MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 129

High-frequency ESR spectroscopy at 94 GHz is used to study the dark-stable
tyrosine radical YD

in single crystals of photosystem II isolated from the thermophilic
cyanobacterium Synechococc

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elongates (Hofbauer, et al., 2001). Magnitude and
orientation of the g tensor of YD • and related information on several proton hyperfine
tensors are deduced from analysis of angular-dependent EPR spectra. The flash-induced
Fourier transform IR (FTIR) difference spectrum of the oxygen-evolving Mn cluster
upon transition indicates that in FS II from Synechocystis 6803 a tyrosine
residue specifically labeled with at the ring-4 position, is coupled to the Mn cluster,
and the vibrational modes of this tyrosine are affected upon formation (Noguchi and
Tang, 1997). It is suggested that the Mn cluster and a tyrosine are linked via chemical
and/or hydrogen bonds and the structural changes of the Mn cluster are transmitted to the
tyrosine through these bonds.
Experiments on the binding of to manganese-depleted photosystem II and
electron donation from the bound to an oxidized tyrosine have showed that only
one tightly bound manganese is essential for electron transfer. Pulsed ESR studies in the
oriented depleted FS II membranes (Mino et al., 2000) indicate that an angle
between the vector connecting the tyrosin radical YD with a center, which exposes a
doublet ESR signal, and the plane of the tylakoid membrane, is about 8°. A functional
role for tyrosine-D in the assembly of the inorganic core of the water oxidation complex
of photosystem II and the kinetics of water oxidation was discussed by Ananyev et al.,
(2002).
Efforts have been undertaken to evolve both state and dynamics of water molecules
in the Mn-cluster. Biphasic rapid isotope exchange beween bulk water and substrate
water in the state of spinach Photosystem II has been detected (Hiller et al., 1998).
Two first order exchange rate constants are determined from measurement of
release after injection of at 10 °C, fast and slow The
activation energies of the fast and slow processes are estimated as 9.3 and 18.6
kcal/mole, correspondingly. These results are considered as evidence that two water
molecules bind at two separate binding sites. Evidence for the presence of a component
of the Mn complex of the Photsystem II which has been exposed to water in the S 2 state
of the water oxidation complex, has been obtained by the ESEEM method (Evans et al.,
2000). The experiments indicated that water protons locate within 3-5 Å of part of the
Mn complex. The use of Fourier transform infrared (FTIR) spectroscopy with a
combination of replacement by and allows the detection O-H
stretching vibrations of a water molecule coupled to the Mn-cluster in the and
states (Noguchi and Sugiura, 2000). It is shown that one of the molecules of the O-H
group is weakly H-bond and other is strongly H-bonded. This H-bonding asymmetry
becomes more prominent upon the transition. The authors suggest that such
structural change might facilitate the proton release reaction by lowering the potential
barrier.
In Photsystem II, the water oxidation with evolution of dioxygen occurs under the
action of a relatively mild oxidant: the cation of chlorophyll which is the product of one-
electron oxidation with redox potential (Anderson, 2001). The potentials of
the oxidation of water by one-, two- and four electron mechanisms are equal to 2.7 V
(hydroxyl radical), 1.36 V (hydrogen proxide), and 0.81 (dioxygen). Enclosed in

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