Page 161 - Algae Anatomy, Biochemistry, and Biotechnology

144 Algae: Anatomy, Biochemistry, and Biotechnology

enzyme that catalyzes the synthesis of ATP is the ATP-synthase or F 0 F 1 -ATPase, one of the most
ubiquitous proteins on Earth. The F 1 F 0 -ATPases comprise a huge family of enzymes with members
found not only in the thylakoid membrane of chloroplasts but also in the bacterial cytoplasmic
membrane and in the inner membrane of mitochondria. The source of energy for the functioning
of ATP-synthase is provided by photosynthetic metabolism in the form of a proton gradient
across the thylakoid membrane, that is, a higher concentration of positively charged protons in
the thylakoid lumen than in the stroma.
The F 0 F 1 -ATPase molecule is divided into two portions termed F 1 and F 0. The F 0 portion is
embedded in the thylakoid membrane, while the F 1 portion projects into the lumen. Each
portion is in turn made up of several different proteins or subunits. In F 0 , the subunits are named
a, b, and c. There is one a subunit, two b subunits, and 9–12 c subunits. The large a subunit pro-
þ
vides the channel through which H ions flow back into the stroma. Rotation of the c subunits,
þ
which form a ring in the membrane, is chemically coupled to this flow of H ions. The b subunits
are believed to help stabilize the F 0 F 1 complex by acting as a tether between the two portions. The
subunits of F 1 are called a, b, g, d, and 1.F 1 has three copies each of a and b subunits which are
arranged in an alternating configuration to form the catalytic “head” of F 1 . The g and 1 subunits
form an axis that links the catalytic head of F 1 to the ring of c subunits in F 0 . When proton trans-
location in F 0 causes the ring of c subunits to spin, the g–1 axis also spins because it is bound to the
ring. The opposite end of the g subunit rotates within the complex of a and b subunits. This rotation
causes important conformational changes in the b subunits resulting in the synthesis of ATP from
ADP and P i (inorganic phosphate) and to its release.
For an image gallery of the three-dimensional models of the ATPase refer to the website of
Michael Bo ¨rsch at the Stuttgart University (www.atpase.de).

ETC Components
Components of the electron transport system in order are plastoquinone, cytochrome b 6 f complex,
plastocyanin, and ferredoxin. Each of the components of the ETC has the ability to transfer an
electron from a donor to an acceptor, though plastoquinone also transfers a proton. Each of
these components undergoes successive rounds of oxidation and reduction, receiving an electron
from the PSII and donating the electron to PSI.
Plastoquinone refers to a family of lipid-soluble benzoquinone derivatives with an isoprenoid
side chain. In chloroplasts, the common form of plastoquinone contains nine repeating isoprenoid
units. Plastoquinone possesses varied redox states, which together with its ability to bind protons
and its small size enables it to act as a mobile electron carrier shuttling hydrogen atoms from
PSII to the cytochrome b 6 f complex.
Plastoquinone is present in the thylakoid membrane as a pool of 6–8 molecules per PSII.
Plastoquinone exists as quinone A (Q A ) and quinone B (Q B ); Q A is tightly bound to the reaction
center complex of PSII and it is immovable. It is the primary stable electron acceptor of PSII,
and it accepts and transfers one electron at time. Q B is a loosely bound molecule, which accepts
two electrons and then takes on two protons before it detaches and becomes Q B H 2 , the mobile
reduced form of plastoquinone (plastoquinol). Q B H 2 is mobile within the thylakoid membrane,
allowing a single PSII reaction center to interact with a number of cytochrome b 6 f complexes.
Plastoquinone plays an additional role in the cytochrome b 6 f complex, operating in a compli-
cated reaction sequence known as a Q-cycle. When Q B is reduced in PSII, it not only receives two
electrons from Q A but it also picks up two protons from the stroma matrix and becomes Q B H 2 .Itis
þ
2
able to carry both electrons and protons (e and H carrier). At the cytochrome b 6 f complex level it
is then oxidized, but FeS and cytochrome b 6 can accept only electrons and not protons. So the two
protons are released into the lumen. The Q-cycle of the cytochrome b 6 f complex is great because it
provides extra protons into the lumen. Here two electrons travel through the two hemes of cyto-
chrome b 6 and then reduce Q B on the stroma side of the membrane. The reduced Q B takes on