Everything about Photosynthetic Reaction Centre totally explained
A
photosynthetic reaction center is a complex of three proteins that's the site where molecular excitations originating from sunlight are transformed into a series of electron-transfer reactions. The reaction center proteins bind functional co-factors, chromophores or pigments such as
chlorophyll and
pheophytin molecules. These absorb
light, promoting an
electron to a higher energy level within a pigment. The
free energy created is used to reduce a chain of
electron acceptors which have subsequently lowered redox-potentials, and is critical for the production of chemical energy during photosynthesis.
Reaction centers are present in all green
plants and in many
bacteria and
algae. Green plants have two reaction centers known as
photosystem I and
photosystem II and the structures of these centres are complex, involving a
multisubunit protein. The reaction centre found in
Rhodopseudomonas bacteria is currently better understood since it has fewer
proteins than the examples in green plants.
Capturing light energy
A reaction center is laid out in such a way that it captures the energy of a photon using pigment molecules and turns it into a usable form. Once the light energy has been absorbed directly by the pigment molecules, or passed to them by
resonance transfer from surrounding
antenna pigments, they release two
electrons into an
electron transport chain.
Light is made up of small bundles of energy called
photons. If a photon with the right amount of energy hits an electron it'll raise the electron to a higher
energy level. Electrons are most stable at their lowest energy level, what is also called its ground state. In this state the electron is in the orbit that has the least amount of energy. Electrons in higher energy levels can return to ground state in a manner analogous to a ball falling down a staircase. In doing so they release energy. This is the process which is exploited by a photosynthetic reaction center.
When an electron rises to a higher energy level this increases the
reduction potential of the molecule that the electron resides in. This means the molecule has a greater tendency to donate electrons, the key to the conversion of light energy to chemical energy. In green plants, the electron transport chain that follows has many electron acceptors including
pheophytin,
quinone,
plastoquinone,
cytochrome bf, and
ferredoxin that ultimately result in the reduced molecule
NADPH. The passage of the electron through the electron transport chain also results in the pumping of
protons (hydrogen ions) from the chlorplast's
stroma into the
lumen resulting in a proton gradient across the
thylakoid membrane that can be used to synthesis
ATP using
ATP synthase. Both the ATP and NADPH are used in the
Calvin cycle to fix carbon dioxide into triose sugars.
Bacteria
Structure
The bacterial photosynthetic reaction centre has been an important model to understand the structure and chemistry of the biological process of capturing light energy. In the 1960s,
Roderick Clayton was the first to purify the reaction centre complex from purple bacteria. However, the first crystal structure was determined in 1982 by
Hartmut Michel,
Johann Deisenhofer and
Robert Huber for which they shared the
Nobel Prize in 1988. This was also significant since it was the first structure for any membrane protein complex.
Four different subunits were found to be important for the function of the photosynthetic reaction centre.
The L and M subunits, shown in blue and purple in the image of the structure both span the plasma membrane. They are structurally similar to one another, both having 5 transmembrane
polypeptide helices. Four
bacteriochlorophyll b (BChl-b) molecules, two
bacteriophaeophytin b molecules (BPh) molecules, two
quinones (Q
A and Q
B), and a ferrous ion are associated with the L and M subunits. The H subunit, shown in gold, lies on the cytoplasmic side of the plasma membrane. A cytochrome subunit, shown in green, contains four c-type hemes and is located on the periplasmic surface (outer) of the membrane. The latter sub-unit isn't a general structural motif in photosynthetic bacteria.
Reaction centres from different bacterial species may contain slightly altered bacterio-chlorophyll and bacterio-pheophytin chromophores as functional co-factors. These alterations causes shifts in the color of light that can be absorbed thus creating specific niches for photosynthesis. The reaction centre contains two pigments that serve to collect and transfer the energy from photon absorption: BChl and Bph. BChl roughly resembles the chlorophyll molecule found in green plants, but due to minor structural differences, its peak absorption wavelength is shifted into the
infrared, with wavelengths as long as 1000nm. Bph has the same structure as BChl, but the central magnesium ion is replaced by two protons. This alteration causes both an absorbance maximum shift and a lowered redox-potential.
Mechanism
The process starts when light is absorbed by two BChl molecules that lie near the
periplasmic side of the membrane. This pair of chlorophyll molecules, often called the "special pair", absorbs photons between 870nm and 960nm, depending on the species and thus is called P870 (for the species rhodobacter sphaeroides) or P960 (for rhodopseudomonas viridis), with
P standing for "pair"). Once P absorbs a photon it ejects an electron, which is transferred through another molecule of Bchl to the BPh in the L subunit. This initial charge separation yields a positive charge on P and a negative charge on the BPh. This process takes place in 10 picoseconds (10
-11 seconds).
The charges on the specialpair
+ and the BPh
- could undergo charge recombination in this state. This would waste the high-energy electron and convert the absorbed light energy in to
heat. Several factors of the reaction centre structure serve to prevent this. First the transfer of an electron from BPh
- to P960
+ is relatively slow compared to two other
redox reactions in the reaction centre. The faster reactions involve the transfer of an electron from BPh
- (BPh
- is oxidised to BPh) to the electron acceptor quinone (Q
A) and the transfer of an electron to P960
+ (P960
+ is reduced to P960) from a heme in the cytochrome subunit above the reaction centre.
The high-energy electron which resides on the tightly bound quinone molecule Q
A is transferred to an exchangeable quinone molecule Q
B. This molecule is loosely associated with the protein and is fairly easy to detach. Two of the high-energy electrons are required to fully reduce Q
B to QH
2 taking up two protons from the cytoplasm in the process. The reduced quinone QH
2 diffuses through the membrane to another protein complex (
cytochrome bc1-complex) where it's oxidised. In the process the reducing power of the QH
2 is used to pump protons across the membrane to the periplasmic space. The electrons from the cytochrome bc
1-complex are then transferred through a soluble cytochrome c intermediate, called cytochrome c
2, in the periplasm to the cytochrome subunit. Thus, the flow of electrons in this system is cyclical.
Green plants
Oxygenic photosynthesis
In 1772, the chemist
Joseph Priestly carried out a series of experiments relating to the gasses involved in respiration and combustion. In his first experiment, he lit a candle and placed it under an upturned jar. After a short period of time, the candle burned out. He carried out a similar experiment with a
mouse in the confined space of the burning candle. He found that the mouse died a short time after the candle had been extinguished. However, he could revivify the foul air by placing green plants in the area and exposing them to light. Priestly's observations were some of the first experiments that demonstrated the activity of a photosynthetic reaction centre.
In 1779,
Jan Ingenhousz carried out more than 500 experiments spread out over 4 months in an attempt to understand what was really going on. He wrote up his discoveries in a book entitled ‘Experiments upon Vegetables’. Ingenhousz took green plants and immersed them in water inside a transparent tank. He observed many bubbles rising from the surface of the leaves whenever the plants were exposed to light. Ingenhousz collected the gas which was given off by the plants and performed several different tests in attempt to determine what the gas was. The test which finally revealed the identity of the gas was placing a smoldering taper into the gas sample and having it relight. This test proved it was oxygen, or as Joseph Priestly had called it, 'de-
phlogisticated air'.
In 1932, Professor Robert Emerson and an undergraduate student, William Arnold, used a repetitive flash technique to precisely measure small quantities of oxygen evolved by chlorophyll in the algae
Chlorella. Their experiment proved the existence of a photosynthetic unit. Gaffron and Wohl later interpreted the experiment and realized that the light absorbed by the photosynthetic unit was transferred. This reaction occurs at the reaction centre of photosystem II and takes place in cyanobacteria, algae and green plants.
Photosystem II
Photosystem II is the photosystem that generates the electron that will eventually reduce NADP
+. Photosystem II is present on the thylakoid membranes inside chloroplasts, the site of photosynthesis in green plants. The structure of Photosystem II is remarkably similar to the bacterial reaction centre and it's theorized that they share a common ancestor.
The core of photosystem II consists of
two subunits referred to as D1 and D2. These two subunits are similar to the L and M subunits present in the bacterial reaction centre. Photosystem II differs from the bacterial reaction centre in that it has many additional subunits which bind additional chlorophylls to increase efficiency. The overall reaction
catalyzed by photosystem II is:
The cooperation between photosystems I and II creates an electron flow from H
2O to NADP
+. This pathway is called the
'Z-scheme' because the
redox diagram from P680 to P700 resembles the letter z.
Further Information
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