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Review
. 1999 Jul;181(13):3869-79.
doi: 10.1128/JB.181.13.3869-3879.1999.

How photosynthetic bacteria harvest solar energy

R J Cogdell  1 , N W Isaacs, T D Howard, K McLuskey, N J Fraser, S M Prince
Affiliations
Free PMC article
Review

How photosynthetic bacteria harvest solar energy

R J Cogdell et al. J Bacteriol. 1999 Jul.
Free PMC article
No abstract available

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Figures

FIG. 1
FIG. 1
Schematic representation of the LH2 holocomplex from Rhodopseudomonas acidophila. (Top) View from the cytoplasmic side of the membrane, looking down the central ninefold axis of symmetry. (Bottom) View from within the membrane. The organization of the two rings of Bchla molecules, arranged between the transmembrane α-helices, are shown; the 9 B800 Bchla molecules parallel to the plane of the membrane and the second ring of 18 B850 Bchla molecules, with their bacteriochlorin rings perpendicular to the plane of the membrane. The bottom view also shows the carotenoid (rhodopin-glucoside) which spans the membrane and comes into van der Waal’s contact with both groups of Bchla. Only the chromophoric portions of the pigments are shown. This and most of the other figures in this minireview were produced with the Molscript program (38).
FIG. 2
FIG. 2
Location and organization of the B800 Bchlas in the LH2 from Rhodopseudomonas acidophila. The B800 Bchlas (nine Bchla molecules) can be seen arranged peripherally between the β-apoprotein α-helices.
FIG. 3
FIG. 3
Comparison of the electron density in the region of the B800 Bchla binding pocket of LH2 from Rhodopseudomonas acidophila at a resolution of 2.5 Å (top) and 2.0 Å (bottom). (Top) With a resolution of 2.5 Å, the extension of the N-terminal methionine residue of the α-apoprotein is clearly seen in the electron density map, together with the “modelled” formyl group. (Bottom) At this improved resolution of 2.0 Å, the N-terminal extension is also clearly seen. Now, however, it can be seen to bifurcate. The modelled formyl group no longer gives a satisfactory fit to this higher-resolution data. The electron density is shown as the white or blue cage.
FIG. 4
FIG. 4
Organization of B850 Bchlas in the LH2 from Rhodopseudomonas acidophila. The 18 Bchla molecules, which from the B850 ring can be seen, edge on, are arranged between the transmembrane α-helices of the α-apoprotein (inner) and β-apoprotein (outer).
FIG. 5
FIG. 5
Two views of a model of the purple bacterial PSU. (Top) A top view, looking perpendicular to the assumed plane of the membrane. This section is taken at a point in the LH complexes where the tightly coupled rings of Bchlas are located. This figure was adapted from reference with permission from Elsevier Science. The reaction center is located in the center of the LH1 complex. The smaller LH2 sits outside the large LH1 complex. (Bottom) A side view, looking from within the assumed plane of the membrane (blue, LH2; green, LH1; yellow, RC). This section is taken exactly perpendicular to the view shown in the top panel. The distances shown between the different pigment groups (shown in orange) are calculated assuming a space-filling model and the closest possible organization. The times shown are energy transfer times as measured in intact membranes (31, 61, 69, 72).
FIG. 6
FIG. 6
Relative orientations of the Qx and Qy transition dipoles in the B800 and B850 Bchlas and the carotenoid in the LH2 complex from Rhodopseudomonas acidophila. (A) Alignment of the Qx dipoles (yellow-green). (B) Alignment of the Qy dipoles (red-white). (C) Alignment of the carotenoid to the Qx dipoles (the transition dipole movement of the S2 state of the carotenoid runs up and down the long axis of the conjugated double bands). (D) Alignment of the carotenoid with respect to the Qy dipoles. The figure was adapted from reference with permission from Elsevier Science and produced with O (32). The distances between the center of a B800 Bchla and the α-bound and β-bound B850 Bchlas in the same αβ-apoprotein pair are 17.4 and 18.2 Å, respectively. Qx and Qy are labels for the two Bchla absorption bands at ∼590 nm and 800 or 850 nm, respectively. The transition dipoles, which correspond to these absorption bands, lie within the plane of the bacteriochlorin rings, at right angles to each other, diagonally between the nitrogen atoms, which coordinate the central Mg2+. One of the factors which controls the rate of excitation energy transfer between two molecules is the angle between the transition dipoles involved. When the dipoles are parallel, energy transfer is favorable; when they are orthoganal, energy transfer is much less favorable.
FIG. 7
FIG. 7
Schematic representation of the two low-lying excited singlet states of S1 and S2 carotenoids. The approximate positions of the S1 and S2 excited singlet states are shown. The S0→S2 represents the optically allowed (one-photon) transition that gives rise to the carotenoid’s well-known, strong absorption spectrum. The approximate times for the S2→S1 and S1→S0 transitions are also shown.
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References

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