Lecture 9 Summary

The bacterial photosynthetic reaction center

The photosystem of purple photosynthetic bacteria provides a good model for illustrating the details of light energy transduction. High-resolution crystal structures have revealed the disposition of protein subunits and the associated prosthetic groups. There is considerable homology between the simpler bacterial systems and the photosynthetic complexes in cyanobacteria and the chloroplasts of plants.

Two structures have been determined for bacterial photosynthetic reaction centers, both from species of the genus Rhodopseudomonas (Rps.). The photosynthetic complex from Rps. viridis was determined to 2.3 Å resolution by Johann Deisenhofer, Robert Huber, and Hartmut Michel in 1984. In fact, this was the first atomic-resolution structure to be determined for a transmembrane protein. These researchers shared the 1988 Nobel Prize in Chemistry in recognition of this milestone achievement

A special pair of bacteriochlorophyll (BChl) molecules (colored green in 2nd figure) becomes excited due to the transfer of energy from light. The porphyrin rings of the special pair bacteriochlorophylls are nearly parallel, and the Mg²⁺ ions are 7 Å apart. This special pair becomes photooxidized as electron transfer to the other prosthetic groups is initiated upon excitation. The other prosthetic groups are accessory bacteriochlorophylls (BChl, blue), bacteriopheophytins (BPh, yellow), and quinones (Q_A and Q_B, grey, with oxygens in red). An iron (II) ion lies between the quinones.

Paralleling the 2-fold "similarity" (pseudosymmetry) between the M and L subunits, there is an approximate symmetry in the arrangement of the cofactors. The rings are very symmetrically arranged, while the long hydrophobic tails of the cofactors show variation. Despite the structural symmetry, electron transfer occurs very asymmetrically, moving through the L subunit side (on the right in figure).

An accessory BChl lies next to the special pair on each side, assisting in electron transfer, without itself being reduced. The electron is actually transferred to bacteriopheophytin (BPh, in yellow, on right). From BPh, electron transfer occurs to the Q_A site. Finally the electon is acquired by Q_B, which is much more exposed than Q_A. Iron assists (but does not itself show redox change) in the transfer.

Q_A passes its electrons to Q_B without becoming fully reduced. That is, it alternates only between the Q (quinone) and Q•^- (semiquinone) form. The Q_B quinone acquires two electrons (sequentially) from semiquinone species in the A site, becoming first a semiquinone (QH•) and the fully-reduced quinol (QH₂). Note that two protons are picked up from the cytoplasm in this reduction, contributing to generation of a pH gradient

The bacterial photosynthetic reaction center operates in a cyclic mode

As noted above, two photons must be absorbed (and thus two photooxidation events must occur) to generate one QH2. At the same time, two protons are taken up from cytoplasm. the reduced quinone released from the photosynthetic reaction center participates in a Q cycle that effectively translocates protons across the membrane while transferring electrons through the cytochrome bc₁ complex (which is quite similar to Complex III of mitochondrial electron transport). Electrons are passed, one at a time, to cytochrome c₂ from the cytochrome bc₁ complex.

Cytochrome c₂ acts as a mobile electron carrier on the periplasmic side of the membrane, and it donotes its electrons back to the photosynthetic reaction center, reducing P870+. (In this figure, the special pair are represented as a special pair of BChl a molecules, which has an absorption maximum of 870 nm.) Cytochrome c₂ can donate an electron directly, or via a c-type cytochrome with 4 heme groups, depending on the species, to neutralize P870⁺. Either way, the circuit is complete, and P870 is ready for another photooxidation event.

Photosystems I and II in plant chloroplasts

When we come to consider the more complex photosynthetic apparatus in plant chloroplasts and cyanobacteria, we find that the photosynthetic reaction center and electron transport apparatus in purple bacteria is a pretty good model to guide us. At first glance, the two cases may seem quite different. The chloroplastic apparatus has not just one, but two light-absorbing reaction centers where photooxidation and electron transport occur. These are known as photosystem II (PS II) and photosystem I (PS I). PS II is the most like the purple bacteria center. The electrons transferred as a result of photooxidation in PS II are not cycled back to itself as in the bacterial photosynthetic system, but instead are transferred in a non-cyclic scheme to the second reaction center in the series, PS I. The reduction of the photoxidized pair in PS II - P680⁺ - is accomplished by the removal of electrons from water - no mean feat. But P680⁺ is such a strong oxidant - much stronger than either p870⁺ or P960⁺ in the bacterial system - that it is able to pull away electrons from the quite electronegative oxygen atom. The second photosystem (PS I) uses light energy to create a species - P700* - with a very negative reduction potential - considerably more negative than P870* or P960* in purple bacteria. This strong reductant is then used to reduce NADP⁺. Thus, in plant chloroplasts, the combined action of two photosystems enables them to operate in a non-cyclic fashion to so that a net oxidation/reduction occurs and to produce energy in the form of a transmembrane proton gradient.

Upon closer examination, similarities between the purple bacterial system and the chloroplast system become evident. As noted above, PS II is similar to the bacterial photocenter. This can clearly be seen in terms of structure, where the two systems share a homologous dimeric core. Although PS II is larger and more complex in composition, the core of PS II consists of protein subunits denoted D1 and D2 that are similar to each other and homologous to the L and M subunits in the bacterial photocenter. They also show highly similar arrangements of chromophores. BChl is replaced by Chl a and BPh is replaced by Ph (pheophytin) in PS II, and the quinones are plastoquinone. The same electron transfer pathway seems to operate in PS II, leading to the reduction of an exchangeable plastoquinone (analogous to Q_B in bacteria) which transfers electrons to and translocates protons via a Q cycle in conjunction with a transmembrane protein complex very similar to mitochondrial Complex III. In chloroplasts, this is the cytochrome bf complex.

Even PS I shows a degree of similarity with the purple bacterial system. Again there is homology between the core subunits (psaA and psaB) and there are large regions of homology with the L and M bacterial core subunits. Although there are distinct differences in electron transfer between PS I, on one hand, and PS II and the bacterial system on the other, again there is a special pair of Chl molecules (P700 in PS I) where photoinduced charge separation occurs, and a quinone is involved in electron transfer within the complex. Another point of similarity to the bacterial photocenter specific to PS I is that it can operate independently of PS II in a cyclic manner, making a greater contribution to development of a transmembrane proton gradient. Since the process is cyclic, there is no net redox reaction associated with this mode, and NADP⁺ is not reduced. Thus, just like the bacterial photocenter, there is no production of reducing power when PS I operates in a cyclic mode

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