Reaction centre structures

Figure 1.9 shows how PSI and PSII are functionally coupled with cyt 6 6 and ATP synthase in the thylakoid membrane. We now know for certain from x-ray crystallographic studies that all reaction centres are characterised by a pseudo-2 fold symmetry axis that relates the cofactors and the proteins that bind them. In Type 11 RCs, this symmetry gives rise to a redox-active branch and an inactive branch, as shown for PSII in Fig. 1.9. Despite intense studies on the purple bacterial RC it is still not clear how Type I1 centres are able to differentiate their active and inactive branches. However, this property has distinct advantages when the terminal acceptor (i.e. QB) requires two electrons to be fully reduced. In Type I reaction centres, where a single, centrally located iron-sulphur centre Fx is the electron acceptor (as for PSI in Fig. 1.9), it seems possible that primary charge separation occurs with similar probability up either branch; this must be so in green sulphur bacterial RCs, which are homodimeric while in the case of PSI the situation is less clear. The two protein subunits that constitute the RCs of PSI, PSII and purple bacteria are not identical, as in the case of green sulphur bacteria, but form a heterodimer. In purple bacteria, the two subunits are called L and M, while in PSII the closely related

subunits are known as the D1 and D2 proteins (see Figs. 3.3 and 3.10). All four proteins show considerable homologies, and all have five transmembrane helices related to each other in their reaction centres by the same pseudo-2 fold axis that relates the cofactors.
 

The two proteins that make up Type I reaction centres, PsaA and PsaB, are also arranged around the pseudo-2 fold axis that relates the cofactors (see Fig. 3.8), but in this case they have eleven transmembrane helices. Interestingly the five transmembrane helices at the C-terminal ends of these Type I RC proteins are arranged in a similar, but not identical, manner as in Type I1 RCs (Rhee et al., 1998; Schubert et al., 1998).
 

The structural details briefly described above have emerged from X-ray crystallographic studies which began with the elucidation of the structure of a Type I1 RC isolated from the purple bacterium R. viridis in the 1980s by Deisenhofer, Huber, Michel and colleagues (Deisenhofer et al., 1984, 1985) and have recently advanced to the determination of the structure of PSI at 2.5 (Jordan et al., 2001) and PSI1 at resolutions ranging from 3.8 8, to 3.5 8, (Zouni et al., 2001; Kamiya and Shen, 2003; Ferreira et al., 2004). As discussed in detail in Chapter 3, these studies have given a structural basis for the interpretation of data obtained by a variety of spectroscopic techniques and for developing general theories of electron transfer in proteins. Moreover, the determination of the structures of the cytochrome bc (Iwata et al., 1998; Zhang et al., 1998) and ATP synthase (Abrahams et al., 1994) complexes of the respiratory membranes allows realistic structural extrapolations to the corresponding complexes of photosynthesis.