Pyrimidines characteristics, structure, functions

The pyrimidines they are molecules in cyclic form, rich in nitrogen. They are part of the nucleotides, which in turn are the fundamental structural constituents of nucleic acids.

In addition to their presence in nucleic acids, the nucleotides formed by pyrimidines have an important role as intracellular messengers and participate in the regulation of glycogen and phospholipid biosynthesis pathways..

The main difference between a pyrimidine and a purine is in the structure: the former are made up of a single ring, while in the latter we find a ring of pyrimidines joined to an imidazole ring.

Pyrimidine rings are also found in some synthetic drugs, such as barbiturates and those used to treat HIV.

Article index

• 1 Characteristics and structure
• 2 Functions
  • 2.1 -Structural blocks of nucleic acids
  • 2.2 -Extracellular messengers
  • 2.3 -Intermediate metabolism
• 3 DNA damage
• 4 Metabolism of pyrimidines
  • 4.1 -Synthesis
  • 4.2 Degradation
  • 4.3 Diet requirements
• 5 References

Characteristics and structure

Pyrimidines are aromatic chemical compounds whose structure is cyclic (a single ring) and flat..

The most abundant pyrimidines in nature are uracil (molecular formula 2,4-dihydroxypyrimidine), cytosine (2-hydroxy-4-aminopyrimidine) and thymine (2,4-dihydroxy-5-methyl pyrimidine).

The molar mass is around 80 g / mol, with a density of 1.016 g / cm. They are soluble in water and thanks to their rings they have the property of absorbing light at a maximum of 260 nanometers..

Features

-Building blocks of nucleic acids

Nucleic acids are biopolymers made up of monomers called nucleotides. In turn, the nucleotides are made up of: (i) a five-carbon sugar, (ii) a phosphate group and (iii) a nitrogenous base.

Pyrimidines in DNA and RNA

Nitrogen bases are flat cyclic compounds that are classified into purines and pyrimidines.

Compared to puric bases, pyrimidines are smaller (remember that the structure of the former comprises two fused rings, and one of them is a pyrimidine ring).

This fact has consequences when it comes to pairing in the DNA double helix: in order to establish a stable structure, purines only pair with one pyrimidine..

As we mentioned earlier, the three most common pyrimidines in nature are uracil, cytosine, and thymine..

One of the fundamental differences between DNA and RNA is the composition of pyrimidines that make up its structure. Uracil and cytosine are part of the nucleotides in RNA. In contrast, cytosine and thymine are found in DNA..

However, small amounts of thymine nucleotides are found in transfer RNAs..

In nucleotides, pyrimidines bind to carbon 1 of ribose through nitrogen located at position 1.

-Extracellular messengers

The nucleotides that contain pyrimidines (and also purines) are molecules that fulfill an extracellular messenger role. They are responsible for regulating various functions in virtually every cell in the body.

These nucleotides are released from damaged cells or they can be secreted by a non-lytic pathway and interact with specific receptors on the cell membrane..

Specific membrane receptors are called P2 receptors and are classified into two families: P2Y or metabotropic and P2X or ionotropic..

-Intermediate metabolism

Pyrimidine nucleotides are involved in biological synthesis pathways for other components. An example of this participation is the glycogen and phospholipid biosynthesis pathway..

DNA damage

One of the most common lesions in the DNA molecule occurs at the pyrimidine level, specifically in the formation of dimers between the thymine bases. That is, a bond is formed between two of these molecules.

This occurs due to ultraviolet radiation (from sun exposure) that DNA receives, or due to exposure to mutagenic agents..

The formation of these pyrimidine dimers distorts the DNA double helix, causing problems when replicating or transcribing. The enzyme responsible for correcting this event is called photolyase.

Pyrimidine metabolism

-Synthesis

Overview

The synthesis of nitrogenous bases - both purines and pyrimidines - is a fundamental element for life, since they are the raw material to synthesize nucleic acids..

The general scheme of the synthesis of pyrimidines differs in a fundamental aspect with the synthesis of purines: the ring of pyrimidines is assembled before anchoring to ribose-5-phosphate.

Reactions

The molecule called carbamoyl aspartate has all the elements (atoms) necessary for the synthesis of a pyrimidine ring. This is formed through a condensation reaction between an aspartate and a carbomoyl phosphate.

The carbomoyl phosphate precursor is formed in the cell cytoplasm by a reaction catalyzed by the enzyme carbamoyl phosphate synthetase, the substrates of which are carbon dioxide (CO_two) and ATP. The compound resulting from the oxidation of carbamoyl aspartate is orotic acid.

It is curious that carbamoyl phosphate synthetase is an enzyme common to the pathway described and to the urea cycle. However, they differ in some aspects related to their activity; For example, this version of the enzyme uses glutamine and not NH as a nitrogen source.₃.

Once the ring has closed, it can be converted to other compounds such as uridine triphosphate (UTP), cytidine triphosphate (CTP), and thymidylate..

Degradation

Catabolic (or breakdown) reactions involving pyrimidines take place in the liver. Unlike purines, the substances produced by catabolism do not form crystals when they accumulate, an event that causes gout in patients who accumulate this waste substance..

The compounds generated are carbon dioxide, water and urea. Cytosine can move to another pyrimidine (uracil) and then continue the path of degradation in multiple intermediates.

Diet requirements

Pyrimidines, like purines, are synthesized by the cell in amounts that meet the requirements of the cell. It is for this reason that there are no minimum requirements for nitrogenous bases in the diet. However, when these molecules are consumed, the body has the ability to recycle them..

References

1. Alberts, B., Bray, D., Hopkin, K., Johnson, A. D., Lewis, J., Raff, M.,… & Walter, P. (2013). Essential cell biology. Garland science.
2. Cooper, G. M., & Hausman, R. E. (2007). The cell: a molecular approach. Washington, DC, Sunderland, MA.
3. Griffiths, A. J. (2002). Modern genetic analysis: integrating genes and genomes. Macmillan.
4. Griffiths, A. J., Wessler, S. R., Lewontin, R. C., Gelbart, W. M., Suzuki, D. T., & Miller, J. H. (2005). An introduction to genetic analysis. Macmillan.
5. Koolman, J., & Röhm, K. H. (2005). Biochemistry: text and atlas. Panamerican Medical Ed..
6. Passarge, E. (2009). Genetics text and atlas. Panamerican Medical Ed..