ATP – Concept, uses, production, importance and cycle of ATP

We explain what ATP is, what it is for and how this molecule is produced. Also, glycolysis, Krebs cycle, and oxidative phosphorylation.

What is ATP?

In biochemistry, the acronym ATP designates Adenosine Triphosphate or Adenosine Triphosphate, an organic molecule belonging to the group of nucleotides, essential for the energy metabolism of the cell. ATP is the main source of energy used in most cellular processes and functions, both in the human body and in the organism of other living beings.

The name of ATP comes from the molecular composition of this molecule, formed by a nitrogenous base (adenine) linked to the carbon atom of a pentose sugar molecule (also called ribose), and in turn with three phosphate ions linked in another carbon atom. All of it is summarized in the molecular formula of ATP: C₁₀H₁₆N₅OR₁₃P₃.

The ATP molecule was first discovered in 1929 in human muscle in the United States by Cyrus H. Fiske and Yellapragada SubbaRow, and independently in Germany by the biochemist Karl Lohmann.

While the ATP molecule was discovered in 1929, its functioning and importance in the different energy transfer processes of the cell were not recorded until 1941, thanks to the studies of the German-American biochemist Fritz Albert Lipmann (winner of the Nobel Prize in 1953, together with Krebs).

What is ATP for?

The main function of ATP is to serve as an energy supply in the biochemical reactions that take place inside the cell, which is why this molecule is also known as the organism’s “energy currency”.

ATP is a useful molecule to momentarily contain the chemical energy released during the metabolic processes of decomposition of food, and release it again when necessary to boost the various biological processes of the body, such as cell transport, promote reactions that consume energy or even to carry out mechanical actions of the body, such as walking.

How is ATP made?

In cells, ATP is synthesized through cellular respiration, a process that takes place in the cell’s mitochondria. During this phenomenon, the chemical energy stored in glucose is released, through an oxidation process that releases CO._two, H_twoO and energy in the form of ATP. Although glucose is the substrate par excellence of this reaction, it should be clarified that proteins and fats can also be oxidized to give ATP. Each of these nutrients from the individual‘s diet have different metabolic routes, but they converge on a common metabolite: acetyl-CoA, which starts the Krebs Cycle and allows the process of obtaining chemical energy to converge, since all cells consume their energy in the form of ATP.

The cellular respiration process can be divided into three phases or stages: glycolysis (a prior pathway that is only required when the cell uses glucose as fuel), the Krebs cycle, and the electron transport chain. During the first two stages, acetyl-CoA, CO_two and only a small amount of ATP, while during the third phase of respiration H is produced_twoO and most of the ATP through a set of proteins called the “ATP synthase complex”.

Glycolysis

As said, glycolysis is a pathway prior to cellular respiration, during which for each glucose (which has 6 carbons) two pyruvates are formed (a compound made up of 3 carbons).

Unlike the other two stages of cellular respiration, glycolysis takes place in the cytoplasm of the cell. The pyruvate resulting from this first pathway must enter the mitochondria to continue its transformation into Acetyl-CoA and thus be able to be used in the Krebs cycle.

Follow on: Glycolysis

Krebs cycle

The Krebs cycle (also citric acid cycle or tricarboxylic acid cycle) is a fundamental process that occurs in the matrix of cellular mitochondria, and which consists of a succession of chemical reactions whose objective is to release the chemical energy contained in the Acetyl-CoA obtained from the processing of the different food nutrients of the living being, as well as the obtaining of precursors of other amino acids necessary for reactions biochemicals of another nature.

This cycle is part of a much larger process that is oxidation of carbohydrates, lipids and proteins, being its intermediate stage: after the formation of Acetyl-CoA with the carbons of said organic compounds, and prior to oxidative phosphorylation where ATP is “assembled” in a reaction catalyzed by an enzyme called ATP synthetase or ATP synthase.

The Krebs Cycle operates thanks to several distinct enzymes that completely oxidize Acetyl-CoA and release two different molecules from each oxidized molecule: CO_two (carbon dioxide) and H_twoOr (water). In addition, during the Krebs cycle, a minimum amount of GTP (similar to ATP) and reducing power in the form of NADH and FADH is generated._two that will be used for the synthesis of ATP in the next stage of cellular respiration.

The cycle begins with the fusion of an acetyl-CoA molecule with an oxaloacetate molecule. This union gives rise to a six-carbon molecule: citrate. Thus, coenzyme A is released. In fact, it is reused many times. If there is too much ATP in the cell, this step is inhibited.

Subsequently, the citrate or citric acid undergoes a series of successive transformations that will successively originate isocitrate, ketoglutarate, succinyl-CoA, succinate, fumarate, malate and oxaloacetate again. Together with these products, a minimum amount of GTP is produced for each complete Krebs cycle, reducing power in the form of NADH and FADH_two and CO_two.

Electron transport chain and oxidative phosphorylation

The last stage of the nutrient utilization circuit uses oxygen and compounds produced during the Krebs cycle to produce ATP in a process called oxidative phosphorylation. During this process, which takes place in the inner mitochondrial membrane, NADH and FADH_two they donate electrons driving them to an energetically lower level. These electrons are finally accepted by oxygen (which when joining with protons gives rise to the formation of water molecules).

The coupling between the electronic chain and oxidative phosphorylation operates on the basis of two opposing reactions: one that releases energy and another that uses that released energy to produce ATP molecules, thanks to the intervention of ATP synthetase. As electrons “travel” down the chain in a series of redox reactions, the released energy is used to pump protons through the membrane. When these protons diffuse back through ATP synthetase, their energy is used to bind an additional phosphate group to an ADP (adenosine diphosphate) molecule, leading to the formation of ATP.

Importance of ATP

ATP is a fundamental molecule for the vital processes of living organisms, such as chemical energy transmitter for different reactions that occur in the cell, for example, the synthesis of complex macromolecules Y fundamental, such as those of DNA, RNA or for protein synthesis that occurs within the cell. Thus, ATP provides the energy necessary to allow most of the reactions that take place in the body.

The utility of ATP as an “energy donor” molecule is explained by the presence of phosphate bonds, rich in energy. These same bonds can release a large amount of energy by “breaking” when ATP is hydrolyzed to ADP, that is, when it loses a phosphate group due to the action of water. The hydrolysis reaction of ATP is as follows:

ATP is key for macromolecule transport to occur through the plasma membrane (exocytosis and cellular endocytosis) and also for synaptic communication between neurons, so its continuous synthesis is essential, from glucose obtained from food. Such is its importance for life, that the intake of some toxic elements that inhibit ATP processes, such as arsenic or cyanide, is lethal and causes the death of the organism in a fulminant way.