Adenosine Diphosphate: Function & Diseases

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

What is adenosine diphosphate?

Adenosine diphosphate, as a mononucleotide, consists of the purine base adenine, the sugar ribose, and a two-part phosphate chain. The two phosphate residues are linked by an anhydride bond. When another phosphate residue is taken up, adenosine triphosphate (ATP) is formed under energy consumption. ATP is in turn the central energy store and energy transmitter in the organism. In energy-consuming processes, it also releases the third phosphate residue under energy dissipation, again forming the lower-energy ADP. However, when ADP releases a phosphate residue, adenosimonophosphate (AMP) is formed. AMP is a mononucleotide of ribonucleic acid. However, ADP can also form from AMP by taking up a phosphate residue. Energy is also required for this reaction. The more phosphate residues the mononucleotide contains, the more energy it has. The negative charge of the phosphate residues in a densely packed space causes repulsive forces, which destabilize the most phosphate-rich molecule (ATP) in particular. A magnesium ion can stabilize the molecule somewhat by distributing the voltage. However, an even more effective stabilization is achieved by the reversion of ADP under the release of a phosphate residue. The energy released is thereby used for energetic processes in the body.

Function, effects, and roles

Although adenosine diphosphate is overshadowed by adenosine triphosphate (ATP), it nevertheless possesses the same great importance for the organism. ATP is called the molecule of life because it is the most indispensable energy transmitter in all biological processes. However, the action of ATP could not be explained without ADP. All reactions depend on the energetic binding of the third phosphate residue with the second phosphate residue in ATP. The release of the phosphate residue always occurs during energy-consuming processes and phosphorylation of other substrates. In this process, ADP is formed from ATP. When a substrate molecule that has been energetically activated by phosphorylation transfers its phosphate residue back to ADP, the more energy-rich ATP is formed. Therefore, the ATP/ADP system should actually be considered in its entirety. Through the action of this system, new organic substances are synthesized, osmotic work is performed, substances are actively transported across biomembranes, and even mechanical movement is induced during muscle contraction. Furthermore, ADP plays its own role in many enzymatic processes. For example, it is a component of coenzyme A. As a coenzyme, coenzyme A supports many enzymes in energy metabolism. For example, 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. ADP also plays a role in the coagulation of blood. By attaching to certain receptors on platelets, ADP stimulates increased platelet aggregation and thus ensures a faster healing process for bleeding wounds.

Formation, occurrence, properties, and optimal values

Adenosine diphosphate is found in all organisms and all cells due to its high importance. Its main importance is together with ATP for energy-transferring processes. ATP and thus also ADP are found in large quantities in the mitochondria of eukaryotes because the processes of the respiratory chain take place there. In bacteria, of course, they are found in the cytoplasm. ADP is originally produced by the addition of a phosphate residue to adenosine monophosphate (AMP). AMP is a mononucleotide of RNA. The starting point of biosynthesis is ribose-5-phosphate, which attaches molecular groups of certain amino acids via various intermediate steps until the mononucleotide inositol monophosphate (IMP) is formed. Via further reactions, AMP is finally formed in addition to GMP. AMP can also be recovered from nucleic acids via the salvage pathway.

Diseases and disorders

Disorders in the ATP/ADP system occur mainly in the so-called mitochondriopathies. As the name suggests, these are diseases of the mitochondria.The mitochondria are cell organelles in which most of the energy-generating processes take place via the respiratory chain. Here, the building blocks of carbohydrates, fats and proteins are broken down to produce energy. ATP and ADP are of central importance in these processes. It has been found that in mitochondriopathies the concentration of ATP is lower. The causes for this are manifold. For example, the formation of ATP from ADP can be disturbed by genetic causes. As a common feature of all possible genetic diseases, the particular impairment of strongly energy-dependent organs was discovered. Thus, the heart, the muscular system, the kidneys or the nervous system are frequently affected. Most diseases are rapidly progressive, and the disease process varies from individual to individual. It is possible that the differences come from the varying numbers of mitochondria affected. Mitochondriopathies may also be acquired. Especially such diseases as diabetes mellitus, obesity, ALS, Alzheimer’s disease, Parkinson’s disease or cancer are also related to disorders of mitochondrial function. The energy supply of the body is impaired, which in turn leads to further damage of highly energy-dependent organs. However, ADP also exerts some important functions beyond energy-transferring processes. For example, its effect on blood clotting can also lead to blood clots in undesirable locations. To prevent thrombosis formation as well as strokes, heart attacks or embolisms, blood can be thinned or ADP inhibited in vulnerable individuals. ADP inhibitors include the drugs clopidogrel, ticlopidine, or prasugrel.