MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 124

In the high-resolution ESR (326 GHz) study of the biradical state in the
Rb. Spheroids, RC determines the exchange integral in the biradical (Calvo
et al., 2001). Because the rate constant of electron transfer from to is essentially
less (Feher et al., 1992; Xu et al, 2000) than expected for an nonadiabatic
activationless ET and the values considerably deviate from the dependence of the
supperexchange attenuation parameter on the distance between donor and acceptor
centers in RCs (Fig. XXX), we can conclude that the ET is adiabatic and requires
thermal activation.
The electron transfer of free energy in RC from Rb. spheroidas is
calculated by applying an electrostatic model using the Poisson Boltzmann equation and
Monte Carlo sampling (Rabinshtein et al., 2000). It is shown that the electron transfer is
energetically uphill for the “dark” structure and downhill for the
“light” structure Another interesting conclusion is that coupled
groups GluL212 and L213 bind one proton in the structure and two protons in
An important role of the hydrogen bond bridges in the long-range electron
transfer between and in photosynthetic reaction centers has been suggested
computationally (Peluso et al., 2000).
The effect of the replacement of native ubiquinone in RC from Rb. spheroids for
naphtaquinones of different redox potential on the two-phase kinetics of the
transition is investigated using spectral technique (Li et al., 2000). The variation of the
naphtoquinone free energy

from -0.090 to -0.250 V leads to a decrease of

characteristic t e of the fast component from 29 to yet no affect on the slow

component kinetics with The former fraction increased with increasing
driving force and decreases as the isoprene tail is elongated from 2 to 10 units. The
above-mentioned experimental data are rationalized in the framework of a model
suggesting that the fast process is related to electron transfer from to with
formation of which is not in the conformational equilibrium with the medium.
The energy difference between and equilibrium state is estimated as 0. 09 V.
The slow transition is assumed to be gated by a conformational change at
Another kinetic model suggests a formation of conformationally excited primary
acceptor followed by fast ET to
The electron transfer from the primary donor P to the secondary acceptor

im

is

coupled to two protons uptake followed by exchange of doubly reduced for
from the cytoplasm (Witt, 1996). The high-resolution x-ray diffraction study revealed in
the “dark” structure two water channels, P1 and P2 leading from the pocket to the
surface of the protein on the cytoplasmic side of the RC. These channels have been
assumed to deliver protons to photo reduced states and The GluH173 in the
“light” structure, located along the P2 channel, is disordered compared to this group in
the “dark” structure.
The pathway for proton transfer to is studied in the reaction center (RC) from Rb.
sphaeroides using two approaches (Ädelroth et al., 2001): 1) the binding of or
to the RC surface at His-H126, His-H128, and Asp-H124 and 2) the replacement of the
histidines for Ala. In the double mutant RC at pH 8.5, the observed rates of proton
uptake associated with both the first and the second proton-coupled electron-transfer