MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 116

subsequent chemical reactions eventually results in the formation of stable compounds
such as ATP and NADPH.
Early results
The primary photochemical processes of photosynthesis take place within membrane
bound complexes of pigments and protein, reaction centers (Shuvalov and Krasnovsky,
1981; Deisenhofer et al., 1986, Rees et al., 1989; Norris and Shiffer, 1990; Kirmaier and
Holten, 1991; Feher et al., 1992; Stowell et al., 1997). One mole of a reaction center
from different bacteria contains 4 moles of bacteriochlorophyl (Bchl), 2 moles of
bacteriopheophytin (Bph), two moles of ubiquinone (Q) and a non-heme Fe atom. In RC
from Rhodobacter speroides, a total of 11 hydrophobic create a framework that
organizes the cofactor and a hydrophobic band approximately 35 Å wide. RC from
Rhodopseudomonus viridus has three polypeptides having pronounced hydrophobic
properties. The molecular mass of the polypeptides are 37 571 (L), 35902 (M) and 28902
(H). The H subunit does not carry pigments but it is sufficient for the photochemical
activity. The protein components of reaction centers from different-bacteria are similar.
A series of early studies based on the use of a whole arsenal of biochemical,
physicochemical and physical methods including ESR, ENDOR, TRIPPLE, ESSEM,
EXAFS, Mössbauer spectroscopy, optically detected magnetic resonance ODMAR,
adsorption detected magnetic resonance (ADMAR), reaction yield detection magnetic
resonance (RYDMER), magnetic field effect on reaction yield (MARY), as well as pico-
and femptosecond optical spectroscopy have established the main features of the
structure of RCs and the kinetics of electron transfer during photoseparation of charges
Hoff, 1992; Hoff and Deisenhofer; 1997;Okamura et al., 2000; Deligiannakis and
Rutherford, 2000; Yakovlev and Shuvalov, 2000; Yakovlev et al. 2001; and references
therein).
The process starts with the accumulation of light quanta by the light-harvesting
complex (LHC), the so-called antenna protein, which is a complex of 12 polypeptides
with 12 Bchl molecules Hoff and Shertz, 1992 and references therein). The distances
between chlorophyll molecules are sufficient for an effective energy transfer by the
Forster mechanism. The singlet electronic excitation migrates along the LHC and enters
the primary acceptor the dimer of bacteriocchlorophyl which also passes over
into the singlet state. This is followed by a chain of events (Fig. 3.15). During time of the
order of a picosecond, an electron from the excited is transferred to
bacteriochlorophyl, DA 1 and then, in picoseconds, to bacteriopheophytin Bph and, in
about 200 ps, to the primary acceptor ubiquinone The next electron transfer from
to the secondary acceptor occurs at a rate in the millisecond range. During this
tune the electron from the secondary donor, e.g. type c chytochrome, has the chance to
be transformed from reduced cytochrome c to As a result, the energy of a solar
quantum is transformed into chemical energy of the reduced secondary acceptor, which
can be involved in consequence reactions.
The author of this monograph has suggested that rapid electron transfer in reaction
centers in the forward direction and significantly slower transfer in the reverse direction
may account for the tunneling (long-distance) mechanism of the photoseparated charges
(Likhtenshtein et al., 1975, 1979a,b; Likhtenshtein, 1988a). The concepts of tunneling