The metabolism of living organisms as they evolved on earth constantly dissipates energy in a non-reusable form. To sustain life this energy has to be replenished. The only external energy source is the sun, so life has to cope with utilizing light to fuel all metabolic demands.
Photons emanated by the sun carry energy depending on the colour of the light. At a wavelength of 800 nm this amounts to 35.7 kcal/mol, which compares favorably with the hydrolysis energy of e.g. phosphoenolpyruvate (14.8 kcal/mol) or adenosine triphosphate (7.3 kcal/mol). So all we need is a machinery to catch the photon's energy and store it in the form of say a phosphoester bond. Nature accomplishes this task with the aid of multienzyme complexes organized in cellular membranes. The simplest systems are found in purple photosynthetic bacteria. The cytoplasmic membranes contain photosynthetic units (PSU) which are made up of reaction centers (RC) and light harvesting complexes (LH I and LH II). Light drives the reduction of quinones by the PSU and the quinones are reoxidized by a cytochrome complex (thus turning the process into a cycle). Turning the quinone cycle expels protons from the cytoplasm to the periplasm. The electrochemical gradient thus generated is used to drive ATP synthesis by the membrane bound F0F1-ATPase.
Besides quinones other ligands are used by the photosynthetic enzymes: chlorophylls are used as photon acceptors, carotinoids mediate the transfer of electrons to the quinones, and hemes are used by the cytochromes to recycle electrons. The exact type of ligands varies according to the bacteria as does the arrangement of the protein subunits in the different complexes. Several structures were resolved to the atomic level giving insight to the mechanisms of energy conversion. The supermolecular organization is so far known by cryo-electron microscopy.
The initial step of the energy conversion sequence is the electronic excitation of a chlorophyll in the reaction center upon receiving a photon's energy. The state of the chlorophyll is relaxed by transferring an electron to a quinone, defining the chlorophyll as the primary donor. In step 1 thus a charge couple D+Q- is generated. In step 2 the chlorophyll is reduced by a cytochrome, and in step 3 the charge of the quinone is passed on to a second quinone. Step 4 repeats steps 1 and 2 resulting in two charged quinones. In steps 5 and 6 protons are taken up to generate an uncharged quinone and a hydroquinone. The hydroquinone diffunds out of the reaction center into an also membrane bound cytochrome bc1 complex and is reoxidized to the quinone state. In steps 8 and 10 protons are liberated. Due to the spatial arrangement of the protein complexes the protons are taken up in steps 5, 6 in the interior of the cell and expelled in steps 8, 10 to the periplasmic space. This process generates an electrochemical gradient across the cytoplasmic membrane. The quinone cycle is closed by transfer of the quinone back to the RC (11).
In the frame to the right the chromophores of the reaction center of Rhodopseudomonas sphaeroides are to be seen. The primary chlorophyll (type a) is shown in yellow. The electrons are passed via another bacteriochlorophyll a (green) and a bacteriopheophytin (light blue) to ubiquinone-10 (red). Magnesium atoms are symbolized in dark green, a single iron ion in orange.
The primary chlorophyll in the reaction center is rather a small target to be hit by photons. To improve the absorption cross section, RCs are surrounded by antenna proteins harbouring more chlorophyll as well as pigments absorbing at other wavelengths thus extending the efficiency even further. These proteins are arranged in circular patterns around the RCs within the bacterial cytoplasmic membrane. The diameter of these complexes extends to ca. 10 nm, they are termed light harvesting complex I (LH I). Most bacteria contain additional photosensitive complexes (LH II). An example of the chromophore arrangement of a LH II is shown at the top of this page. Below two different complexes are shown with their additional carotenoid chromophores and the supporting protein structure. In the bacterial membranes several LH II complexes are situated close enough to LH I to mediate fast energy transfer via LH I to the RC.
A low resolution crystal structure of a reaction center with its surrounding light harvesting complex I shows the spatial pattern of LH I proteins with the enclosed chlorophyll around the RC of Rhodopseudomonas palustris (RC in blue, LH I proteins in light violet (helices only), chlorophyll in green). Note a gap in the otherwise complete oval arrangenment of LH I around the RC
(view perpendicular to the membrane). This gap is used for diffusion of the ubiquinone cofactor as a shuttle to the cytochrome complex. One of the helical proteins
is discussed to facilitate the ubiquinone movement.
Reaction center from Rhodopseudomonas viridis
Protein chain L - chain M - chain H - Cytochrome chain C - Chlorophyll b - Pheophytin b - Quinone - Heme - Iron - Magnesium
L and M only
The core proteins of the RCs of purple bacteria are the L and M chains which give the complex a quasi symmetric structure. They span the membrane with five helices each and harbour the active ligands. In most of these bacteria an asymmetrically placed H chain is found which also spans the membrane with one helix. Only in some species a cytochrome c chain is integrated into the RC.
Despite the nearly symmetrical arrangement of the chromophores there is a functional difference: the a branch is 200 times more used in electron transport than the b branch.
Within the membranes RCs and cytochrome bc1 complexes are found in a stochiometry of 2:1. Electron micrographs of native membranes containing two-dimensionally ordered PSUs from LH I, RC and cyt bc1 led to a model showing two RCs placed symmetrically beneath a cyt bc1 surrounded by c-shaped LH I-complexes comprised of 12 subunits in Rhodobacter sphaeroides.
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RE Blankenship, http://photoscience.la.asu.edu/photosyn/education/antenna.html
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S Iwata et al, Complete structure of the 11-subunit bovine mitochondrial cytochrome bc1 complex, Science 281 (1998) 64-71
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AW Roszak et al, Crystal structure of the RC-LH1 core complex from Rhodobacter palustris, Science 302 (2003) 1969-1972
About the basics of membrane proteinology:
G v Heijne, A day in the life of Dr. K. or how I learned to stop worrying and love lysozyme: A tragedy in six acts, J. Mol. Biol. 293 (1999) 367-379