LIGHT REACTIONS

LIGHT REACTIONS

5.   LIGHT REACTIONS

Mechanisms of electron transport and ATP synthesis in the chloroplast
The process initiates with the absorption of light energy by PS II that catalyzes photolysis of water, arising a pair of electrons and protons, the protons reside in the lumen, but the electrons are transferred to pheophytin which reduces plastoquinone (PQ) molecules A and B sequentially. After getting reduced, PQB converts to PQB2- and leaves D1 protein of PS II to enter in lipid bilayer where it accepts two protons from stroma and becomes plastohydroquinol (PQH2). Then it binds to large multi-protein complex cytochrome b6f, where each PQH2 molecule donates its two electrons but protons are released in lumen and the oxidized PQ returns back to PS II, while two additional protons are translocated through the complex from the stroma. The electrons from cytochrome b6f are then transferred to copper containing membrane protein called plastocyanin (PC) via an iron-sulphur protein. Plastocyanin carries electrons to the luminal side of PS I where they are transferred to P700. The chlorophyll molecule of P700 i.e. A0 accepts a pair of electrons and get reduced. The electrons are now transferred to ferredoxin protein on the surface of stromal side via co-factors like phylloquinone (A1) and three iron clusters (Fx, Fa and Fb respectively) and finally, it reduces NADP+ to NADPH with the involvement of membrane-associated flavoprotein ferredoxin-NADP reductase (FNR).  Movement of electrons is categorized into two ways. When the lateral movement of electrons occurs from the oxidation of water molecule till reduction of NADP, it is regarded as non-cyclic electron flow and if the electron returns back to cytochrome b6f complex from ferredoxin, it circulates via electron carriers and mediates transfer of protons from stroma to lumen for ATP synthesis then it reflects cyclic electron movement. As a result of the electron flow through the thylakoid membrane, it includes translocation of protons from the stroma to the lumen of thylakoid. Accumulation of protons generates a gradient that drives movement of protons through ATP synthase enzyme as suggested by P. Mitchell in his theory of chemiosmotic model for the synthesis of ATP in the chloroplast.  further, this can be elaborated through the structure of ATP synthase and binding change mechanism proposed by  Paul Boyer in 1972. It states that three catalytic sites on the enzyme bind ADP and phosphate in sequence followed by a conformational change so as to form tightly bound ATP. These sites then re-configures to release newly produced ATP. These changes are accomplished by rotational catalysis driven by rotating inner core of enzyme.

 
5.1.    Photosynthesis inhibiting herbicides 
The use of herbicides to kill unwanted plants is widespread in modern agriculture. Some herbicides, like dichlorophenyldimethylurea (DCMU, also known as diuron) and paraquat, block photosynthetic electron flow. DCMU blocks electron flow at the quinone acceptors of photosystem II, by competing for the binding site of plastoquinone that is normally occupied by PQ-b. Paraquat accepts electrons from the early acceptors of photosystem I and then reacts with oxygen to form superoxide, O²¯, a species that is very damaging to chloroplast components, especially lipids.
5.2.    Cyclic-photophosphorylation
For assimilation of CO2, the yield of ATP from non-cyclic photophosphorylation is not sufficient, thus cyclic photophosphorylation takes place for synthesizing the lacking ATP. Cyt-b6f complex accepts an electron from ferredoxin. When no NADP+ is available its means the full concentration of NADPH. So cyclic return electron is by so create proton gradient to derive ATP synthesis. The gradient of protons that is formed is utilized for the synthesis of ATP. Oxidation of water and reduction of NADP+ does not take place in cyclic photophosphorylation. 

Q-cycle:- Q-cycle is series of oxidation-reduction reaction takes place between coenzyme Q10 and Ubiquinol.
Z-point:- The photosynthesis can be broadly studied under light reaction and dark reaction. The light dependent reaction happens in the thylakoid of chloroplast and is light dependent. It involves the photosystem I and II. These photosystem absorb light.
Dark reaction is the light independent reaction. The ATP and NADPH formed in the light reaction are utilized in dark reaction to reduce the CO2 into glucose dark reaction occur in the stroma.
It was proposed by Mitchell. It is based on the transfer of electrons by plastoquinone in the electron transport chain. 
Thus a total of 6 protons are contributed for ATP synthesis: 4 from Q cycle, and 2 from hydrolysis of water. Similarly, a total of 4H+ per pair of electrons are contributed to cyclic photophosphorylation.
5.3.    ATP synthesis
ATP synthase was discovered as small membrane particles by electron microscopy. It was found to consist of two parts: 
1.    Soluble part F1 responsible for ATP synthesis.
2.    Membrane-bound Fo particle responsible for the flux of protons.
F1 particles were analyzed by X-ray crystallization Its major subunits α and β were observed to be arranged in an alternating manner forming a hexagonal array.
One α and β subunit comprise a binding site for adenine nucleotide and its β subunit is responsible for ATP synthesis. The γ subunit protrudes through the centre of F1 particle and is bent and loaded with ADP.
Various investigations on the working of ATP synthase shows that γ and ε subunits with 12 c-subunit of F0 forms a rotor and its γ subunit rotates. A stator in which this rotor rotates consist of (αβ)3, \delta, a, b, b’, changes the conformation of catalytic centers.
a-subunit of the stator possesses a channel through which protons move from outside of the membrane reaches a glutamate residue of c-subunit. The second channel of the stator is present on its another site. Protons bound to c-subunit are released inside. This Brownian movement of protons occurs in one direction because of the spatial separation of two channels and arginine residue of a stator i.e. positively charged repels the protons and prevent the rotor to move in a backward direction.


5.4.    Binding Change Hypothesis
It is proposed for the synthesis of ATP. There are three identical sites in the F1 protein. Loose (L) form, tight form (T) and an open form (O).
These conformational changes are brought about by the proton gradient that is created by the subunits of Fo particle and one full revolution of F1 causes its conformational changes to form a total of three ATP. 

5.5.    Inhibition of electron flow
Many herbicides have been developed that act by inhibiting the electron flow. Some examples are 
5.6.    Uncouplers
These are the compounds that uncouple electron from ATP synthesis. In the presence of an uncoupler, electron transport is carried out at a high rate without formation of ATP from ADP and phosphate. These are major of two types:


 


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