2.1 Photosynthesis
2.1.2 The photosynthetic unit (PSU)
The peripheral light-harvesting 2 (LH2) complex
The structures of the LH2 complexes from Rhodopseudomonas (Rps.) acidophila [2, 4] and Rhodospirillum (Rs.) molischianum [3] were determined by x-ray crystallography and that from Rhodobacter (Rb.) sphaeroides by electron-microscopy [7]. Remarkably, all peripheral LH complexes form circular oligomers of the two hydrophobicα- andβ- apoproteins that non-covalently bind three BChlamolecules and one or two carotenoids, featuring a nonameric (Rps.
acidophila,Rb. sphaeroides) or octameric (Rs. molischianum) quaternary protein structure.
2.1: Photosynthesis 11
(A) (B)
1000 900 800 700 600 500 400 300
200 wavelength (nm)
B800 B800
B850 B850
BChl Qy BChl Qx
Car BChl Soret protein z
x
y
x 10 Å
Figure 2.4: X-ray structure and absorption spectrum of the LH2 complex fromRs. molischianum. (A) The left part displays the whole pigment-protein complex whereas in the right part only the BChlamolecules are drawn.
The upper part shows a side view, the lower part a top view. The pigments are arranged in two concentric rings commonly termed B800 (light-grey) and B850 (black). The atomic coordinates were taken from the Protein Data Bank, identification code 1LGH. (B) Absorption spectrum clearly featuring the B800 and B850 absorption bands in the near infrared.
In the left part of Fig. 2.4A the structure of the LH2 pigment-protein complex ofRs. molischia-numderived from x-ray crystallography with an resolution of 2.4 ˚A is shown as a whole whereas in the right part only the BChla molecules are depicted. Two rings of BChl a molecules can be distinguished. One ring consists of eight repeating pairs of oneα- and oneβ-bound pigment which are oriented like the blades of a turbine. Due to its absorption in the near infrared at around 850 nm this pool is termed B850 ring. The other ring consists of eight well-separated pigments — each bound by a heterodimer — arranged in a C8 symmetry that have their molec-ular planes perpendicmolec-ular to the symmetry axis. It is labelled B800 ring as its maximum absorp-tion occurs around 800 nm. In Fig. 2.4B the room temperature absorpabsorp-tion spectrum is shown in which the B800 and the B850 bands are clearly distinguishable. Upon excitation, energy is transferred from the B800 to the B850 pigments in 1 to 2 ps [72–75] while energy transfer among the B850 molecules is an order of magnitude faster [76–78]. The lowest excited state of the B850 pigment pool has a relative long fluorescence lifetime of approximately 1 ns [79].
The light-harvesting 1 (LH1) complex
For LH1 is has not been possible yet to obtain high-quality crystals and therefore the three-dimensional arrangement of its subunits remains to be ascertained. Based on the homology between the LH1 and LH2 proteins, the basic LH1 subunit is believed to contain only two closely coupled BChlamolecules contributing to its absorption around 870 nm and presumably one carotenoid [80]. In analogy to its absorption maximum the pigment pool is termed B875 band.
Analysing the number of pigments per RC showed that this number varies between 23 and 33 for different strains of purple bacteria [81–83]. These values are higher than the 16 or 18 pig-ments that are present in the B850 rings of LH2 suggesting that the main difference between LH2 and LH1 is the size of the ring. This assumption is strengthened by two different 2-D pro-jection maps from electron-microscopy experiments on LH1 fromRb. sphaeroides [7] and LH1 reconstituted fromαβ-dimers obtained by detergent treatment of native LH1 complexes from Rs. rubrum [6], the latter is depicted in Fig. 2.5A. These experiments revealed a closed-ring structure featuring C16 symmetry of the αβ-subunits which would be just large enough to in-corporate a reaction centre [6, 11]. However, also non-circular structures of the LH1 complexes were observed as can be seen from the electron density map of a special type of LH1 fromRb.
sphaeroides in Fig. 2.5B [8].
The room temperature absorption spectrum of LH1-RC complexes from Rb. sphaeroides is shown in Fig. 2.5C.
20 Å 20 Å
(A) (B) (C)
absorption (a.u.)
1000 900 800 700 600 500 400 300
wavelength (nm)
B875
BChl Qy BChl Qx
Car BChl Soret
RC
Figure 2.5: Structures of two LH1 complexes and typical absorption spectrum. (A) Projection map at 8.5 ˚A resolution of reconstituted LH1 complexes fromRs. rubrumderived by electron microscopy, from [6]. A reaction centre is schematically drawn inside the ring. (B) Projection map at 20 ˚A resolution of LH1-RC complexes from Rb. sphaeroides, from [8]. (C) Absorption spectrum of LH1-RC complexes fromRb. sphaeroides.
2.1: Photosynthesis 13 The reaction centre (RC)
The structures of the reaction centre fromRps. viridis [1] andRb. sphaeroides [84] (the latter is depicted in Fig. 2.6) are known at atomic resolution. The RC ofRb. sphaeroides comprises four bacteriochlorophyll (BChl)a and two bacteriophytin (BPheo) a molecules, a carotenoid, a menaquinone (QA), an ubiquinone (QB) and a non heme-iron all held in place by a protein scaffold (not shown).
Cyclic electron transport inside and outside the PSU
The final goal of photosynthesis is the conversion of solar into chemical energy. This is achieved by a cyclic electron transport which sets in after the light-harvesting and transport of excitation energy to the reaction centre. The whole process of excitation and charge separation starting from light absorption and finishing with an ATP molecule will be described briefly in the fol-lowing (see also [29, 49, 51, 56, 85, 86]).
1 Upon the absorption of a photon by a BChla or a carotenoid molecule in one of the pe-ripheral antenna complexes the excitation energy is transferred within about 50 ps (B800
0.7 ps
−→ B850 100 fs−→ B850 −→3 ps B875−→80 fs B875−→35 ps RC) [12, 49, 54, 67, 68, 87] via neigh-bouring complexes to the reaction centre, more precisely to the special pair (P) in the RC
PM
Figure 2.6:First steps in photosynthesis and subsequent cyclic electron transport [49, 51]. The atomic coordinates of the RC from Rb. sphaeroides were taken from the Protein Data Bank, identifier 1PSS. For more details see text.
which comprises two BChlamolecules, PLand PM. This process occurs with a quantum efficiency of about 95%.
2 The relaxation of the excited P, which occurs in about 3 ps initiates achargetransfer to the BPheoL resulting in an electron-hole pair P+BPheo−L. The role of the BChlL in this step is not completely clear. From here the electron is transferred to the menaquinone QAin 200 ps and in the final step in about 100µs to the ubiquinone QB which picks up a proton from the cytoplasmic side to become QBH. This stepwise electron transfer leads to a stable charge separation across the membrane as the back reaction leading to RC relaxation is at least a factor of 104 slower than the forward reactions.
3 After two electron transfer steps, the reduced hydroquinone (QBH2) which is only loosely bound to the RC leaves into the lipid phase of the membrane towards the ubiquinone-cytochromebc1 complex.
4 The bc1 complex oxidises the hydroquinone with help of the cytochrome c2 complex and uses the exothermic reaction to establish a proton gradient across the membrane by pumping the protons to the periplasmic side. The electrons are shuttled back to the special pair in the RC by the cytochromec2complex and the quinone QB returns to the RC.
5 The proton gradient is needed to drive the synthesis of ATP from ADP by the ATPase complex. With this last step, the conversion of solar energy into chemical energy that can be used by the metabolic processes of the organism is finished.