MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 125

and

reactions

3.5.2. PLANT PHOTOSYNTHESIS

In oxygenic photosynthetic organisms, plant and green bacteria, the reaction centers of
two systems PS I and PS II convert the absorbed light energy into energy of stable
products, i.e. ferredoxin and dioxygen (Witt 1996 and references therin) PS I from plants
and cyanobacteria mediates light-induced electron transfer from plastocyanin to
ferredoxin (flavodoxin) at the stromal membrane side, while PS II is a photoenzyme that
catalyzes oxidation of the water in a water-splitting Mn-containing system (Dismukes
al., 2000). Subsequent absorption of four light quanta by PS I and PS II results in
evaluation of dioxygen from a two water molecule. The overall process occurs by the
following scheme:

The key step of the process is the water splitting under absorption of light quanta of
relative low energy. Here we will focus mainly on the latter process which appears to be
one of the most enigmatic reactions in chemistry and photochemistry and will only
briefly consider the light energy conversion reaction centers of PS I and PS II.

Reaction centers of PS I and PS II.
PS I from cyanobacteria consist of 11 protein subunits and several cofactors. After the
photoexcitation of the primary donor, a dimer of chlorophyll a P700, an electron

is transferred via a chlorophyll to a phylloquinone and then to the iron sulfur

clusters, and (Witt, 1996;Itoh et al., 2000)).
Data on chrystallographic models PS I from cyanobacteria have been reported
(Jordan et. al., 2001). PS I from S. elongates contains nine protein subunits featuring
transmembrane and three stromal sububits. The organic cofactors are arranged
in two branches along the pseudo-C2 axis. The distances between adjacent donor and
acceptor centers of the system vary from 8.2 to 14.9 Å for the “right” brunch (A) and

for the “left” brunch (B). Therefore, “non-conducting” zones similar to
those in the bacterial RC separate the centers.
The time scale of different steps of electron transfer along the PS I cascade system is
also similar to those of the electron jump in bacterial RCs. The primary transfer from the
excited chlorophyll dimer, primary donor P*, to takes place with a time constant of
about 25 ps. The next step from to a secondary acceptor occurs in 200-600 ps. The
recombination time constants of with reduced intermediate acceptors increase as the
electron moves along the chain, and range from nanoseconds for transition to
millseconds for transition reduce FX to (Shuvalov and Krasnovsky, 1981; Schloder et
al., 1998; Shmidt et al., 2000; Shmidt et al., Guergova-Kuras et. al., 2001; Setif et al.,
2001; Vassiliev et all., 2001; Gobets et al., 2001 and references therein). Kinetic and
spectral inhomogenity of samples of PS I has been reported (Shmidt et al., 2000;

from 8.6 to 22 Å