On the spatial organization of hemes and chlorophyll in cytochrome b(6) f - A linear and circular dichroism study

The organization of chromophores in the cytochrome b(6)f from Chlamydomonas reinhardtii has been studied spectroscopically. Linear dichroism (LD) meas urements, performed on the complex co-reconstituted into vesicles with phot osynthetic reaction centers as an internal standard, allow the determinatio n of the orientations of the chromophore with respect to the membrane plane . The orientations of the b(H)- and b(L)-hemes are comparable to those dete rmined crystallographically on the cytochrome bc(1). The excitonic CD signa l, resulting from the interaction between b-hemes, is similar to that repor ted for the cytochrome bc(1). LD and CD data are consistent with the differ ences between the b(6)f and bc(1) leaving the orientation of the b-hemes un affected. By contrast, the LD data yield a different orientation for the he me f as compared either to the heme c(1) in the crystallographic structures or to the heme fas studied by electron paramagnetic resonance. This differ ence could either result from incorrect assumptions regarding the orientati ons of the electronic transitions of the f-heme or may point to the possibi lity of a redox-dependent movement of cytochrome f. The chlorophyll a was o bserved in a well defined orientation, further corroborating a specific bin ding site for it in the b(6)f complex.