ATP ( adenosine triphosphate) is the compound that acts as the energy currency of the cell. The body requires energy to carry out metabolism or any function. This topic deals with the intricate details of mitochondria that lead to the production of energy in the form of adenosine triphosphate. It is important to note that the energy produced is the conserved energy of the respiration process. Electron transfer releases and the proton motive force conserves more than enough energy about 200kJ per mole of electron pairs to drive the formation of one mole of ATP which requires about 50kJ. Thus it is very clear that ATP formation in the cell by oxidative phosphorylation is thermodynamically favourable.
The chemical mechanism that couples the proton flux with phosphorylation can be described by the model known as the chemiosmotic model.
It was proposed by Peter Mitchell, which predicts the role of electron transfer in ATP synthesis in mitochondria. According to this model, the role of electron transfer in mitochondrial ATP synthesis is to pump protons to create a PMF (proton motive force), which drives ATP production in mitochondria. ATP synthesis can occur in vivo if the proton motive force is created artificially without electron transfer, that is without the need for oxidation of the substrate. It is important to note that ATP synthesis is a coupled reaction that is the oxidation of the substrate and phosphorylation reaction are coupled together, neither reaction can occur without the other. Because the energy of substrate oxidation drives ATP synthesis, inhibitors of the electron transfer will block ATP synthesis, the examples of such inhibitors include cyanide, carbon monoxide and, antimycin A. To emphasize the crucial role of proton motive force in mitochondria and ATP synthesis the equation of ATP synthesis can be written as:
ADP + Pi + nHp -------> ATP + H2O + nHn
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The diagram illustrated here depicts the correlation between the electron transfer chain and oxidative phosphorylation (ATP production in mitochondria). I< II< III, IV are the complexes of the electron transport chain, these complexes are located in the inner mitochondrial membrane of an animal cell. Each complex undergoes a reduction and oxidation reaction to pass the electrons to the subsequent complex. Simultaneously for each electron transfer protons are transported from the mitochondrial matrix to the intermembrane space. This accumulation of protons in the intermembrane space leads to two things, the first generation of chemical gradient (alkaline in the matrix and acidic in intermembrane space) second electrical gradient due to charge separation. Both of these gradients together create proton motive force, which is responsible for the ATP production in mitochondria. Synthesis of ATP in mitochondria requires an oxidizable substrate, the electron transfer complex, protons, and ATP synthase and, ADP + Pi.
ATP Synthase in Mitochondria
ATP synthase is the enzyme that catalyzes ATP production in mitochondria, it is the enzyme that catalyzes the reaction by binding the ADP to it. The mechanism by which the enzyme works is known as rotational catalysis.
Mitochondrial ATP synthase is an F-type ATPase, it is also known as the complex V of oxidative phosphorylation. It has two functional domains known as F0 and F1.
The Fo component of the enzyme is an integral protein in the membrane. Fo serves as a proton pore, the proton enters the matrix from intermembrane space through the fo pore. Fo is designated its name because of its oligomycin sensitive nature.
F1 is the functional domain that is responsible for reversible binding of ADP + Pi and ATP. The reversible binding of ATP and ADP+ Pi takes place at the enzyme surface, it stabilizes the binding of ATP relative to ADP + Pi by binding tightly to ATP. This tight association of ATP with ATP synthase releases enough energy to counterbalance the cost of the ATP formation in mitochondria. The major energy barrier of ATP synthesis in mitochondria is the release of the ATP from ATP synthase in mitochondria, the proton motive force is the major cause that drives the release of ATP from the enzyme surface.
Reaction coordinate of the ATP synthase
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In a typical enzyme-catalyzed reaction reaching the transition state (ES) is the major energy barrier but in the ATP synthase catalyzed reaction the release of ATP from the enzyme surface is the major energy barrier to overcome. The free energy change for the formation of the ATP from ADP and Pi in an aqueous solution is large and positive, but on the enzyme surface, the very tight bounding of the ATP provides sufficient binding energy to bring the free energy of the enzyme-bound ATP close to that of ADP + Pi. This renders the reaction thermodynamically reversible. The free energy used to release ATP is provided by the proton motive force.
Structure of ATP Synthase in Mitochondria
ATP production in the mitochondria is carried out by the enzyme ATP synthase, as mentioned earlier this enzyme has two functional domains F0 and F1. F0 has three subunits namely a,b, and c. There is a single copy of a, there are two copies of b and ten to twelve copies of. The b subunit binds to the delta subunit of the F1 complex whereas all the copies c combined form a disc-like structure that is hydrophobic helical, there is a 2 transmembrane helix that spans the inner mitochondrial membrane.
F1 is the catalytic site where the ATP is bound. It contains alpha, beta, gamma, delta, and ε submit. There are three copies of α and, there are three copies of β subunit. Gamma γ acts as the shaft that is attached to the β subunit. β and α are arranged in an alternative knob-like structure. This leads to the formation of the β- conformation.
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ATP synthesis in mitochondria and chloroplasts is achieved by rotational catalysis of the ATP synthase. It is the key to the binding change mechanism for ATP formation in mitochondria. The mechanism can be explained as follows, all the F1 complex has three nonequivalent ATP binding sites, one for each pair of alpha-beta pair. At any point of these are in beta- ATP conformation, a second in beta-ADP, and a third is in beta empty. The PMF causes the rotation in the central shaft making it in contact with each alpha-beta pair. This induces a cooperative conformational change leading to the release of ATP from the enzyme surface. The rotation of the shaft occurs in 3 discrete steps of 120 degrees.
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Inhibitor of Mitochondrial ATP Synthase
ATP synthesis in chloroplast and mitochondria can be inhibited by the destruction of ATP synthase in mitochondria, there are some examples of such inhibitor of mitochondrial ATP synthase
Aurovertin- It inhibits the F1 domain of the ATP synthase
Oligomycin - It inhibits the Fo domain of the enzyme
Ventriuricidin- It inhibits the Fo domain of the chloroplast ATP synthase
DCCD- It blocks the proton flow through the form of mitochondrial and chloroplast ATP synthase.