Reaction centre can define as the site of photosynthetic reactions. Chlorophyll and pheophytin are the pigments found in a reaction centre. It comprises protein pigments that performs light absorption and excitation of an electron to the higher energy state. Reaction centre is generally seen in photosynthetic organisms like green plants, many bacteria and algae.
Photosystem I and II are the two common RCs. A reaction centre includes a multisubunit protein complex, which makes its structure more complex. The light energy captured by the RC is used to reduce an electron acceptor to produce ATP that is further used by a cell to, which is crucial for the production of chemical energy in photosynthesis.
Content: Reaction Centre
A reaction centre is the active site of the organisms that carry out photosynthesis by converting light energy into chemical one via the association of ascesary pigment proteins or antenna molecules. The photosynthetic organisms use either of the two photosystems (PS-I or PS-II) or both to carry out the light absorption, energy transfer and transfer of electrons. Photosystem is the basic component of the light reaction:
- In plants and algae, the photosystems are located inside the chloroplasts.
- In photosynthetic bacteria, the photosystem is found in the cytoplasmic membrane.
A reaction centre lies within the photosystems, whose function is to reduce the carrier molecules by using the light energy. Light-harvesting pigments surround the RC, which is altogether called as antenna complex that mediates the absorption of light and transfer of photons to charge the reaction centre. In a photosystem, two kinds of RCs generally exist.
- Type I RCs: It includes PS-I (P700) and green-sulphur bacteria.
- Type II RCs: It includes PS-II (P680) and non-sulphur purple bacteria.
Mechanism of Capturing Light Energy
The arrangement of the reaction centre facilitates capturing of photon molecules through light-harvesting pigment molecules. Firstly the light energy is captured by the pigment molecules, and then the antenna molecules transfer the light energy to the reaction centre via resonance transfer.
As the light energy reaches the RC, it passes 2e– into the ETC. Here, the light is considered as the form of energy because it is composed of small bunches of energy termed as photons. The photons from the light energy of a specific wavelength help in the excitation of an electron to a higher energy level.
Electrons are most stable at their ground state. In-ground state an electron possess the least amount of energy in its orbit. The excited electrons return to the photosyntem, by releasing some kind energy that later harnessed by the cell in the dark reaction. Thus, this is the mechanism of a photosynthetic reaction centre.
Reaction Centre of PS-I and PS-II
As we know, the reaction centre plays a significant role in photosynthesis of bacteria, cyanobacteria and higher plants. The photosynthesis occurs in two ways, namely oxygenic and non-oxygenic photophosphorylation.
Cyanobacteria and higher plants carry out oxygenic photosynthesis by using both photosystems I and II. Photosynthetic bacteria undergoes non-oxygenic photosynthesis, and uses only one photosystem (either PS-I or PS-II).
The mechanism of photosynthetic RC involves three consecutive stages:
Light absorption: The photopigments of the RC first absorb the light of specific wavelength.
Energy transfer: Auxillary antenna pigments then passes the photons to the reaction centre, where the photochemical reaction occurs.
Electron excitation: The charged reaction centre releases the high energy electrons by exciting the electrons from a ground to a higher energy state. The high energy electron is accepted by the primary cofactors like plastoquinone or ferredoxin and then goes through a series of the chemical reaction. At last, these electrons are either goes back to the antenna molecules (cyclic photophosphorylation) or may enter to the PS-I (Noncyclic photophosphorylation).
A reaction centre possesses a multisubunit complex (containing >24 or >33 protein subunits). The RC in photosynthetic bacterial photosynthetic acts as a model organism to research the mechanism of capturing light energy.
Hartmut Michel, Johann Deisenhofer and Robert Huber were the scientists who proposed the crystal structure of RC. On further study, it was found that different subunits and few accessory molecules participate in the photosynthetic reaction centre.
Subunits: A bacterial photosynthetic RC comprises the following distinct subunits.
- L and M subunits: These are associated with the cell membrane, both are structurally identical, and each possesses 5 transmembrane polypeptide helices. The ferrous ions are associated with these subunits.
- H subunit: It is associated with the cytoplasmic side of the cell membrane.
- Cytochrome subunit: It is associated with the periplasmic space of the cell membrane, and comprising four c-type hemes.
Pigment molecules: A bacterial photosynthetic RC comprises the following pigment proteins.
- Four bacteriochlorophyll b (BChl-b) molecules: It is the primary protein pigment that facilitates energy transfer via photon absorption. Bacteriochlorophyll roughly resembles the plant chlorophyll.
- Two bacteriopheophytin b molecules (BPh) molecules: BPh possesses a more or less same structure as BChl-b, only the central Mg2+ is replaced by the 2H+.
- Two quinones (QA and QB) are the cofactor protein molecules.
Types of Reaction Centre
There are six kinds of photosynthetic RCs, depending on the following factors:
- Size and nature of the light harvesting pigment
- The associated photosystem
- Strength of the reaction centre pigment to excite an electron
The six classes of RC can categorize into two groups:
Type-I reaction centre: Heliobacteria, green sulfur bacteria, and photosystem-I possesses type-I RC. In this group, the iron-sulfur clusters function as the electron acceptors.
Type-II reaction centre: Purple bacteria, green filamentous bacteria, and photosystem-II possess type-II RC. In this group, quinones act as the primary electron acceptors.
In a photosynthetic reaction, the reaction centre functions as a primary donor that passes the electron to the cofactors accepting an electron. The electron donors generally comprise of Tyr residue (TyrZ), which comprises:
- Cluster of 4 manganese ions for PSII
- Plastocyanin molecule for PSI
- Cytochrome-c for the bacterial RCs
The structure and arrangement of the antenna pigments, associated with the RC differs variably in different photosynthetic organisms. The RC of heliobacteria possess a simple organization by having a single protein complex (contains 40 chlorophyll g pigments) that lacks auxiliary peripheral antenna proteins.
The RC of photosystem-I and green sulfur bacteria comprises light-harvesting pigments (around 100), while the RC of purple bacteria and photosystem-II comprises around six to eight pigment proteins that possesses C2 symmetry.