matter on earth. One website devoted to cytochrome b(6)f is found at http://
http://www.biology.purdue.edu/people/faculty/cramer/Cramer/html/cytbf.html. Four
multi - subunit membrane proteins take part in the conversion of the sunlight
into chemical energy. These are: photosystem I, photosystem II, cytochrome
b(6)f complex, and F - ATPase.^71 Photosystem I generates the most negative
redox potential in nature and thus largely determines the global amount of
enthalpy in living systems. The cytochrome b(6)f complex interacts with pho-
tosystems I and II (PSI and II) by receiving electrons from PSII and passing
them to PSI, at the same time pumping protons across the membrane and
powering the Q cycle. The Q cycle is the name give to the movement of lipo-
philic quinone species across membranes coupled to redox reactions and
proton uptake and release.^72 Figure 1 of reference 73 shows a schematic
diagram of the PSII, cytochrome b(6)f, PSI complex. X - ray crystallographic
structures for PSI (PDB: 1JB0)^74 and PSII (PDB: 1FE1, 1IZL)^75 have been
published.
The cytochrome b(6)f complex mediates electron transfer between the PSI
and PSII reaction centers by oxidizing lipophilic plastoquinol (PQH 2 ) (see
Figure 7.24 ) and reducing the enzymes plastocyanin or cytochrome c 6. The
electronic connection also generates a transmembrane electrochemical proton
gradient that can support adenosine triphosphate (ATP) synthesis instead of
electron transport.
The b(6)f complex consists of a total of four hemes, two belonging to the
cytochrome b family, one belonging to the cytochrome f family, a heme “ x, ”
and a [2Fe – 2S] Rieske (RISP or ISP) iron – sulfur cluster. The iron – sulfur
protein (ISP) component of the cytochrome b(6)f and cytochrome bc 1 (Section
7.6) complexes, now called the “ Rieske ” ISP, was fi rst discovered and isolated
by John S. Rieske and co - workers in 1964 (in the cytochrome bc 1 complex).
The homologues of the Rieske proteins include ISP components of cyto-
chrome b(6)f and cytochrome bc 1 complexes, aromatic - ring - hydroxylating
dioxygenases (phthalate dioxygenase, benzene, naphthalene and toluene 1,2 -
dioxygenases), and arsenite oxidase. Comparison of amino acid sequences has
revealed the following consensus sequence: cys – X – His – (X) 15 – 17 – cys – X – X – his,
where X stands for any amino acid. The crystal structures of a number of
Rieske proteins, including cytochrome b(6)f (discussed in this section) and
cytochrome bc 1 (Section 7.6), are known. The overall Rieske protein fold is
dominated by an antiparallelβ - structure and contains only one α - helix. The
Figure 7.24 The plastoquinol/plastoquinone reaction.- 2H+, - 2 e-
OO(CH 2 -CH=C(CH 3 )-CH 2 ) 9 -HOHOH(CH 2 -CH=C(CH 3 )-CH 2 ) 9 -Hdihydroplastoquinone,
plastoquinol, PQH 2plastoquinone, PQCYTOCHROME b(6)f: A GREEN PLANT CYTOCHROME 383