Electron and Proton Transfer Pathways

The arrangement of the redox prosthetic groups in the complex is shown in Figures 7.1c and 7.2a. Starting on the lumenal p-side ofthe complex, the edge-edge intramonomer distances (to the nearest A) are: [2Fe-2S] cluster - cyt f 26 A; [2Fe-2S] - heme bp, 24A; heme bp - heme bn, 7A; heme bp - Chl a, 12 A; Chl a - heme bn, 6 A; Chl a - P-carotene, 14 A. The edge-edge intermonomer distances are: heme bp - heme bp, 13 A; heme bn - heme bn, 29 A. In the perspective ofthe structure of the complex (Figure 7.2a), the electron transfer scheme described in Figure 7.1c shows part of a Q- cycle model for electron and proton transfer through the b6 f complex. The model describes a bifurcated electron transfer scheme originating with the lumenal p-side oxidation of the quinol (PQH2) which initially transfers one electron in the high potential chain downhill in redox potential to the [2Fe-2S] cluster (Em s +0.3 V) while liberating 2H+ to the lumenal p-side aqueous phase. The electron is subsequently transferred to cytochrome f (Em s +0.35 V), plastocya-nin (Em s +0.38 V) (Figures 7.1c and 7.4, Table 7.1), (see also Chapter 8 for details on the soluble lumenal electron carriers), and the primary donor of PSI (P700) (Em s +0.45 V) (Table 7.1) (see also Chapters 2 and 3). Details ofthe interaction of the quinol with the Rieske [2Fe-2S] cluster at the lumenal p-side interface, and of proton-coupled electron transfer from the quinol to the cluster, are discussed for the cytochrome bc1 complex in the Rb. sphaeroides photosynthetic bacterium using structure data for the bc1 complex from the respiratory chain [27, 28].

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