ATP-Synthase

The enzyme ATP synthase or FoF1 ATPase is a transmembrane protein. ATP synthase occurs depending on the ratio of the substrates and products as either ATP-consuming proton pump or as a proton- driven ATP synthase. Under physiological conditions, the main function of the enzyme, however, is to catalyze the synthesis of ATP. ATP is an energy- rich compound, whose formation requires the input of energy:

In order to apply this energy, the ATP synthase couples the ATP formation with the energetically favorable transport of protons (or other ions ) along a proton Fælles across a membrane. ATP synthase is thus a power converter that converts one form of energy into another. The enzyme plays in the metabolism of almost all known organisms play a central role, because ATP is continuously needed as an energy carrier.

The ATP synthase consists of 8 to 20 different subunits together. They are grouped into two complexes:

The water-soluble complex F1 catalyzes the formation of ATP.
The water-insoluble, built in a membrane complex Fo transports protons.

The enzyme is therefore also referred to by both of its subunits as FoF1 -ATPase.

4.1 Structure of Fo
4.2 Structure of F1
4.3 Overview of human ATPase

5.1 Hypothetical mechanism of the rotary motion
5.2 ATP synthesis in F1 ( αβ ) 3 complex

Importance and occurrence

Virtually all processes in organisms require adenosine triphosphate ( ATP). It provides all kinds of metabolic processes energy. The majority of the ATP is consumed is regenerated by the ATP synthase in animals, plants, and most of the bacteria. The daily turnover of ATP in humans is partially well over 80 kilograms.

The ATP synthase also known as F-type ATPase occurs

In the plasma membrane of prokaryotes ( "Bacteria " )
In the inner mitochondrial membrane of eukaryotes ( cells of plants and animals ).
In the thylakoid membrane of the chloroplasts of plant cells.

Primal organisms from the kingdom of archaea have an A -type ATPase, which differs in structure somewhat "normal" by the ATP synthase. The reason could be related to the different structure of the cell membrane and cell wall of these organisms.

ATP synthases use the energy of an ion gradient, which exists between the two sides of the membrane. Usually it involves a proton gradient. In alkaliphilic bacteria and ATP synthase, which is used instead of a sodium proton gradient for ATP synthesis exist. ( EC 3.6.3.15 )

History

The elucidation of the function and mechanism of ATP synthase was provided primarily by researchers who dealt with mitochondria. Although this enzyme plays an important role in photosynthesis of the plants and under aerobic bacteria fell ATP synthase as a component of cell respiration and the respiratory chain of the people in the focus of biochemistry.

The complex V

Beginning of the 60s of the 20th century could look back on tremendous progress biochemistry. The energy metabolism was almost cleared up in a few decades. The citric acid cycle has been known as the physiological role of NADH as a hydrogen carrier and the role of ATP as an energy supplier.

It was known that the NADH hydrogen is oxidized with oxygen to form water. And it was clear that in this process the bulk needed in the cell ATP is produced. In addition, it was known that the oxidation of NADH proceeds in steps. Had been found in the mitochondria membranes enzymes and coenzymes, which form an electron transport chain from the NADH to oxygen.

The elucidation of these so-called respiratory chain turned out, however, becoming increasingly difficult. The enzymes of the respiratory chain were difficult to isolate and investigate because they are membrane proteins. In addition, they form large enzyme complexes. Four of these complexes were (and are ) still further elucidated. Complex V, the ATP forms remained after isolation of its water-soluble component in 1961, in the dark. The elucidation of the respiratory chain, and thus the biochemistry at all, so had a " blemish ".

From the research on the sugar degradation ( glycolysis ) was known already at least a mechanism ( substrate chain ), which is produced from ADP and phosphate ATP. It concluded that it would behave similarly somehow in the respiratory chain and believed to be tight before the final enlightenment. There was "only"

The exact connection of the complexes I-IV for ATP synthesis, ie the coupling of O2 consumption and ATP formation
The explanation of why found complex V ATP indeed split, but none produced as soon as they had isolated him
A " high-energy intermediate" of the respiratory chain as a substrate of the ATP synthase

It was missing "only" the entire relationship between NADH oxidation and ATP production. But you already had a final name for the metabolism pathway: oxidative phosphorylation.

In this situation, Peter D. Mitchell introduced in 1960 before a hypothesis that has long been attacked violently. Because Mitchell postulated one for those Biochemistry " unimaginable " mechanism.

The Mitchell hypothesis

Mitchell himself had indeed since the beginning of the forties mainly studied the respiratory chain, but also knew the research results of the transport processes in membranes. His group is focused in the study of the respiratory chain to the electron carrier ubiquinone (Q- cycle). The results were not compatible with the idea of " energy-rich intermediate product " of the respiratory chain as the motor of ATP synthase. Mitchell instead postulated that the enzyme gets its power from a pH gradient.

The rejection of the professional world was overwhelming. Even as Mitchell in 1978 for his chemiosmotic theory received the Nobel Prize (Chemistry), spoke renowned biochemist nor pejorative of " Mitchell " hypothesis.

The elucidation of the mechanism

Even scientists who officially faced the " Mitchell hypothesis" very skeptical covered in their research laboratory in the chemiosmotic theory into serious consideration. While the accumulated evidence of Mitchell's theory, it was also in the experimental treatment of the ATP synthase on. In membrane vesicles could study them "in action".

Paul D. Boyer, originally a " Mitchell " skeptics, explained to the molecular mechanism of ATP synthase. John E. Walker and co-workers were able to crystallize the ATP synthase and educate their spatial structure. Both were awarded the Nobel Prize in 1997 for chemistry. They shared the prize with Jens C. Skou, who had already discovered in 1957, the first proton pump and thus laid the foundation for the " Mitchell " hypothesis.

Position to other enzymes

A plurality of enzymes consumes ATP, which is supplied by the ATP synthase, as cosubstrate. Among the enzymes, the ATP synthase thus occupies a key position.

Of the remaining ATPases, the ATP synthase differs in several respects:

It is the major source of ATP. With its function as ATP - producer, the ATP synthase differs from the others ( consistently ) ATP - consuming ATPases such as a power generator from an electric motor.
It consists of the two subunits F1 and Fo. Their function requires both units in a specific arrangement. It is therefore often referred to as the ATP synthase F-type ATPase. In contrast, the ATP -splitting ATPases V- type ATPases.
Like all ATPases, the ATP synthase in principle also act as a proton pump, thereby consuming ATP. The fact that these " back " reaction in vivo in mitochondria plays an important role in ATP synthase, but is questionable. The ATP synthase has a different " ratio" as the proton pump ATPases. The latter pump ATP per consumed approximately two protons to the outside. In the ATP synthase, the energy of a single ATP molecule, however, would spread over three to four protons. As proton pump ATP synthase could therefore not establish such a large pH gradient.

The ATP synthase has according to the IUBMB Enzyme Nomenclature EC number 3.6.3.14 and the belongs to the category of hydrolases.

Blueprint of the ATP synthase

According to their position relative to the membrane (Fig. 4, bottom, bright area ) distinction is a membrane-bound Fo and a water-soluble F1 subunit.

Mechanically allows the enzyme in a rotating rotor (Fig. 4, reddish to purple) and a stator (Fig. 4, green) members.

Because of the fluid nature of the membrane of the stator performs a rotary movement in opposite directions from the rotor.

Subsequently, the blueprint of the ATP synthase of the bacterium E. coli is described because it is intensively studied and built relatively simple.

Structure of Fo

Fo consists of hydrophobic peptides ( water-insoluble protein chains ) and is located in the membrane. This part of the enzyme is composed of three different subunits:

Foa is used for power transmission to Fob and is also part of the mechanism that converts the proton motion into a rotary motion.
Fob combines membrane and the F1 component. Fob is used for power transmission and consists of two peptide chains such as Foa.
Foc is E. coli twelve coils arranged in a ring. Inside the ring are believed to be the phospholipids from which the cell membrane is composed. It forms an insulating layer, so that there is not a proton flux possible.

Each Foc peptide chain has an active center. When it is removed from this center H , then the structure of the peptide chain is changed to a mechanically unstressed condition. Then takes the active site again an H on, the peptide chain turns back. This rotation exerts a force on Foa.

Structure of F1

F1 is the water soluble component of the ATP synthase. It is located on the inside of the membrane. Five different peptides ( α and ε ) are the subunits of this component:

The three F1α and F1β peptides form the F1 ( αβ ) 3Komplex. Its still occasionally used name as globular ( αβ ) 3- ATPase indicates that here the conversion of ADP to ATP takes place. Between the peptides exist three pores, can enter and exit through the substrate and product. In three catalytic sites, the ATP is produced in the interior of F1 ( αβ ) 3Komplexes.
F1γ is the rotary axis of the system. It transmits the rotational movement of the membrane placed in the ring of the three catalytic ( αβ ) 3 centers.
F1δ and F1ε are not shown in the picture on the right. The former peptide is a link between fob and the ( αβ ) 3 complex. F1ε probably connects the Foc ring with the F1γ axis.

Overview of human ATPase

Human ATPase consists of at least 16 subunits, of which two are encoded by mitochondrial genes. Both of F1γ as of F0C are several gene loci, the different precursors, but encode identical subunits.

Movement and reaction processes

The function of the " molecular machine " is described here by the example of the E. coli ATP synthase.

Hypothetical mechanism for the rotary movement

The location of the authorities responsible for the movement of the center is shown in figure 5. The mechanism of the rotary motion is shown in Figure 6. ( 10/ 2005)

Each Foc - peptide from the rotor ring is at position 61 an aspartic acid ( ASP ) residue. Are in the idle state of the engine, with one exception, the ASP carboxyl groups protonated ( -COOH peptide ).

In the rotor - peptide 1, which is adjacent to the stator Fob, the ASP group is adjacent to an arginine - group of the stator. The positive charge can be obtained by ionic interactions stabilize a negative charge, so that the Asp residue present in deprotonated form ( -COO- peptide ).

The negatively charged peptide rotor 1 differs from that in its spatial structure of the other peptides Foc. In a hairpin loop, it has built up like a coil spring, a strong mechanical stress.

( Which way does the proton is not yet clearly understood. Foc The peptides might create the space that it can penetrate to the decisive position 61. According to the " Single Channel Theory" the stator peptide creates access. )

Chemical energy is transformed here into kinetic energy. The rotation of the peptide 1 is the driving of the motor.

The motion of the rotor - peptide, however, has simultaneously taken the next Foc - 2 peptide chain in the " spell" of the positively charged arginine group.

1 transmitted through the rotation of the rotor, kinetic energy, 2 a proton from outer pushed through to the inside.

ATP synthesis in F1 ( αβ ) 3 complex

The actual conversion of ADP to ATP is biochemically, as compared to proton -driven rotation of the Foc ring nothing unusual. It will take place structural changes of the peptide chains involved ( here ADP and phosphate) bring the substrates for the reaction.

In F1 ( αβ ) 3 complex, there are three catalytic sites. Successively take three forms:

The energy-consuming step is to form the open mold, so to remove the reaction product of ATP from the enzyme. Exactly what the rotation.

The rotation of the rotor by 360 ° in three steps provides three molecules of ATP. Since the "machine" rotates 30 ° at each proton passage as provided in this model for each four ATP H consumed.

ATP synthase in various organisms

The ATP synthase of the bacterium E. coli consists of eight different proteins. The ATP synthase in the mitochondria of the yeast Saccharomyces cerevisiae is in principle the same design from 20 different proteins.

A full range ( 9-14) is also in the number of Foc rotor peptides. From these result in various proportions between the spent H and ATP produced.