Application of exciton coupling theory to the structure of mitochondrial cytochrome b

The orientation of the chromophores in the cytochrome bc 1 of Rhodospirillum 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 orientation of the three hemes with respect to the membrane plane. The orientations of the b H -and b L -hemes of the R. sphaeroides and beef heart mitochondrial complexes are similar to those determined by crystallographic studies of the mitochondrial cytochrome bc 1 . On the other hand the orientations of the b-hemes of the R. rubrum complex lead to the conclusion that the 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 subunit 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 different orientation of the heme c 1 of the three studied complexes in comparison to crystallographic studies. This difference may arise from the above hypothesis on the transitions of the heme or from flexibility of this subunit in function of its redox state.The quinol-cytochrome c (or -plastocyanin) oxidoreductases (cytochrome bc 1 -b 6 f complexes) are a ubiquitous class of enzymes found in mitochondrial and bacterial respiratory chains, as well as in the green plant and bacterial photosynthetic chains (for reviews see also Refs. 1-4). These enzymes couple the two-electron oxidation of quinol (ubi-, mena-, or plasto-) and the one-electron reduction of cytochrome (Cyt) 1 c (or Cyt c 2 , HiPIP, plastocyanin) to the electrogenic translocation of protons across the membrane bilayer. These complexes therefore contribute to the electrochemical proton gradient that drives ATP synthesis.The catalytic heart of the enzyme consists of three subunits, Cyt b, Cyt c 1 , and the iron sulfur protein (termed the Rieske protein) that carry four redox centers. The Cyt b subunit contains a high potential and a low potential b-type heme (termed b H and b L , respectively); the Cyt c 1 subunit contains a c-type heme, and the Rieske protein contains a 2Fe-2S cluster. The complete atomic structure of the mitochondrial enzyme from two species has recently been published (5, 6), but no such structure is presently available for any of the bacterial counterparts. For structural and functional studies of the Cyt bc 1 complex, the photosynthetic bacteria provide, however, several experimental advantages as follows: (i) the bacterial enzyme shows a much simpler organization than the mitochondrial enzyme. Although the mitochondrial Cyt bc 1 complex contains 11 subunits (the functions of which are mostly unknown except those of the catalytic one), the complex from Rhodobacter (Rb.) sphaeroides is reduced to four subunits (7,8), and the enzyme from Rhodospirillum (Rs.) rubrum represents the simplest composition with...

Section: Discussionsupporting

confidence: 58%

Relative Orientation of the Hemes of the Cytochromebc 1 Complexes from Rhodobacter sphaeroides, Rhodospirillum rubrum, and Beef Heart Mitochondria

Schoepp¹,

Breton²,

Parot³

et al. 2000

“…53-54 and references therein). The strength of the signal is a function both of the distance between the hemes and of the relative orientation of their planes (54). At variance with LD, CD spectra therefore have the potential to detect relative rearrangements that would leave the tilt of each heme with respect to the membrane plane unchanged.…”

Section: Arrangement Of the Two Hemes In Cyt Bmentioning

confidence: 99%

“…Interpreting the more intense CD signal of Cyt b 6 f in terms of relative rearrangement of hemes b H and b L also raises difficulties. According to the theoretical treatment of heme/heme interactions given by Palmer and Degli Esposti (54), moving the two b-hemes apart by ϳ2 Å (the likely increase in distance between histidine ␣-carbons were the intervening ␣-helix elongated by one residue) is predicted to decrease the intensity of the excitonic signal by ϳ15%. In the crystallographic structure of chicken muscle Cyt bc 1 (2), the planes of hemes b H and b L make one with another an angle of ϳ48°, close to that (45°) considered to permit optimal excitonic interaction (54).…”

Section: Arrangement Of the Two Hemes In Cyt Bmentioning

confidence: 99%

“…According to the theoretical treatment of heme/heme interactions given by Palmer and Degli Esposti (54), moving the two b-hemes apart by ϳ2 Å (the likely increase in distance between histidine ␣-carbons were the intervening ␣-helix elongated by one residue) is predicted to decrease the intensity of the excitonic signal by ϳ15%. In the crystallographic structure of chicken muscle Cyt bc 1 (2), the planes of hemes b H and b L make one with another an angle of ϳ48°, close to that (45°) considered to permit optimal excitonic interaction (54). Except if heme planes happened to be rotated by 90°with respect to each other, in which case no intensity change would result, any departure from the position observed in the bc 1 complex would therefore be expected to decrease the CD signal.…”

Section: Arrangement Of the Two Hemes In Cyt Bmentioning

confidence: 99%

See 1 more Smart Citation

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

Schoepp¹,

Chabaud²,

Breyton³

et al. 2000

The organization of chromophores in the cytochrome b 6 f from Chlamydomonas 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 -and b L -hemes are comparable to those determined crystallographically on the cytochrome bc 1 . The excitonic CD signal, resulting from the interaction between b-hemes, is similar to that reported for the cytochrome bc 1 . LD and CD data are consistent with the differences between the b 6 f and bc 1 leaving the orientation of the b-hemes unaffected.

“…The two hemes are bis-histidine-coordinated (20) and span the membrane bilayer approximately perpendicularly to the membrane plane (21) Numerous mutations of cytochrome b have been obtained in photosynthetic bacteria (Rhodobacter species) and mitochondria (reviewed in Ref. 23).…”

mentioning

confidence: 99%

Molecular Genetic Identification of a Pathway for Heme Binding to Cytochrome b 6

Kuras¹,

Vitry²,

Choquet³

et al. 1997

100

Heme binding to cytochrome b 6 is resistant, in part, to denaturing conditions that typically destroy the noncovalent interactions between the b hemes and their apoproteins, suggesting that one of two b hemes of holocytochrome b 6 is tightly bound to the polypeptide. We exploited this property to define a pathway for the conversion of apo-to holocytochrome b 6 , and to identify mutants that are blocked at one step of this pathway.