complex, the resolution limit was limited and the three-dimensional struc-
ture of photosystem II could not be determined in detail. The core protein
subunits and cofactors have been found to form the same symmetrical
arrangement, as found in the other complexes. The position of the man-
ganese cofactor has been identified but the molecular arrangement of
this crucial cofactor has not been elucidated. Despite these limitations,
however, the structure does provide a foundation for a discussion of the
mechanism of water oxidation on a molecular level.
To oxidize water, photosystem II must have several specific chemical
capabilities. First, water is a very stable molecule and, correspondingly,
the oxidation/reduction midpoint potential at pH 7 is high, at +0.82 V,
and is +0.93 V at pH 5, which is the pH of the thylakoid lumen where the
reaction occurs (Chapter 6). To oxidize water, the primary electron donor,
P680, must have a higher oxidation /reduction midpoint potential. Although
the midpoint potential is too high to measure experimentally, P680 has
been estimated to have a midpoint potential of about 1 V, making P680
the strongest oxidant of any natural biological system. To generate this
potential, light is absorbed at 680 nm, which has an energy of 1.82 eV using
eqn 20.4. Thus, both the excitation energy and the oxidation/reduction
potential are much higher than those of the bacterial reaction center
as P680+performs difficult oxidation reactions while the reaction center
simply moves protons and electrons across the cell membrane. At a value
of 1.1 V, the oxidation/reduction midpoint potential of P680 is much
higher than that of chlorophyll ain organic solutions, which is less than
0.9 V. Presumably, the increase in potential of the chlorophyll amole-
cule in the protein is due to the presence of specific protein– chlorophyll
interactions such as hydrogen bonds that can increase the potential of
bacteriochlorophyll (Chapter 8). Compared to the midpoint potential of
0.93 V needed for water oxidation, the energy available is sufficient but
with little room to spare as the difference in potentials of YZ(see below)
and P680 is estimated to be 20 –100 mV for the different S states.
Second, the electron-transfer process must be coupled to proton trans-
fer. In photosystem II, the oxidized electron donor, P680+, is reduced rapidly
by a tyrosine residue, identified as YZor TyrZ, that is located about 10 Å
away from P680. Amino acid residues are normally thought of as passive
components of proteins but in some cases they participate in oxidation/
reduction reactions. The oxidation/reduction midpoint potential of YZis
high, at an estimated 0.9 –1.0 V, as indeed it must be to retain the oxidation
strength, but P680+is an even stronger oxidant and will take an electron
from the tyrosine. By using an intermediate electron carrier, the electron
transfer in photosystem II occurs with a high yield, as found in the
bacterial reaction center. More importantly, protonated oxidized tyrosines
are not energetically stable and the oxidation is coupled to the release of the
phenolic proton, forming a neutral tyrosyl radical following mechanisms
similar to those discussed for quinones (Chapter 5).
singke
(singke)
#1