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The porphyrin handbook - Kadish K.M.

Kadish K.M. The porphyrin handbook - Academic press, 2000. - 368 p.
Download (direct link): kadishsmishgulilard2000.djvu
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systems not directly related to electron transfer or photon harvesting.
Another interesting example of a multiporphyrin assembly is that of
cytochrome c-,, a hemoprotein isolated from the sulphate-reducing
bacteria of the genus Desnlfovibrio╦1 The X-ray structure of this
electron storage and exchange protein was solved as early as I979.'1 The
structure shows that the four hemes of the system are arranged in a non-
parallel fashion with Fe Ľ Ľ Fe distances ranging from ~ I I A to ~ 17 A
(Figure I). Cytochrome c, was the lirst

Copyright 2000 by Academic Press All rights of reproduction in any form
reserved.

ISBN 0-12-393200 9/J30.00
2
Chambron et al.
Figure 1. A drawing of the main polypeptide chain and of the heme
arrangement in cytochrome c3 from Desulfovibrio desulfuricans,5
Hydrogenase is the normal physiological electron donor and/or electron
acceptor for cytochrome c3. The protein of D. d. also has a sulphur
reductase activity. The four hemes (in black; the coordinated histidyl
groups have been removed for clarity) lie near the surface of the
protein. They are connected to the apoprotein by two thioether bonds
involving well-resolved cysteins. The short iron-to-iron distances (from
10.9 A to 1 7.3 A) reflect the close packing of the four nonparallel
porphyrin rings.
multiheme cytochrome to be studied by X-ray crystallography, but other
cytochrome structures have later been solved, with one of the most
complex and recent systems being that of bovine heart cytochrome ˝
oxidase (oxidized form) which has two hemes and a dinuclear copper
complex.6 Undoubtedly, the most complex and spectacular sets of
tetrapyrrolic assemblies belong to photosynthesis. These tetrapyrrolic
assemblies are found either in the photosynthetic reaction centers (RC)
or in the various light-harvesting complexes.
In 1984, Deisenhofer and colleagues reported an X-ray structure of the
RC of the photosynthetic bacterium Rhodopseudomonas viridis.7S As
explained in subsequent review articles,4-11 the real tour de force of
the work was their attempt to crystallize the membrane protein. The
magnificent crystallographic work which led to the structure is of course
equally important. This structure determination can no doubt be regarded
as a major scientific event not only because of its direct link to
bacterial photosynthesis but also for the many studies which it inspired
in various fields of research, from biology to biophysics and chemistry.
Figure
2 shows a schematic view of the photosynthetic RC from Rhodopseudomonas
viridis, with its special pair (SP) of bacteriochlorophylls, its two
accessory bacteriochlorophylls (BCh), and the two bacteriopheophytins
(BPh). Above the special pair, a tetraheme cytochrome also plays an
important role.
That the RC of another photosynthetic bacterium, Rhodobacter
sphaemides has been studied by X-ray
crystallography,12-14 following the initial work of Deisenhofer et al. is
noteworthy.7-8 Striking similarities exist between the RCs of the two
different purple bacteria investigated.
The great importance of the RC is obvious since this protein complex
leads to charge separation and thus is responsible for the conversion
between electronic energy
Hemes
Figure 2. Arrangement of the cofactors associated with the reaction
center (RC) protein subunits of the photosynthetic purple bacterium
Rhodopseudomonas viridis.10,11 The four heme groups belonging to a
cytochrome form a linear chain that points to the so-called special pair
(SP), which is a closely associated dimer of bacteriochlorophylls-b. It
is the origin of two branches of cofactors, roughly exchanged by a C2
symmetry axis, each consisting of another bacteriochlorophyll-b (the
"accessory" bacteriochlorophyll BChL or BChM), a bacterio-pheophytin
(BPhL or BPhM), and a quinone (Q A or Q B). In addition, a non-heme iron
sits between the quinones. The special pair is the starting point for a
light-driven electron transfer reaction across the membrane. Only the L
branch is photoactive. Direct photonic excitation, or energy transfer
from the light-harvesting complexes in the membrane (see Figure 3) puts
SP into an excited state which is able to reduce BChL with a time
constant of about 3 ps. Actually, BChL further transfers its electron to
BPhL at such a rate (subpicosecond timescale) that it was believed that
BPhL was the primary electron acceptor. The electron then moves to the
menaquinone QA with a time constant of a; 200 ps. From Qa the electron is
further transferred to the ubiquinone QB in "100 /is. The hole left on SP
is filled from an electron provided by the cytochrome with a time
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