On the Spatial Organization of Hemes and Chlorophyll in Cytochrome b 6 f

The organization of chromophores in the cytochrome b 6 f fromChlamydomonas reinhardtii has been studied spectroscopically. Linear dichroism (LD) measurements, performed on the complex co-reconstituted into vesicles with photosynthetic reaction centers as an internal standard, allow the determination of the orientations of the chromophore with respect to the membrane plane. The orientations of the b H- andb L-hemes are comparable to those determined crystallographically on the cytochrome bc 1. The excitonic CD signal, resulting from the interaction betweenb-hemes, is similar to that reported for the cytochromebc 1. LD and CD data are consistent with the differences between the b 6 f andbc 1 leaving the orientation of theb-hemes unaffected. By contrast, the LD data yield a different orientation for the heme f as compared either to the heme c 1 in the crystallographic structures or to the heme f as studied by electron paramagnetic resonance. This difference could either result from incorrect assumptions regarding the orientations of the electronic transitions of the f-heme or may point to the possibility of a redox-dependent movement of cytochrome f. The chlorophyll a was observed in a well defined orientation, further corroborating a specific binding site for it in theb 6 f complex.