Adenosine diphosphate (ADP) is a mononucleotide with the purine base adenine and plays a central role in all metabolic processes. Together with adenosine triphosphate (ATP) it is responsible for the energy expenditure in the organism. Most disorders in the function of ADP are mitochondrially conditioned.

What is adenosine diphosphate?

Adenosine diphosphate is a mononucleotide of the purine base adenine, the sugar ribose and a two-part phosphate chain. The two phosphate residues are linked by an anhydride bond. Upon absorption of another phosphate residue, adenosine triphosphate (ATP) is produced under energy consumption.

ATP is in turn the central energy storage and energy carrier in the organism. In energy-consuming processes, there is also the third phosphate residue with the release of energy, whereby the lower-energy ADP forms again. However, when ADP releases a phosphate residue, adenosimonophosphate (AMP) is produced. AMP is a mononucleotide of ribonucleic acid. ADP can also be formed from AMP by incorporation of a phosphate moiety. Also in this reaction energy is necessary. The more phosphate moieties the mononucleotide contains, the more energetic it is.

The negative charge of phosphate residues in densely packed space causes repulsive forces, which destabilizes especially the most phosphate-rich molecule (ATP). Through a magnesium ion, the molecule can be somewhat stabilized by a distribution of tension. However, even more effective stabilization is achieved by the reformation of ADP with the release of a phosphate moiety. The released energy is used for energetic processes in the body.

Function, effect & tasks

Although adenosine diphosphate is in the shadow of adenosine triphosphate (ATP), it has the same great importance for the organism. ATP is referred to as the molecule of life because it is the most indispensable energy carrier in all biological processes. However, the effect of ATP could not be explained without ADP.

All reactions are dependent on the high-energy binding of the third phosphate residue with the second phosphate residue in ATP. The release of the phosphate residue always occurs in energy consuming processes and the phosphorylation of other substrates. This creates ADP from ATP. When a substrate molecule energetically activated by phosphorylation transfers its phosphate residue back to ADP, the more energetic ATP is formed. Therefore, the system ATP / ADP should be considered in its entirety.

The effect of this system is to synthesize new organic substances, perform osmotic work, actively transport substances through biomembranes, and even induce mechanical movement in muscle contraction. Furthermore, ADP plays a role in many enzymatic processes. So it is part of coenzyme A. Coenzyme A supports as a coenzyme many enzymes in the energy metabolism. So it is involved in the activation of fatty acids.

It is composed of ADP, vitamin B5 and the amino acid cysteine. Coenzyme A directly influences fat metabolism and indirectly carbohydrate and protein metabolism. Another function ADP takes on the coagulation of blood. By attaching to certain platelet receptors, ADP stimulates enhanced platelet aggregation, ensuring a faster healing process for bleeding wounds.

Education, occurrence, properties & optimal values

Adenosine diphosphate is found in all organisms and all cells due to its high importance. Its main importance, together with ATP, is energy-transferring processes. ATP and thus also ADP occur in large quantities in the mitochondria of the eukaryotes, because the processes of the respiratory chain take place there. Of course, they are in the cytoplasm of the bacteria.

ADP is originally produced by the addition of a phosphate residue to adenosine monophosphate (AMP). AMP is a mononucleotide of the RNA. The starting point of the biosynthesis is ribose-5-phosphate, which accumulates molecular groups of specific amino acids through various intermediate steps until the mononucleotide inositol monophosphate (IMP) is formed. After further reactions, in addition to GMP, AMP is finally formed. AMP can also be recovered via the salvage pathway from nucleic acids.

Diseases & Disorders

Disturbances in the system ATP / ADP occur mainly in the so-called mitochondriopathies. As the name implies, these are diseases of the mitochondria. The mitochondria are cell organelles in which most of the energy-producing processes take place via the respiratory chain.

Here, the building blocks of carbohydrates, fats and proteins are broken down while generating energy. Central to these processes are ATP and ADP. It has been found that the concentration of ATP is lower in mitochondriopathies. The causes are manifold. For example, genetic causes may disrupt the production of ATP from ADP. As commonality of all possible genetically caused diseases, the special impairment of strongly energy-dependent organs was discovered. This often affects the heart, the muscular system, the kidneys or the nervous system. Most diseases are rapidly progressive, with the disease process being individually different.

The differences may be due to the different numbers of affected mitochondria. Mitochondrial disorders can also be acquired. Especially diseases such as diabetes mellitus, obesity, ALS, Alzheimer's disease, Parkinson's disease or cancer are also associated with disorders of mitochondrial function. The energy supply of the body is impaired, which in turn leads to further damage to strongly energy-dependent organs.

However, ADP also performs some important functions beyond energy-transferring processes. Its effect on blood coagulation can also lead to blood clots in unwanted place. To prevent the formation of thrombosis as well as strokes, heart attacks or embolisms, the blood can be diluted in endangered persons or ADP inhibited. ADP inhibitors include the drugs clopidogrel, ticlopidine or prasugrel.

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