Page 162 - Algae Anatomy, Biochemistry, and Biotechnology

Photosynthesis 145

two protons from the stroma, becoming Q B H 2 , which migrates to the lumen side of the cytochrome
b 6 f complex where it is again oxidized, releasing two more protons into the lumen. Thus the
Q-cycle allows the formation of more ATP. This Q-cycle links the oxidation of plastoquinol
(Q B H 2 ) at one site on the cytochrome b 6 f complex to the reduction of plastoquinone at a second
site on the complex in a process that contributes additional free energy to the electrochemical
proton potential.
The cytochrome b 6 f complex is the intermediate protein complex in linear photosynthetic
electron transport. The cytochrome b 6 f complex essentially couples PSII and PSI and also provides
the means of proton gradient formation by using cytochrome groups as redox centers in the ETC
thereby separating the electron/hydrogen equivalent into its electron and proton components.
The electrons are transferred to PSI via plastocyanin and the protons are released into the thylakoid
lumen of the chloroplast. The electron transport from PSII to PSI via cytochrome b 6 f complex
occurs in about 7 ms, representing the rate limiting step of the photosynthetic process.
The cytochrome b 6 f exists as a dimer of 217 kDa. The monomeric complex contains four large
subunits (18–32 kDa), including cytochrome f, cytochrome b 6 , the Rieske FeS iron-sulfur protein
(ISP), and subunit IV, as well as four small hydrophobic subunits, PetG, PetL, PetM, and PetN. The
monomeric unit contains 13 transmembrane helices: four in cytochrome b 6 (helices A to D); three
in subunit IV (helices E to G); and one each in cytochrome f, the ISP, and the four small hydro-
phobic subunits PetG, PetL, PetM, and PetN. The monomer includes four hemes, one [2Fe-2S]
cluster, one chlorophyll a, one b-carotene, and one plastoquinone. The extrinsic domains of
cytochrome f and the ISP are on the luminal side of the membrane and are ordered in the crystal
structure. Loops and chain termini on the stromal side are less well ordered. The ISP contributes
to dimer stability by domain swapping, its transmembrane helix obliquely spans the membrane
in one monomer, and its extrinsic domain is part of the other monomer. The two monomers
form a protein-free central cavity on each side of the transmembrane interface.
Cytochrome c 6 is a small soluble electron carrier. It is a highly a-helical heme-containing
protein. It is located on the luminal side of the thylakoid membrane where it catalyzes the electron
transport from the membrane-bound cytochrome b 6 f complex to PSI. It is the sole electron carrier
in some cyanobacteria.
Plastocyanin operates in the inner aqueous phase of the photosynthetic vesicle, transferring
electrons from cytochrome f to PSI. It is a small protein (10 kDa) composed of a single poly-
peptide that is coded for in the nuclear genome. Plastocyanin is a b-sheet protein with copper as
the central ion that is ligated to four residues of the polypeptide. The copper ion serves as a
one-electron carrier with a midpoint redox potential (0.37 eV) near that of cytochrome f.
Plastocyanin shuttles electrons from the cytochrome b 6 f complex to PSI by diffusion. Plasto-
cyanin is more common in green algae and completely substitutes for cytochrome c 6 in the
chloroplasts of higher plants. In cyanobacteria and green algae where both cytochrome c 6
and plastocyanin are encoded, the alternative expression of the homologous protein is regulated
by the availability of copper.
Ferredoxin is a small protein (11 kDa), and has the distinction of being one of the strongest
soluble reductants found in cells (midpoint redox potential ¼ 20.42 eV). The amino acid sequence
of ferredoxin and the three-dimensional structure are known in different species. Plants contain
different forms of ferredoxin, all of which are encoded in the nuclear genome. In some
algae and cyanobacteria, ferredoxin can be replaced by a flavoprotein. Ferredoxin operates in
the stromal aqueous phase of the chloroplast, transferring electrons from PSI to a membrane
associated flavoprotein, known as FNR. A 2Fe2S cluster, ligated by four cysteine residues,
serves as one-electron carrier.
Once an electron reaches ferredoxin, however, the electron pathway branches, enabling redox
free energy to enter other metabolic pathways in the chloroplast. For example, ferredoxin can
transfer electrons to nitrite reductase, glutamate synthase, and thioredoxin reductase.