ATP (ADENOSINE TRIPHOSPHATE): STRUCTURE, FUNCTIONS, HYDROLYSIS - BIOLOGY - 2025

The ATP (adenosine triphosphate) is an organic molecule with high energy bonds formed by a ring of adenine, ribose and three phosphate groups. It has a fundamental role in metabolism, since it transports the energy necessary to keep a series of cellular processes working efficiently.

It is widely known by the term "energy currency", since its formation and use occurs easily, allowing it to quickly "pay" for chemical reactions that require energy.

Source: By User: Mysid (Self-made in bkchem; edited in perl.), Via Wikimedia Commons

Although the molecule to the naked eye is small and simple, it stores a significant amount of energy in its bonds. Phosphate groups have negative charges, which are in constant repulsion, making it a labile bond that is easily broken.

The hydrolysis of ATP is the breakdown of the molecule by the presence of water. By this process the contained energy is released.

There are two main sources of ATP: substrate level phosphorylation and oxidative phosphorylation, the latter being the most important and most used by the cell.

Oxidative phosphorylation couples the oxidation of FADH ₂ and NADH + H ⁺ in the mitochondria, and phosphorylation at the substrate level occurs outside the electron transport chain, in pathways such as glycolysis and the tricarboxylic acid cycle.

This molecule is responsible for providing the energy necessary for most of the processes that occur inside the cell to take place, from protein synthesis to locomotion. In addition, it allows the traffic of molecules through membranes and acts in cell signaling.

Structure

ATP, as its name implies, is a nucleotide with three phosphates. Its particular structure, specifically the two pyrophosphate bonds, make it an energy-rich compound. It is composed of the following elements:

- A nitrogenous base, adenine. Nitrogen bases are cyclic compounds that contain one or more nitrogen in their structure. We also find them as components in nucleic acids, DNA, and RNA.

- Ribose is located in the center of the molecule. It is a pentose type sugar, as it has five carbon atoms. Its chemical formula is C ₅ H ₁₀ O ₅. Carbon 1 of ribose is attached to the adenine ring.

- Three phosphate radicals. The last two are the "high energy bonds" and are represented in the graphical structures by the tilt symbol: ~. The phosphate group is one of the most important in biological systems. The three groups are called alpha, beta and gamma, from closest to furthest.

This link is very labile, so it divides quickly, easily and spontaneously when the physiological conditions of the body warrant it. This occurs because the negative charges of the three phosphate groups constantly try to move away from each other.

Features

ATP plays an indispensable role in the energy metabolism of virtually all living organisms. For this reason, it is often referred to as an energy currency, as it can be continuously spent and replenished in just a few minutes.

Directly or indirectly, ATP provides energy for hundreds of processes, in addition to acting as a phosphate donor.

In general, ATP acts as a signaling molecule in the processes that occur inside the cell, it is necessary to synthesize the components of DNA and RNA and for the synthesis of other biomolecules, it participates in trafficking through membranes, among others.

The uses of ATP can be divided into main categories: transport of molecules through biological membranes, the synthesis of various compounds and finally, mechanical work.

The functions of ATP are very broad. Furthermore, it is involved in so many reactions that it would be impossible to name them all. Therefore, we will discuss three specific examples to exemplify each of the three mentioned uses.

Energy supply for sodium and potassium transport across the membrane

The cell is a highly dynamic environment that requires maintaining specific concentrations. Most molecules do not enter the cell randomly or by chance. For a molecule or substance to enter, it must do so by its specific transporter.

Transporters are membrane-spanning proteins that function as cell “gatekeepers”, controlling the flow of materials. Therefore, the membrane is semi-permeable: it allows certain compounds to enter and others do not.

One of the best known transports is the sodium-potassium pump. This mechanism is classified as an active transport, since the movement of ions occurs against their concentrations and the only way to carry out this movement is by introducing energy into the system, in the form of ATP.

It is estimated that one third of the ATP formed in the cell is used to keep the pump active. Sodium ions are constantly pumped out of the cell, while potassium ions are pumped in the reverse direction.

Logically, the use of ATP is not restricted to the transport of sodium and potassium. There are other ions, such as calcium, magnesium, among others, that need this energy currency to enter.

Participation in protein synthesis

Protein molecules are made up of amino acids, linked together by peptide bonds. To form them requires the breaking of four high-energy bonds. In other words, a considerable number of ATP molecules must be hydrolyzed for the formation of a protein of average length.

Protein synthesis occurs in structures called ribosomes. These are able to interpret the code that the messenger RNA has and translate it into an amino acid sequence, an ATP-dependent process.

In the most active cells, protein synthesis can direct up to 75% of the ATP synthesized in this important work.

On the other hand, the cell not only synthesizes proteins, it also needs lipids, cholesterol, and other essential substances and to do so it requires the energy contained in the ATP bonds.

Provide energy for locomotion

Mechanical work is one of the most important functions of ATP. For example, for our body to be able to contract muscle fibers, the availability of large amounts of energy is necessary.

In muscle, chemical energy can be transformed into mechanical energy thanks to the reorganization of the proteins with the ability to contract that form it. The length of these structures is modified, shortened, which creates a tension that translates into the generation of movement.

In other organisms, the movement of cells also occurs thanks to the presence of ATP. For example, the movement of cilia and flagella that allows the displacement of certain unicellular organisms occurs through the use of ATP.

Another particular movement is the amoebic one that involves the protrusion of a pseudopod at the cell ends. Several cell types use this locomotion mechanism, including leukocytes and fibroblasts.

In the case of germ cells, locomotion is essential for the effective development of the embryo. Embryonic cells travel important distances from their place of origin to the region where they must originate specific structures.

Hydrolysis

The hydrolysis of ATP is a reaction that involves the breakdown of the molecule by the presence of water. The reaction is represented as follows:

ATP + Water ⇋ ADP + P _i + energy. Where, the term P _i refers to the inorganic phosphate group and ADP is adenosine diphosphate. Note that the reaction is reversible.

The hydrolysis of ATP is a phenomenon that involves the release of an immense amount of energy. The breaking of any of the pyrophosphate bonds results in the release of 7 kcal per mole - specifically 7.3 from ATP to ADP and 8.2 for the production of adenosine monophosphate (AMP) from ATP. This equates to 12,000 calories per mole of ATP.

Why does this release of energy occur?

Because the hydrolysis products are much more stable than the initial compound, that is, ATP.

It should be mentioned that only the hydrolysis that occurs on the pyrophosphate bonds to give rise to the formation of ADP or AMP leads to the generation of energy in significant quantities.

The hydrolysis of the other bonds in the molecule does not provide as much energy, with the exception of the hydrolysis of inorganic pyrophosphate, which has a large amount of energy.

The release of energy from these reactions is used to carry out metabolic reactions inside the cell, since many of these processes need energy to function, both in the initial steps of the degradation routes and in the biosynthesis of compounds..

For example, in glucose metabolism, the initial steps involve phosphorylation of the molecule. In the following steps, new ATP is generated, to obtain a positive net profit.

From the energy point of view, there are other molecules whose release energy is greater than that of ATP, including 1,3-bisphosphoglycerate, carbamylphosphate, creatinine phosphate and phosphoenolpyruvate.

Obtaining ATP

ATP can be obtained by two routes: oxidative phosphorylation and phosphorylation at the substrate level. The former requires oxygen while the latter does not. About 95% of the ATP formed occurs in the mitochondria.

Oxidative phosphorylation

Oxidative phosphorylation involves a two-phase nutrient oxidation process: obtaining reduced coenzymes NADH and FADH ₂ derived from vitamins.

The reduction of these molecules requires the use of hydrogens from nutrients. In fats, the production of coenzymes is remarkable, thanks to the enormous amount of hydrogens that they have in their structure, compared to peptides or carbohydrates.

Although there are several pathways of coenzyme production, the most important route is the Krebs cycle. Subsequently, the reduced coenzymes are concentrated in the respiratory chains located in the mitochondria, which transfers the electrons to oxygen.

The electron transport chain is made up of a series of membrane-coupled proteins that pump protons (H +) to the outside (see image). These protons enter and cross the membrane again by means of another protein, ATP synthase, responsible for the synthesis of ATP.

In other words, we have to reduce coenzymes, more ADP and oxygen generate water and ATP.

Source: By Bustamante Yess, from Wikimedia Commons

Substrate level phosphorylation

Phosphorylation at the substrate level is not as important as the mechanism described above and, as it does not require oxygen molecules, it is often associated with fermentation. This way, although it is very fast, extracts little energy, if we compare it with the oxidation process it would be about fifteen times less.

In our body, fermentation processes occur at the muscle level. This tissue can function without oxygen, so it is possible that a glucose molecule is degraded to lactic acid (when we are doing some exhaustive sports activity, for example).

In fermentations, the final product still has energy potential that can be extracted. In the case of fermentation in muscle, the carbons in lactic acid are at the same level of reduction as those of the initial molecule: glucose.

Thus, energy production occurs by the formation of molecules that have high-energy bonds, including 1,3-bisphosphoglyrate and phosphoenolpyruvate.

In glycolysis, for example, the hydrolysis of these compounds is linked to the production of ATP molecules, hence the term “at the substrate level”.

ATP cycle

ATP is never stored. It is in a continuous cycle of use and synthesis. This creates a balance between the ATP formed and its hydrolyzed product, ADP.

Source: By Muessig, from Wikimedia Commons

Other energy molecules

ATP is not the only molecule composed of nucleoside bisphosphate that exists in cellular metabolism. There are a number of molecules with structures similar to ATP that have comparable energy behavior, although they are not as popular as ATP.

The most prominent example is GTP, guanosine triphosphate, which is used in the well-known Krebs cycle and in the gluconeogenic pathway. Others less used are CTP, TTP and UTP.

References

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