With the recent presentation of the high-resolution (3.0 Å) crystal structure of the RC-LH1 complex from T. tepidum [9] and the reasonably resolved (4.8 Å) structure of the RC-LH1 complex from Rps. palustris [24], one can basically dis-tinguish between two types of monomeric core complexes: For the first type (T.
tepidum) the LH1 complex completely surrounds the RC, but for the second type (Rps. palustris) an additional protein out of register with the array of inner LH1 α-apoproteins, creates a gap in the LH1 aggregate (Figure 2.5). Nevertheless, the general construction principle of these two types of LH1 complexes is assumed to be equivalent: Two BChl a molecules and one carotenoid (presumably for Rps.
palustris, vide infra) are non-covalently bound between two low-molecular-weight apoproteins α and β. These subunits then oligomerize around the RC, forming a closed array of 16 subunits for T. tepidum, whereas for Rps. palustris one subunit is replaced by the so called ‘W’ protein. Thereby, it has to be noted that due to the rather low resolution of the palustris structure, the macrocycles of the BChl a molecules in the LH1 complex are positioned with low accuracy and it was not possible to locate the carotenoid molecules [24] (Figure 2.5a,c). This was different for the LH1 complex from T. tepidum [9] (Figure 2.5b,d). Here, it was shown that the arrangement of the BChl a molecules in the LH1 complex is quite analogous to the association of the B850 BChls in LH2, with an average Mg-Mg distance of 9.04Åwithin a LH1 subunit and8.46Åbetween adjacent subunits. As for the LH2 complex, the two BChls in a LH1αβ-heterodimer are liganded to histidine residues, one coming from the α-apoprotein and one from the β-apoprotein. Additionally, it was possible to locate the positions of the 16 carotenoid molecules (spirilloxan-thin) in the LH1 complex, one per each αβ-heterodimer. In both types of LH1 complexes the BChls form a strongly coupled, elliptical array of 30 pigments in the case of Rps. palustris and of 32 pigments for T. tepidum, giving rise to a single, strong Qy absorption band in the 870−890 nm region, referred to as B880 band (Figure 2.1c). For T. tepidum, Ca2+-binding to the LH1 apoproteins (Figure 2.5b) results in a shift of this band to 915nm. Overall, the two RC-LH1 complexes have elliptical structures with comparable dimensions, with the long axis of the ellipse amounting to 110 Å for Rps. palustris and to 105 Å for T. tepidum, measured as the distance between the centers of opposingβ-apoproteins. The RC ofT. tepidum has a permanent cytochrome unit, missing for the Rps. palustris complex which has a Rb. sphaeroides-type RC without this subunit (Figure 2.5a,b).
It has been suggested that the reason for the different types of LH1 complexes is the transfer of ubiquinone/ubiquinol (QB/QBH2) through the barrier of the LH1 complex, as part of the cyclic electron transport in purple bacterial photosynthesis [8]. Thereby, for the RC-LH1 complex from Rps. palustris it was proposed that the W protein may provide a portal through which QBH2 can traverse the LH1 complex. This notion is reinforced by the specific orientation of the LH1 complex
2.4 RC-LH1 complexes
Figure 2.5: Two types of monomeric core complexes from the photosynthetic purple bacteria.
The LH1 complex from the species Rps. palustris (a,c; PDB file: 1PYH) is interrupted by a protein called ‘W’, whereas the LH1 complex from T. tepidum (b,d; PDB file: 3WMM) completely surrounds the RC. The following color scheme was used in all panels: α-apoproteins, cyan; β-apoproteins, green; W protein, pink; RC proteins, gray; BChls and BPheos, red;
Mg2+-ions, blue dots; carotenoids, orange; haems, brown; quinones, light green; Ca2+-ions, red spheres and Fe2+-ion, gray sphere. The top part of the figure (a,b) shows the complete complexes, whereas in the lower part (c,d) the proteins have been removed to allow a closer view on the pigment molecules. (a,b) For the RC-LH1 complex from T. tepidum the RC has a permanent cytochrome subunit which extends into the periplasmic space. Rps. palustris expresses Rb. sphaeroides-type RCs without this subunit. Another peculiarity of the RC-LH1 complex from T. tepidum are the Ca2+-ions bound to theαβ-apoproteins on the periplasmic side. (c,d) In contrast to Rps. palustris, for T. tepidum both the positions of the carotenoids and of the two quinones in the RC are included in the PDB file. The figure was created with
2 Light-harvesting complexes
with respect to the RC, such that W is located opposite to the QB binding site in the RC, with the hydrophobic tail of QB pointing towards the gap in the LH1 aggregate [24] (Figure 2.6). Another bacterial species for which a similar princi-ple for the traffic of QB/QBH2 through the LH1 complex was proposed, is Rb.
sphaeroides. This species forms dimeric RC-LH1 complexes in which a S-shaped LH1 complex surrounds two RCs and two proteins called ‘PufX’ take over the role of the W protein [37].
Figure 2.6: Specific orientation of the LH1 complex from Rps. palustris with respect to the binding site of the secondary electron acceptor, QB, in the RC. Color scheme: protein helices, gray; W protein, red; pigment molecules, black; ubiquinones, red; Fe2+-ion, red sphere. Since the ubiquinones are not included in the PDB file: 1PYH of the RC-LH1 complex from Rps.
palustris, a RC structure including these molecules had to be found. Thus, the RC structure in this figure was taken from the PDB file: 1PCR [38] and overlaid over the initial RC of the Rps. palustris RC-LH1 structure. It can be seen that the hydrophobic tail of the secondary ubiquinone,QB, points towards the W protein which might form a gate, allowing the reduced ubiquinone, QBH2, to escape into the membrane lipid phase. The figure was created with PyMOL; Schrodinger, 2010.
In the crystal structure of the RC-LH1 complex from T. tepidum channels have been revealed in the LH1 complex that may facilitate the shuttling of QB/QBH2 through the closed LH1 aggregate [9]. These channels are located on the interface between each pair of adjacent αβ-heterodimers and have an average size approx-imately equal to the size of the benzoquinone head of an ubiquinone. Based on molecular-dynamics simulations, a similar pathway for QB/QBH2 was already pro-posed earlier for Rsp. rubrum, another purple bacterial species with a ‘closed’ LH1 complex around the RC [22].
The diffusion of QB/QBH2 through the LH1 complex, might also be the explana-tion for the increased structural flexibility of LH1 complexes as compared to LH2,