iron-sulfur protein [51,55], and the consensus membrane folding pattern of Cyt b 6 and subunit IV
[51,56–58]. The monomeric b 6 ƒ complex is inferred to contain 11 transmembrane helices: 1 (Cyt ƒ) he-
lix, 4 (Cyt b 6 ) helices, 3 (subunit IV) helices, and 3 helices of three smaller subunits. Several types of data
support the idea that in vivo cytochrome b 6 ƒ is associated in a dimer [51].
B. Subunits of Cytochrome b 6 ƒ Complex
- PetA Subunit (Cyt ƒ)
The largest cytochrome b 6 ƒ subunit is the organelle petAgene product, Cyt ƒ, with a molecular mass
of about 32 kDa [59]. It covalently binds c-type cytochrome and contains a docking site of the electron
acceptor plastocyanin. Martinez et al. [54,60] reveled that Cyt ƒ has an elongated (2.5 3.5 75 nm)
structure with a large and a small domain with a predominant -strand motif of the large domain. The
extended structure may be necessary for Cyt ƒ to make contact with the plastocyanin. The c-heme is
ligated by the N-terminal -amino group, which is unique to all heme proteins. Another unusual struc-
ture is an internal extended (1.1 nm) linear water chain, which may function as a luminal exit port for
protons translocated by the Cyt b 6 ƒ complex. In addition, it was found that the interface between large
and small domains contains five basic residues: Lys58, 65, and 66 (large domain) and Lys187 and
Arg209 (small domain). Because the complementary plastocyanin has two corresponding regions with
negative surface-situated residues, it was inferred that in the beginning successful docking of Cyt ƒ and
plastocyanin involves a long-range electrostatic attraction. This is followed by rearrangement of the
protein structure during which metal centers become close enough for rapid intracomplex electron
transfer [61].
- PetC Subunit
The Rieske protein is a product of the nuclear petCgene. The product PetC has a molecular mass of
about 19 kDa [62]. It binds a high-potential iron-sulfur center. Information about its secondary struc-
ture was derived from the known structure of the Cyt bc 1 mitochondrial protein. However, despite the
similarities in the Rieske protein family, the existing differences make the complete Rieske–Cyt b 6 ƒ
protein secondary structure still unclear [51]. The existing data based on the circular dichroism spec-
trum suggest that Rieske protein has only one -helix, while -sheets are 52–60%, -turns 7–25%, and
random coils up to 40% [51,63]. The crystal structure of the luminal part of Rieske protein has been
determined [64]. It shows two domains: a small “cluster-binding” subdomain that comprises the 2Fe-
2S cluster and a large subdomain. Two cysteines and two histidines coordinate the Fe-S cluster. The
two histidines are exposed to the solvent, whereas the rest of the cluster is shielded by two loops co-
valently linked by a disulfide bridge and by a third proline loop [64]. In the arrangement of the Rieske
protein along the luminal surface of the membranes, the Fe-S cluster faces the plastoquinol binding site,
which would be formed by cytochrome b 6 , subunit IV, and the Rieske protein motifs. It was proposed
that a flexible hinge at the luminal side of its helix could allow the Rieske protein to orient in two con-
figurations with respect to the membranes: a “relaxed” configuration in which it is close enough to the
cytochrome ƒ in the lumen to allow electron transfer between the two and a “tight” configuration in
which it is close enough to cytochrome b 6 at the thylakoid membrane surface to allow electron trans-
fers from the plastoquinol binding site [65].
- PetB Subunit
The second larger Cyt b 6 ƒ complex protein is cytochrome b 6. It is a 24-kDa product of the organelle petB
gene. Cyt b 6 binds two b-type hemes. It is capable of interacting with plastoquinol as well as with prod-
ucts of its oxidation—semiquinone and plastoquinone [51]. Some authors suggest that it could also bind
the chlorophyll amolecule [50]. The consensus membrane-folding pattern of Cyt b 6 reveals four trans-
membrane helices.
- Other Subunits
The structure and function of other proteins (subunit IV, PetG, PetL, and PetM) are unclear. It was found
that subunit IV is a product of the petDorganelle gene [51] with a molecular mass of 17.5 kDa and three
transmembrane helices. The three other proteins have molecular masses between 3.4 and 4 kDa and one
transmembrane helix. They appear to be necessary for stable assembly of the complex [51].
PHOTOSYNTHETIC MEMBRANES IN HIGHER PLANTS 287