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Photosynthetic reaction centre

2007 Schools Wikipedia Selection. Related subjects: General Biology

   A photosynthetic reaction centre is a protein that is the site of the
   light reactions of photosynthesis. The reaction centre contains
   pigments such as chlorophyll and phaeophytin. These absorb light,
   promoting an electron to a higher energy level within the pigment. The
   free energy created is used to reduce an electron acceptor and is
   critical for the production of chemical energy during photosynthesis.

   Reaction centres are present in all green plants and in many bacteria
   and algae. Green plants have two reaction centres 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 centre 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 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 will raise the
   electron to a higher energy level. Electrons are most stable at their
   lowest energy level or ground state, the orbit in which the electron
   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 centre.

   When an electron rises to a higher energy level it increases the
   reduction potential of the molecule. This means it 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 phaeophytin, 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

   Bacterial photosynthetic reaction centre.
   Bacterial photosynthetic reaction centre.

   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 1960's, Roderick Clayton was
   the first to purify the reaction centre complex from purple bacteria.
   However, the first crystal structure was determined 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 reaction centre contains two pigments that serve to collect and
   transfer the energy from photon absorption: BChb and Bph. BChb 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 BChb, but the central magnesium ion is replaced by two
   protons.

Mechanism

   The light reaction
   Enlarge
   The light reaction

   The process starts when light is absorbed by two BChl-b molecules that
   lie near the periplasmic side of the membrane. This pair of chlorophyll
   molecules, often called the "special pair", absorbs photons at roughly
   960nm, and thus is called P960 (with P standing for "pigment"). Once
   P960 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 P960 and a
   negative charge on the BPh. This process takes place in 10 picoseconds
   (10^-11 seconds).

   The charges on the P960^+ 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 bc[1]-complex) where it is
   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 is 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:

   \begin{matrix}\ &light &\ \\ 2Q + 2H_2 O &\Longrightarrow & O_2 +
   2QH_2\end{matrix}

   Q represents plastoquinone, the oxidized form of Q. QH[2] represents
   plastoquinol, the reduced form of Q. This process of reducing quinone
   is comparable to that which takes place in the bacterial reaction
   centre. Photosystem II obtains electrons by oxidizing water in a
   process called photolysis. Molecular oxygen is a byproduct of this
   process and it is this reaction that supplies the atmosphere with
   oxygen. The fact that the oxygen from green plants originated from
   water was first deduced by the Canadian-born American biochemist Martin
   David Kamen. He used a radioactive isotope of oxygen, O[18] to trace
   the path of the oxygen, from water to gaseous molecular oxygen. This
   reaction is catalyzed by a reactive centre in photosystem II containing
   four manganese ions.

   The reaction begins with the excitation of a pair of chlorophyll
   molecules similar to those in the bacterial reaction centre. Due to the
   presence of chlorophyll a, as opposed to bacteriochlorophyll,
   photosystem II absorbs light at a shorter wavelength. The pair of
   chlorophyll molecules at the reaction centre are often referred to as
   P680. When the photon has been absorbed the resulting high-energy
   electron is transferred to a nearby pheophytin molecule. This is above
   and to the right of the pair on the diagram and is coloured grey. The
   electron travels from the pheophytin molecule through two plastoquinone
   molecules, the first tightly bound, the second loosely bound. The
   tightly bound molecule is shown above the pheophytin molecule and is
   coloured red. The loosely bound molecule is to the left of this and is
   also coloured red. This flow of electrons is similar to that of the
   bacterial reaction centre. Two electrons are required to fully reduce
   the loosely bound plastoquinone molecule to QH[2] as well as the uptake
   of two protons.

   The difference between photosystem II and the bacterial reaction centre
   is the source of the electron that neutralizes the pair of chlorophyll
   a molecules. In the bacterial reaction centre, the electron is obtained
   from a reduced compound heme group in a cytochrome subunit.

   A difference between photosystem II and the bacterial reaction centre
   is the source of the electron which neutralizes the pair of pigment
   molecules. Once photoinduced charge separation has taken place, the
   P680 molecule carries a positive charge. P680 is a very strong oxidant
   and extracts electrons from two water molecules which are bound at the
   manganese centre directly below the pair. This centre, below and to the
   left of the pair in the diagram, contains four manganese ions, a
   calcium ion, a chloride ion, and a tyrosine residue. Manganese is used
   because it is capable of existing in four oxidation states; Mn^2+,
   Mn^3+, Mn^4+ and Mn^5+. Manganese also forms strong bonds with
   oxygen-containing molecules such as water.

   Every time the P680 absorbs a photon, it emits an electron, gaining a
   positive charge. This charge is neutralized by the extraction of an
   electron from the manganese centre which sits directly below it. The
   process of oxidizing two molecules of water requires four electrons.
   The water molecules which are oxidized in the manganese centre are the
   source of the electrons which reduce the two molecules of Q to QH[2].

Photosystem I

   After the electron has left photosystem II it is transferred to a
   cytochrome b6f complex and then to plastocyanin, a blue copper protein
   and electron carrier. The plastocyanin complex carries the electron
   that will neutralize the pair in the next reaction centre, photosystem
   I.

   As with photosystem II and the bacterial reaction centre, a pair of
   chlorophyll a molecules initiates photoinduced charge separation. This
   pair is referred to as P700. 700 is a reference to the wavelength at
   which the chlorophyll molecules absorb light maximally. The P700 lies
   in the centre of the protein. Once photoinduced charge separation has
   been initiated, the electron travels down a pathway through a
   chlorophyll α molecule situated directly above the P700, through a
   quinone molecule situated directly above that, through three 4Fe-4S
   clusters and finally to an interchangeable ferredoxin complex.
   Ferredoxin is a soluble protein containing a 2Fe-2S cluster coordinated
   by four cysteine residues. The positive charge left on the P700 is
   neutralized by the transfer of an electron from plastocyanin. Thus the
   overall reaction catalyzed by photosystem I is:

   \begin{matrix} \ & light & \ \\ Pc(Cu^+ ) + Fd_{ox} & \Longrightarrow &
   Pc(Cu^{2+}) + Fd_{red} \end{matrix}

   The cooperation between photosystems I and II creates an electron flow
   from H[2]O to NADP^+. This pathway is called the 'Z-scheme' because the
   redox diagram from P680 to P700 resembles the letter z.

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