BACTERIAL PHOTOSYNTHETIC CENTER
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First isolated from the purple bacterium: Rhodopseudomonas virids.
Converts energy from sunlight into electrical and chemical energy by pumping protons from one
side of a membrane to the other.
o The first steps of photosynthesis
 The bacteria are filled with photosynthetic vesicle, which are hollow,
membrane-enveloped spheres. The bacterial photosynthetic center is
embedded into these membranes.
 One end faces the interior of the vesicle and is called the periplasmic side and
the other faces cytosolic side.
 Generally a lot of these vesicles are bunched together giving the membrane a
very large surface area and therefore increasing the amount of light that the
bacteria can absorb -> these bunches of vesicles are called thylakoids.
Around each reaction center there are about 100 small membrane proteins, the antenna
pigment molecules, each one containing several chlorophyll molecules which capture photons
over a wide area and transfer them to the photoreaction center, this allows the photoreaction
center to be more efficient and capture more photons.
BACTERIAL PHOTO-REACTION CENTER STRUCTURE
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The reaction center is built up from four polypeptide chains, three which are called L, M, and H.
the fourth subunit is a cytochrome
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The reaction center also contains pigment molecules including:
o 4 bacteriochlorophyll molecules
 2 of which are arrange in a dimer called the “Special Pair” close to the
periplasmic side of the of the membrane.
 The chlorophyll is what initiates electron transfer and by exciting electrons via
photon.
o Two quinone molecules
o Two bacteriohphytin molecules -> which are chlorophyll molecules without the central
Mg2+
o And the cytochrome subunit contains has four bound heme groups.
o The arrangement of these pigments is a pseudo two-fold symmetry axis -> coordinated
by a center Iron atom.
o This pigment arregment allows two possible pathways for electron transfer across the
membrane: one on side of if the symmetry.
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The L and the M subunit are firmly anchored into the membrane, each by five hydrophobic
transmembrane alpha-helices.
o The structure of the L and the M subunits are quite similar except some the loops have
different sequence.
The H subunit in addition has one alpha helix which crosses the membrane.
 So the total amount number of transmembrane alpha helices is 11, all which
help anchor the L,H, and M subunits into the membrane.
 Note: The Cytochrome is not attached to the membrane -> it is only associated
with the protein via Protein-Protein interactions.
The L and the M subunits each contain alpha helices D and E in the core of the protein, which
coordinate a central iron atom via Histadine side chains. The central iron helps to stabilize the
four helices -> these helices pack in an arrangement similar to a soluble 4 helix bundle.
o Note: none of the helices in the membrane are perpendicular to the membrane: instead
they all have a tilt of 20 – 25 degrees, as seen one would expect from a helical bundle or
transmbrane helical proteins. (also seen in bacteriorhodopsin and K+ channel).
The L and the M subunits form a pseudo-twofold symmetry.
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Mechanism of the bacterial photoreaction center.
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In photosynthesis light converted into electrical energy by an electron flow which causes the
separation of negatively and positively charged molecules. (a membrane voltage potential
formed by a proton gradient) -> This separation is a storage of energy (like a battery) because if
the charges were to come back together the energy would be lost.
o Note: this is why the hydrophobic membrane is required for photosynthesis, to keep
the charges separated across the membrane.
This requires an electron donor, which must be able to absorb photons and excite an electron
and then transfer to another molecule, the electron acceptor. After this process the Donor will
become positively charged and the acceptor negatively charged.
First a photon gets absorbed the special pair of chlorophylls on the periplasmic side which
excites an electron and transfers it to the Pheophyin molecules -> from the pheophytin the
electron moves to a molecule of quinine -> the quinine is then released from the protein to
perform later steps in photosynthesis.
o Note: this process takes picoseconds and is extremely efficient, about 98 to 100%
efficacy.
BACTERIAL ANTENNA COMPLEX / Light harvesting complex
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Surrounds the reaction center and increases the efficiency of photosynthesis and allow much
more photon capture.
o These pigments are firmly bound to small hydrophobic protein molecules that are
embedded in the membrane which assemble into two types of multimeric complexes,
called LH1 and LH2.
LH2 Structure.
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A Ring of nine identical hetrodimeric units (not subunits) -> each containing and alpha and a
beta chain. (two different subunits, not referring to motif here) -> both chains span the
membrane as transmbrane alpha-helices
o each unit binds: three chlorophyll molecules and two carotenoids (an accessory
molecules)
o The nine hetrodimeric units form a hollow cylinder with the alpha chains forming the
inner wall and the beta chains forming the outer wall -> the center contains only lipids
that make up the bilayer.
Two of the chlorophylls are bound in between the alpha and beta chains close to the
periplasmic side and are perpendicular to the plane of the membrane and each central
magnesium is coordinated by a His residue from the alpha or beta chain.
One of the chlorophylls is parallel to the plane of the membrane and is bound in between the
beta chains -> this chlorophyll is bound to an oxygen on the Beta chain is therefore in more
polar environment and absorbs light of shorter wavelength.
o This arrangement allows the pigments to capture much broader band energy of
sunlight.
In purple bacterium the LH1 complex is very similar except it is built from 16 units inside of 9
units, in addition to the difference in binding the 3rd chlorophyll, which changes the absorption
spectrum of the chlorophylls slightly. (LH1 absorbs higher wavelengths/lower Energy than LH2)
o The LH1 is assumed to surround the reaction center and is known as the core or inner
antenna since it is directly associated with the Reaction center, and the LH2 is know as
the peripheral antenna.
o Once a photon is absorbed there is a downhill flow of energy from the LH2 to the LH1
and finally to the reaction center -> this transfer of energy is very efficient with almost
no loss of energy. (compare with solar panels which operate with 20% efficacy.)
Here yousee the LH2 surrounding the LH1 which is is surrounding the Light harvesting complex