Lecture 10 Summary

The pathway of electron flow in from water to NADP⁺ in photosynthesis is known as the Z-scheme (see Fig.19.22, p.538, BTS). The net "uphill" path of electrons (decrease in reduction potential) is essentially the reverse of that occurring in oxidative phosphorylation, and is made possible by two separate inputs of light energy. Electrons are extracted from water by the manganese center, reducing the extremely strong oxidant P680⁺. (P680⁺ is the strongest oxidant in the Z-scheme.)

P680⁺ arises from the photooxidation of P680*, the excited state of the P680 special pair caused by the absorption of light energy. Electrons are transferred through phototosystem II (PS II) from P680* to a mobile quinone (Q). The reduced Q carries two electrons to cytochrome bf complex. The latter is homologous to Q-cytochrome c oxidoreductase (Complex III) of the mitochondrial electron transport chain, and electron transport is coupled to proton translocation by the operation of a Q cycle. The "output" of electron transport through cytochrome bf complex is the reduced form of plastocyanin (Pc). This small, soluble protein is analogous to cytochrome c, but diffrs in that it uses a copper prosthetic group to carry its electron. In its reduced [Cu (I)] form, Pc acts to reduce P700⁺, the oxidant that arises from photooxidation of P700*. The latter - which is the strongest reductant in the Z-scheme - is the excited state of the P700 special pair caused by the absorption of light energy. This second light-energy harvesting step is carried out by photosystem I (PS I). Electron transfer through PS I occurs via chlorophyll (A0) and bound quinone (A1) prosthetic groups, and then through a series of iron-sulfur clusters of the 4Fe-4S type. These electrons pass from PS I to the one-electron carrier ferredoxin (Fd), a small, soluble 2Fe-2S protein that has a lower reduction potential than NADPH. This means that the electrons from 2Fd(red) can reduce NADP⁺ to NADPH. This reduction is carried out by ferredoxin-NADP⁺ reductase, a flavoprotein that can accept electrons one at a time from Fd(red) and pass an electron pair to NADP⁺.

The bioinorganic chemistry of water-splitting

One of the most fascinating - and not fully resolved - issues in photosynthesis is the water-splitting reaction that gives rise to oxygen. We have seen that P680⁺ is the strongest oxidant in the Z-scheme - in fact it is one of the strongest oxidizing agents in all of biochemistry, powerful enough to drive the oxidation of oxygen (in its –2 oxidation state in water) itself. The actual water-splitting reaction occurs in a part of PS II called the oxygen-evolving complex (OEC), which contains the manganese center alluded to earlier. Many experimental approaches have been brought to bear on the question of how the OEC accomplishes its feat. Only recently has a PS II structure been obtained at sufficient resolution to define the architecture of the manganese center. The 3.5-Å structure obtained by Ferreira, et al. of PS II from Thermosynechococcus elongatus shows that the Mn center is distorted cube with three Mn, four oxygen atoms, and one Ca. An additional Mn is located external to the cube.

References

Ferreira, et al. (2004) Science 303: 1831-1838