MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 118

between and in RC from Rhodopseudomonas spheroidos was shown to be
32-35 Å. This value was obtained employing the method based on measurement of the
effect of on the spin-lattice relaxation time of On the basis of this result and
analysis of quantitative data on exchange interactions (exchange integral J values)
between other RC components (Tiede et al., 1976; Shuvalov and Asadov, 1979; Schepler
et al., 1975; Klevanik et al., 1980;) and the experimental dependence of the spin-
exchange integral on the distance between paramagnetic centers (Section 2. ??, Fig.
Xxxx), a scheme of the spatial localization of the electron donors and acceptors in
reaction centers has been composed (Likhtenshtein et al., 1979,1981,1982). As one can
see from Figs. 3.13-3.14, the RC model proposed on the basis of a physico-chemical
investigation shows similar principle features just as a subsequent crystalographic model
does, namely, that the pigments in the reaction center from R. viridis are located at
distance of 7-11 Å and are separated by non-conducting protein media. The center-center
distance between between and is about 30 Å.
The linear dependence of the logarithm of the rate constant of the electron transfer in
RCs of purple bacteria and plant photosystem I on the edge-edge distance
between the donor and acceptor centers (R) was observed (Fig. 2.9) (Likhtenshtein,
1996, 2000). The slope of the dependence corresponds to the slope predicted for long-
distance spin superexchange orbital overlap through non-conducting media by the
shortest pathway (Eqs. 2.6 with As one can see from Fig. 2 9, the values
of for the transfer from to Bph and from to pheophytin acceptor (Ph)
markedly deviate from the general log - R plot. Such deviation is explained by
assuming the participation of intermediate acceptors located between and Bph,
and between and (Ph) (Michel and Deisenhofer et al., 1985; Kirmaier et al., 1991;
Yakovlev and Shuvalov, 2000). Another deviation is related to ET from the primary
quinone acceptor

to the secondary quinone acceptor The process takes place at

, but the centers are connected with two hydrogen
bonds and two aromatic imidazol groups. On the basis of estimation of the resonance
integral of the energy reorganization, it was concluded that the process runs adiabatically
and is controlled by media reorganization (Likhtenshtein, 1988a, 1996).
The first experimental evidences that electron transfer from to and from to
in reaction centers are controlled by the protein conformational dynamics, was
obtained in the late 1970's (Berg 1978a,b; Likhtenshtein et al., 1979 a, b) This
conclusion was confirmed in subsequent experimental studies in which molecular
dynamics of RC and the photsynthetic membrane were determined with a whole set of
physical labels. (Kotelnikov et al., 1983, Kochetkov et al., 1984; Parak et al., 1983). It
was shown that the electron transfer from reduced primary acceptor to secondary
acceptor takes place only under conditions in which the labels record the mobility of
the protein moiety in the membrane with the correlation frequency

an edge-edge distance of about 14 Å

(Fig.

3.16).
This fact was explained in the framework of two models. The first model is based on
the concept of dynamic adaptation of a protein matrix in every step of an enzymatic
reaction. Concerning the transition, fast reversible conformational transitions
can provide dipolar relaxation favourable for the media reorganization process
(Likhtenshtein, 1976, 1979 a, 1988 a). Such reorganization is necessary to release