Remarkably, this research has been the subject of 3 Nobel Prizes in Chemistry, awarded to Mitchell in , then to Boyer and Walker in Cover image. Plant, Cell and Environment 28, ATP formation caused by acid-base transition of spinach chloroplasts. PMC PMID Establishing a transmembrane pH gradient 1. Protons and linear electron transfer Figure 1. Overview of proton and electron transfers across the thylacoid membrane.
From proton gradient to electrochemical gradient In , Peter Mitchell [4] proposed a mechanism for coupling electron transfer to ATP synthesis. Structure and mechanism of ATP synthase. The F0 part would function as a rotary proton motor, the rotor being constituted by the sub-units c, arranged in a ring. ATP synthase: a nano-machine The determination of the molecular structure of ATP-synthase allowed us to understand how it works, which had been imagined several years earlier by Paul Boyer.
Mitchell recognized that this represents a large energy difference, because the chemiosmotic potential is actually composed of two components. The proton gradient results in a state where the intermembrane space is positive and acidic relative to the matrix.
The shorthand for this situation is: positive out, negative in; acidic out, basic in. Quantitatively, the energy gradient across the membrane is the sum of the energies due to these two components of the gradient:.
The combination of the two components provides sufficient energy for ATP to be made by the multienzyme Complex V of the mitochondrion, more generally known as ATP synthase. See Figure 1.
The mechanism of ATP synthase is not what one would naively predict. This is borne out by two experimental observations: An artificial proton gradient can lead to ATP synthesis without electron transport, and molecules termed uncouplers can carry protons through the membrane, bypassing ATP synthase.
In this case, the energy of metabolism is released as heat. ATP cannot be stored and so its synthesis is closely linked to its consumption. These enzymes are found in the cristae and the inner membrane of mitochondria, the thylakoid membrane of chloroplasts, and the plasma membrane of bacteria [ 5 ]. Usually, there is a general understanding that ATP generation occurs in mitochondria.
However, in the case of bacteria and archaea that lack mitochondria, ATP synthase is found in their plasma membrane. Additionally, ATP synthases are licensed to inhabit the chloroplast of plant cells. Production and Utilization of Acetyl CoA: The complex macromolecules present in dietary food are processed through various metabolic pathways into Acetyl CoA. Acetyl CoA in the mitochondria is then oxidized to carbon dioxide and water through the Citric Acid Cycle and Oxidative Phosphorylation.
ATP synthesis is the most widespread chemical reaction inside the biological world. ATP synthase is the very last enzyme in oxidative phosphorylation pathway that makes use of electrochemical energy to power ATP synthesis [ 7 , 8 , 9 , 10 ]. The mitochondrial ATP synthase is a multi-subunit protein complex having an approximate molecular weight of kDa. This enzyme is the smallest known biological nanomotor and plays a crucial role in ATP generation.
In plants, energy acquired from photons is transferred through photosynthetic electron transport chain ETC , which induces an electrochemical gradient to build up across the membrane. Basically, protons are pumped across the inner mitochondrial membrane as electrons pass through the electron transfer chain.
This induces a proton gradient, with a decreased pH in the intermembrane space and an increased pH in the matrix of the mitochondria. The proton gradient and membrane potential are the major forces involved in ATP synthesis.
It is well established that the electrochemical potential of protons delivered by electron transfer chains across the mitochondrial, chloroplast or bacterial membrane provides the energy for ATP synthesis [ 14 ]. Cellular respiration in the mitochondria is a widely studied process that incorporates chemiosmosis for the production of ATP.
Mitochondria, the chief organelles producing ATP, are absent in prokaryotic organisms. In the absence of mitochondria, archaea and bacteria maneuver chemiosmosis to produce ATP through photophosphorylation.
The electrochemical energy built through the difference in proton concentration and separation of charge across inner mitochondrial membrane translates to the proton motive force PMF. This also satisfies a main criterion stated by Mitchell for the chemiosmotic coupling to occur: the inner mitochondrial membrane must be impermeable to protons. Thus, protons are compelled to re-enter matrix through F 0 while F 1 catalyzes the synthesis of ATP [ 16 ].
The Electron transport chain composed of four different multi-subunit complexes transfer electrons e- in a sequential manner ultimately reducing O 2 to H 2 O. Electron transfer is coupled to a vectorial proton translocation outdoor into the matrix via three of the four complexes I, III and IV.
Protons gather and create an electrochemical gradient throughout the inner mitochondrial membrane. This osmotic potential is used to power ATP synthesis when protons re-enter the mitochondrial matrix through ATP synthase [ 13 ]. The equation for reaction catalyzed is:. There are only slight variations in its structure in the chloroplast and in the mitochondria. The chloroplast ATPase has two isoforms and in the mitochondria it has additional subunits.
Besides these differences, ATPases are structurally and functionally similar. The F 0 part, bound to inner mitochondrial membrane is involved in proton translocation, whereas the F 1 part found in the mitochondrial matrix is the water soluble catalytic domain. F 1 is the first factor recognized and isolated from bovine heart mitochondria and is involved in oxidative phosphorylation.
F 0 was named so as it is a factor that conferred oligomycin sensitivity to soluble F 1 [ 18 ]. Schematic subunit composition of ATP synthase. The structure of enzyme ATP synthase mimics an assembly of two motors with a shared common rotor shaft and stabilized by a peripheral stator stalk. Bacterial F 0 has the simplest subunit structure consisting a 1 , b 2 and c subunits.
Other additional subunits such as subunit e, f, g, and A6L extending over the membrane cohort with F 0 [ 5 , 10 , 20 ]. Paul Boyer proposed a simple catalytic scheme, commonly known as the binding change mechanism, which predicted that F-ATPase implements a rotational mechanism in the catalysis of ATP [ 21 ].
The movement of subunits within the ATP synthase complex plays essential roles in both transport and catalytic mechanisms. Another subsequent change in conformation brings about the release of ATP. These conformational changes are accomplished by rotating the inner core of the enzyme. The core itself is powered by the proton motive force conferred by protons crossing the mitochondrial membrane. The binding-change mechanism as seen from the top of the F 1 complex.
There are three catalytic sites in three different conformations: loose, open, and tight. As a result, ATP is released from the enzyme. In step 2, substrate again binds to the open site, and another ATP is synthesized at the tight site [ 25 ]. Masamitsu et. Conformational transitions that are significant in rotational catalysis are directed by the passage of protons through the F 0 assembly of ATP synthase.
On the other hand, when the proton concentration is higher in the mitochondrial matrix, the F 1 motor reverses the F 0 motor bringing about the hydrolysis of ATP to power translocation of protons to the other side of membrane. A team of Japanese scientists have succeeded in attaching magnetic beads to the stalks of F 1 -ATPase isolated in vitro , which rotated in presence of a rotating magnetic field.
Additionally, ATP was hydrolyzed when the stalks were rotated in the counterclockwise direction or when they were not rotated at all [ 26 ]. Defects or mutations in this enzyme are known to cause many diseases in humans. The first defect in ATP synthase was reported by Houstek et.
It was postulated that mutations in some factors explicitly involved in the assembly of ATP synthase could have caused the defect [ 27 ]. Kucharczyk et. A mutation in one or many of the subunits in ATPase synthase can cause these diseases [ 28 ]. These diseases also result decrement in intermediary metabolism and functioning of the kidneys in removing acid from the body due to increased production of free oxygen radicals. Dysfunction of F 1 specific nuclear encoded assembly factors causes selective ATPase deficiency [ 31 ].
Similar inborn defects in the mitochondrial F-ATP synthase, termed ATP synthase deficiency, have been noted where newborns die within few months or a year. Current research on ATP synthase as a potential molecular target for the treatment for some human diseases have produced positive consequences. Recently, ATPase has emerged as appealing molecular target for the development of new treatment options for several diseases. ATP synthase is regarded as one of the oldest and most conserved enzymes in the molecular world and it has a complex structure with the possibility of inhibition by a number of inhibitors.
In addition, structure elucidation has opened new horizons for development of novel ATP synthase-directed agents with plausible therapeutic effects. More than natural and synthetic inhibitors have been classified to date, with reports of their known or proposed inhibitory sites and modes of action [ 30 ]. We look to explore a few important inhibitors of ATP synthase in this paper. A drug, diarylquinoline also known as TMC developed against tuberculosis is known to block the synthesis of ATP by targeting subunit c of ATP synthase of tuberculosis bacteria.
Another such diarylquinone, Bedaquiline, is used for the treatment of multidrug resistant tuberculosis.
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