenoid

The structure of the LH2 complex from Rps. acidophila is presented, with special emphasis on the detailed arrangement of the pigments (BChl a and the carotenoid, rhodopin-glucoside). The BChl a/s are arranged into two distinct groups, 9 monomeric ones absorbing at 800 nm at 18 tightly coupled ones absorbing of ∼860 nm. The carotenoids connect these two groups of BChl a/s. Recent fs and ps time-resolved energy transfer experiments, designed to probe the mechanism of carotenoid to BCh1 a, singlet-singlet energy transfer in LH2 are described. Possible mechanisms of these energy transfer events are discussed.

The Structure and Function of the LH2 Antenna Complex from Rhodopseudomonas Acidophila Strain 10050

Since the last conference on Photosynthetic Prokaryotes the structure of the LH2 antenna complex from the purple non-sulphur photosynthetic bacterium Rhodopseudomonas acidophila strain 10050 has been determined (1). This integral membrane pigment-protein complex consists of a circular array of heterodimers, each comprising and α- and β-apo-protein which non-covalently bind three molecules of Bchla and, most probably, two molecules of the carotenoid rhodopin glucoside (Figure 1). The pigments are enclosed by two concentric rings of transmembrane α-helices. The inner ring, radius 13A, is composed of α-apoproteins and the outer ring, radius 34A, is formed from the β-apoproteins. The long axes of these transmembrane α-helices are approximately normal to the presumed membrane plane. The structure is ‘capped’ top and bottom by the N- and C-termini of the apoproteins which fold over and interact with each other at the hydrophilic surfaces of the complex. The Bchla molecules form two spectrally distinct sets. Eighteen Bchka’ s, with their central Mg++ liganded to the protein via conserved histidine residues (α his 31 and β his 30), form a strongly coupled array with the plane of their bacteriochlorin rings normal to the membrane plane (parallel to the long axes of the transmembrane α-helices). These Bchla molecules form the so called B850 Bchls. Nine monomeric Bchla molecules, separated by a centre to centre distance of 21.2A, lie with their bacteriochlorin rings parallel to the membrane place between the β-apoproteins. Originally it was suggested (1) that the central Mg++ stem of these Bchla’ s was ligated to the N-formyl group on α-met 1. However as described below this tentative conclusion is now called into question. These 9 Bchla s form the so called B800 Bchla’ s.