The photosynthetic reaction centers
is a special pigment-protein complex, that functions as a photochemical
trap. The function of the reaction center is to convert solar energy
into biochemical amenable energy. Following the initial capture
of a photon by antenna pigments, the photon is transferred to the
RC pigments, where it gives rise to a separation and stabilization
of charge across the photosynthetic membrane. Figure 1 depicts this
process and illustrates the time scales typically involved.
Figure 1. Scheme of the primary processes in
the photosynthetic RC. Here, P represents the charge-separating
(bacterio) chlorophyll pigments (the primary electron donor) and
A represents the first stable acceptor. Energy transfer from the
antenna pigments leads to photoexcitation of P on the fs-ps time
scale (left). Charge separation produces oxidized P+ and A- on the
ps-ns scale (center). The recombination of P+A- to produce PA, heat
and potentially damaging chemical species is efficiently prevented
by further forward electron transfer that is now proton coupled.
These more complex chemical processes ultimately produce stable
photosynthetic products and occur, initially, on the ns and µs
time scales (right)
The photosynthetic reaction center (RC) is the
first membrane protein whose three-dimensional structure was revealed
at the atomic level by X-ray crystallograph more than fifteen years
ago. Structural information about RC made a great contribution
to the understanding of the reaction mechanism of the complicated
membrane protein complex [5,6].
High-resolution structures of RC's from three photosynthetic bacteria
are now available, namely, those from two mesophilic purple non-sulfur
bacteria, Blastochloris viridis and Rhodobacter sphaeroides,
and that from a thermophilic purple sulfur bacterium, Thermochromatium
tepidum. In addition, a variety of structural studies, mainly
by X-ray crystallography, are still being performed to give more
detailed insight into the reaction mechanism of this membrane protein.
The structural data from three RC's and their electron donors
provided reliable models for molecular recognition in the primary
step of bacterial photosynthesis.
Figure 2. Three-dimensional structure of tetrameric
complex of RC penetrate through the membrane. Note that the water
and Cytochrome molecules (blue, yellow color) do not penetrates
the membrane spanning domain (black lines) of the reaction center
complex. The chains of green and white blue color described M, L
and H subunits of RC. (Cox&Lehninger, Principles of Biochemistry,
Worth Publishing, 3rd ed, 2000).
Figure 3. X-ray structure arrangement of co-factors
of RC embedded in the membrane protein complex (Cox &Lehninger,
Principles of Biochemistry, Worth Publishing, 3rd ed, 2000).
Electron transfer coupled with the
uptake of protons across the membrane is a fundamental feature of
bioenergetic processes such as oxidative phosphorylation and photosynthesis,
and the resulting electrochemical gradient of protons is finally
utilized for ATP synthesis (Fig. 8). Key players
in bioenergetics are integral membrane proteins and co-factors embedded
in the membrane protein complexes, where polypeptide chains spanning
across the membrane provide a scaffold for the specific arrangement
of co-factors in membrane protein complexes. Hence, structural data
about membrane protein complexes contribute greatly to obtaining
a profound understanding of reaction mechanisms .
In fact, a great deal of effort
has been made to elucidate the tree-dimensional structures of membrane
proteins involved in bioenergetics for the sake of functional analyses
(Figs.2, 3). Of such structural
studies, crystallographic studies of the photosynthetic reaction
center (RC) provided the first successful description of the three-dimensional
structure at an atomic resolution, and the methodology established
in this structural work has had a strong influence on subsequent
structural studies of membrane proteins.
In addition, structure analyses of RC complexes remain
one of the most active fields in membrane protein structural biology.
This is because it is not yet clear how the RC complex regulates
the electrochemical properties of co-factors, especially that of
the bacteriochlorophyll dimer (the special pair) that acts as the
initiator of photosynthetic electron transfer, how the RC complex
takes up protons to reduce quinone, which acts as the final electron
acceptor in the complex, and how the RC complex accepts electrons
from the electron carrier protein to reduce the photo-oxidized special-pair
(Fig. 8). Consequently, structure analyses of
some modified RC complexes, such as mutants or complexes with substrate
analogues, have been carried out extensively so as to relate structural
information to physical and chemical properties, in spite of the
fact that the three-dimensional structure of the native RC complex
has already been determined precisely.
The RC complex maintains a number of prosthetic groups
in the protein subunit scaffold. The prosthetic groups in the trans-membrane
region apparently form two branches (A and B) that are related by
a pseudo twofold axis perpendicular to the membrane plane.
These two branches run from the
special-pair of bacteriochlorophyll (DA and DB) to the non-heme
iron. Each branch consists of a bacteriochlorophyll monomer (BChlA
or BChlB), bacteriopheophytin (BPhA
or BPhB), and quinone (QA
or QB). A carotenoid molecule is present
in the trans-membrane region near BChlB.
For more details on structural arrangement, see .
Branch A, mainly associated with the L subunit, is selectively utilized
as the pathway of electron transfer, which is induced by charge-separation,
in this process, an electron is emitted from the excited special-pair
and transferred through BPhA and QA
[11,12]. The involvement of
BChlA, which is located between the special-pair
and BPhA, in the electron transfer remains
a matter of debate. Branch B, is associated with the M subunit and
is inactive in electron transfer. QB is the
final electron acceptor at this stage, and the reduced QB
molecule serves as the electron carrier to the Cyt bc1 complex.
The RC structures from photosynthetic bacteria are described schematically
Figure 4. Schematic representation of the X-ray
structure of Rp. viridis RC showing two symmetrical subunits
of the RC
Since prosthetic groups play a central role in photosynthetic
energy conversion, it is necessary to describe the three-dimensional
arrangement of these prosthetic groups and their interactions with
protein subunits precisely, this will lead to a better understanding
of the functional aspects of RC and probably give answer to the
questions why electron transfer through the RC prefers only one