Relative orientation of the hemes of the cytochrome bc(1) complexes from Rhodobacter sphaeroides, Rhodospirillum rubrum, and beef heart mitochondria - A linear dichroism study

The orientation of the chromophores in the cytochrome bc(1) of Rhodospirill um rubrum, Rhodobacter sphaeroides, and beef heart mitochondria is reported , The combination of redox-resolved absorption spectrophotometry and linear dichroism experiments at low temperature allows the determination of the o rientation of the three hemes with respect to the membrane plane. The orien tations of the b(H)-and b(L)-hemes of the R. sphaeroides and beef heart mit ochondrial complexes are similar to those determined by crystallographic st udies of the mitochondrial cytochrome bc(1). On the other hand the orientat ions of the b-hemes of the R. rubrum complex lead to the conclusion that th e b(H)-heme is more perpendicular to the membrane plane than the b(L)-heme. This could be explained by a specific organization of the b-hemes due to s ubunit composition of the complex or, alternatively, to a different spatial position of the heme transitions with respect to the porphyrin macrocycle compared with the other complexes. Moreover, our results demonstrate a diff erent orientation of the heme c(1) of the three studied complexes in compar ison to crystallographic studies. This difference may arise from the above hypothesis on the transitions of the heme or from flexibility of this subun it in function of its redox state.