A nucleoside always consists of a nucleic base, which is linked via an N-glycosidic bond with the monosaccharide ribose or deoxyribose. All five nucleobases - the building blocks of DNA and RNA double and single helices - can be enzymatically converted into nucleosides. Some glycosides are physiologically important, such as adenosine, which is the building block of ADP and ATP in the energy metabolism of cells.

What are nucleosides?

The double helices of DNA and the single helices of RNA are formed from sequences of only five different nucleobases in the form of nucleotides.

All five nucleobases, of which adenine and guanine are based on the five- and six-membered ring of purine, and cytosine, thymine, and uracil on the pyrimidine aromatic six-membered ring, can combine with the monosaccharide ribose or deoxyribose N-glycosidic. The hydroxyl group (-OH) on C-atom 1 of the pentose reacts with the formation and removal of an H2O molecule with the amino group (-NH2) of the nucleobase. When a ribose or deoxyribose residue is attached, adenine forms adenosine or deoxyadenosine.

Similarly, the purine base guanine is converted into guanosine or deoxyguanosine. The three purine bases thymine, cytosine and uracil are transformed by the addition of the ribose residue in thymidine, cytidine and uridine or are each given the prefix "deoxy-" if the attached sugar residue is deoxyribose. In addition, there are a large number of modified nucleosides, some of which play a role in the transfer DNA (tDNA) and in the ribosomal RNA (rRNA).

Artificially produced, modified, nucleosides, so-called Nukleosidanaloga act z. T. as antivirals and are used specifically for the control of retroviruses. Some nucleoside analogues have a cytostatic effect so that they are used to combat certain cancer cells.

Function, effect & tasks

One of the most important functions of the five basic nucleosides is to transform into nucleotides by attaching a phosphate group to the pentose and to form as nucleotides the building blocks of DNA and RNA.

Some nucleosides, in a modified form, also perform tasks in the catalysis of certain metabolic processes. For example, the so-called "active methionine" (S-adenosyl-methionine) serves as a donor of methyl groups. In some cases, nucleosides in their nucleotide form also act as building blocks of group-transferring coenzymes. Examples include riboflavin (vitamin B2), which serves as a precursor for many coenzymes and thus plays a central role in many metabolic processes.

Adenosine plays a very important role in the energy supply of the cells as adenine diphosphate (ADP) and adenosine triphosphate (ATP). ATP can be described as a universal energy source and also serves as a phosphate donor in many metabolic processes involving phosphorylation. Guanosine triphosphate (GTP) is the energy carrier in the so-called citrate cycle in the mitochondria. Nucleotides are also components of coenzyme A and vitamin B12.

The nucleosides uridine and cytidine are used in combination as drugs for the treatment of neuritis and muscle diseases. For example, the remedy for nerve root inflammation in the spine and lumbago is used. Modified nucleosides, so-called Nukleosidanaloga, z. T. virostatic effects against retroviruses. You will find in medicines use against z. B. against the herpes simplex virus and against HI viruses. Other nucleoside analogues with cytostatic effects play a role in the fight against cancer.

Education, occurrence, properties & optimal values

Nucleosides are composed exclusively of carbon, hydrogen, oxygen and nitrogen. All substances are abundant on Earth practically everywhere. Trace elements and rare minerals are not needed for the construction of nucleosides. However, the body does not synthesize nucleosides from the ground up, because the synthesis is complex and energy-consuming.

The human body therefore goes the opposite way, he gains nucleosides mainly from degradation processes in the intermediate purine and pyrimidine metabolism (salvage pathway). Nucleosides participate in pure form or in the phosphorylated form as nucleotides in a variety of enzymatic-catalytic metabolic processes. Particularly worth mentioning is the function of adenosine in the form of ATP and ADP in the so-called respiratory chain. The nucleotide guanine triphosphate plays a crucial role in the so-called citrate cycle.

In cycles, processes take place within the mitochondria of the cells. Since nucleosides are almost always present in bound form or as functional carriers in virtually all body cells in large quantities, there is no general limit or benchmark for optimal concentration. Determining the concentration of certain nucleosides or nucleotides in the blood plasma can be helpful for diagnoses and differential diagnoses.

Diseases & Disorders

Nucleosides are an active part of many metabolic processes and their functions are seldom isolated. Disorders usually involve complex enzymatic-catalytic processes that are interrupted or inhibited at specific sites and lead to corresponding symptoms.

Diseases that cause metabolic abnormalities of the nucleosides usually also affect purine or pyrimidine metabolism because the five basic nucleosides carry either a purine or a pyrimidine backbone. A known disorder in purine metabolism is caused by the well-known Lesch-Nyhan syndrome, a hereditary disease causing hypoxanthine-guanine phosphoribosyltransferase deficiency (HGPRT). The enzyme deficiency prevents the recycling of certain nucleobases, resulting in a cumulative accumulation of hypoxanthine and guanine.

This in turn triggers hyperuricemia, an elevated uric acid level, which leads to gout. The increased uric acid level leads to deposits on joints and tendon sheaths, which can trigger painful symptoms. A very rare hereditary disease manifests itself in adenylosuccinate lyase deficiency, which leads to problems in purine metabolism. The disease leads to muscle twitching and delayed, child development with a serious course.

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