III. COMPLEX CYTOCHROME b 6 ƒ
A. Supramolecular Organization
The cytochrome b 6 ƒ complex acts as plastoquinol:plastocyanin oxidoreductase. The complex contains as
many as seven polypeptide subunits. The four “large” subunits of 18–32 kDa, products of the petA–D
gene, are cytochrome ƒ (Cyt ƒ), cytochrome b 6 (Cytb 6 ), the Rieske iron-sulfur protein, and subunit IV.
They bind a c-type heme, two b-type hemes, and a high-potential iron-sulfur center, respectively. Subunit
IV does not contain a prosthetic group [50]. The latter three subunits are involved in the binding of the
electron donor plastoquinol. There are three smaller subunits, each with one transmembrane helix [50,51].
Furthermore, a single chlorophyll amolecule with so far unknown function is part of the complex [52,53].
The anticipated approximate cross-section view of the cytochrome b 6 ƒ complex is presented in Fig-
ure 4. It is based on the solved structure of Cyt ƒ [54], dimensions of the extrinsic domain of the Rieske
286 DENEV AND MINKOVFigure 3 Functional scheme of photosynthetic water oxidation. (From Ref. 41.)
Figure 4 Anticipated cross section of the cytochrome b 6 ƒ complex orthogonal to the plane of the membrane.
(According to Ref. 51.)