Biomedical Engineering Reference
In-Depth Information
in the presence of dihydrobiopterin, the ferrous-dioxy decay occurs more slowly and is
not associated with Arg hydroxylation. The authors have come to the reasonable
conclusion that
transfers electrons to the active site to form a center with strong
oxidizing power.
A 10-step kinetic model has been developed (Santolini et al., 2001). Crystal
structures of xyNOS show that a Tyr-409 indol nitrogen atom forms a strong hydrogen
bond with the heme thiolate (Crane et al., 1988; Raman et al.1998; Fishmann et al.,
1999). The Try-409 mutation suggests that the heme potential controls the NOS
reactions (Adak et al. 2001). Suppression of this hydrogen bond through the mutation
lowers the reduction potential of the heme, inhibits heme reduction and accelerates
oxidation of the Fe(II) heme-NO complex. The Arg binding increases the reduction
potential of the NOS heme.
3.5.
Light energy conversion and water-oxidation systems in photosynthesis
The main outcome of photosynthesis is the oxidation of water and the synthesis of
glucose from carbon dioxide at the expense of sunlight energy. The immensity of this
process on the scale of the Earth is evident by the fact that annually about 50 billion tons
of carbon from carbon dioxide is bound into forms that provide energy and structural
material for all living organisms on Earth (Dismukes, 2001; Blankenship, 2001;
Rutherford and Faller, 2001; Witt, 1996).
The problems of photosynthesis embrace practically all aspects of modern
biochemistry, biophysics and molecular biology. Here, we shall briefly consider two
aspects of fundamental importance not only for biology but for modern chemistry as
well: (1) the structure and action mechanism of the system of conversion of light energy
into chemical energy in the primary charge photoseparation in bacterial and plant
photosynthesis and (2) the possible mechanisms of the participation of polynuclear
manganese systems in the photooxidation of water. The first system is a remarkable
example of the appearance of qualitatively new properties upon combination of active
groups into an ordered structure. The second system accomplishes one of the most
surprising reactions that occur in nature, the production of a strong reducing agent from
water on account of the quanta of low energy.
3.5.1.
REACTION CENTERS FROM PHOTOSYNTHETIC BACTERIA
The primary photochemical and photophysical processes in the donor-acceptor pair D-A
lead to charge photoseparation, i.e. an appearance of the pair of two charges
where the cation-radical is a strong oxidant and is the anion-radical is a strong
reducing agent (Fyfe and Jones, 2000). Therefore, in the D-A pair, the light absorption
energy is converted to chemical energy accumulated in the photoseparated pair. The
most important problems are the structure and action mechanism of biological
photosynthetis, which prevent fast recombination of and centers of high chemical
reactivity and provide relatively long lifetime for these centers. Involving it in
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