Thymine

Thymine is one of the four nucleobases from which DNA strands, the seat of genetic information, are built. The complementary base in the double helix is ​​always adenine.

Chemically, it is a heterocyclic aromatic compound with a pyrimidine skeleton. In addition to being a nucleobase in DNA to code the amino acid sequence for protein synthesis, thymine plays a role in the body's metabolism as a component of certain bioactive nucleotides.

What is thymine?

The basic structure of thymine is formed by a heterocyclic aromatic six-membered ring, the pyrimidine basic structure. Thymine is one of a total of 4 nucleobases from which the DNA strands are composed. Strictly speaking, it is the nucleotide of thymine.

First a deoxyribose molecule is attached so that the nucleoside deoxythymidine is formed from the nucleic base. The nucleoside is then converted into the nucleotide deoxythymidine monophosphate (dTMP), deoxythymidine diphosphate (dTDP) or deoxythymidine triphosphate (dTTP) by adding one to three phosphate groups. Thymine does not normally appear in RNA because thymine is replaced there by the nucleobase uracil. Uracil is the complementary base to adenine in RNA. However, thymine occurs as a special glycoside (ribothymidine) with an attached ribose molecule in the transfer RNA (tRNA).

The chemical formula C5H5N2O2 shows that thymine is composed exclusively of carbon, hydrogen, nitrogen and oxygen, i.e. substances that are ubiquitous. No rare minerals or trace elements are involved in the composition of thymine. Thymine is preferably obtained by the body from metabolizing proteins that contain thymine or thymidine. Thymine can be completely broken down into carbon dioxide and water by the body's metabolism.

Function, effect & tasks

The main task of thymine is to be present in one of the strands of the double helix of DNA at the designated points and to establish a connection with the complementary nucleobase adenine via a double hydrogen bond.

For the fulfillment of its main task, thymine does not intervene directly in the metabolism, but rather, together with the other three nucleobases, only determines which amino acids are assembled into proteins in which order through its position on the corresponding section of the double helix strand. After making a copy of the corresponding section of a DNA base strand, the so-called messenger RNA (mRNA), this is transferred from the nucleus of the cell into the cytoplasm.

In the cytoplasm, on the ribosomes, the base sequences are translated into the type and order of the amino acids, which are assembled into the intended protein via peptide bonds. The exact function and tasks of thymine and deoxythymidine in the metabolism are not known. Animal experiments have shown that thymidine administration improves the blood count in pernicious anemia, anemia caused by a B12 deficiency. The vitamin B12 deficiency can probably be linked to a disturbance in the synthesis of nucleosides.

Education, occurrence, properties & optimal values

The body can synthesize thymine itself if necessary. However, since the synthesis is time-consuming and energy-intensive, the vast majority of the nucleic base is obtained through a kind of reprocessing of used thymine or thymidine compounds or from the breakdown of proteins that contain thymine or thymidine. This route of synthesis is known as the Salvage Pathway.

It is always used when the body has to use less energy for the breakdown of higher molecules than for biosynthesis. Thymine forms shiny needle-shaped or prism-shaped crystals that taste bitter and can be dissolved in hot water, but hardly in alcohol or ether. Since the basic structure of thymine consists of a six-membered ring, thymine can occur in six different tautomers, each with the same chemical molecular formula, but with a different arrangement of the double bonds and / or the associated groups or molecules.

Since the nucleobase hardly occurs in free form in the organism, there is no optimal level or concentration that could be used as a reference value for pathological deviations and disorders. On the other hand, thymine is used as a basic medicinal product in the manufacture of drugs that are used to treat certain viral diseases such as AIDS and hepatitis B.

Diseases & Disorders

When making copies of the DNA strands in the form of the creation of mRNA, errors can occur such as too frequent replication of a triplet, a sequence of three nucleic bases that determine the type of amino acid, or a loss of a sequence or it leads to a point mutation with potentially serious consequences.

All problems that arise from the creation of the mRNA have in common that the errors are not caused by the nucleobases themselves. However, only thymine makes a certain exception because it is susceptible to DNA mutation under the influence of UV light. If two thymine bases are directly adjacent on a DNA strand, the methyl groups (CH3 group) can form a stable bond with the neighboring thymine under the influence of UV light (sunlight), so that a dimer results, chemically one Derivative of cyclobutane corresponds. This changes the DNA at this point in such a way that a shortened version with fewer DNA bases is produced when the DNA strand is replicated.

If a transcription takes place, the error previously copied from the mRNA is translated into an incorrect amino acid sequence. A modified protein is then produced which, in the worst case, has no biological effectiveness or is unstable and is immediately metabolized again. It is a gene mutation that is mainly seen in skin cells that are exposed to direct sunlight. It is therefore discussed among experts whether such dimers can cause skin cancer.