Differences between purines and pyrimidines

Purines and pyrimidines are two groups of nitrogenous bases that form an essential part of nucleic acids, that is, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) . In nucleic acids, the nitrogenous bases are attached to a pentose, that is, to a 5-carbon sugar that can be deoxyribose (in the case of DNA) or ribose (in the case of RNA).

The existence of the double helix that makes up the structure of DNA and that contains all our genetic information encoded is precisely due to the formation of hydrogen bonds between the purines of one of the chains with a complementary pyrimidine in the other.

Despite being treated in both cases as nitrogenous bases, purines and pyrimidines present key differences between them, both in their structure and in their function at the cellular level. These differences are explained in the following sections.

purines

Purines are a group that includes two nitrogenous bases whose fundamental structure is an aromatic heterocycle consisting of two fused rings. One of these is a six-membered ring while the other is a five-membered ring, and in each ring there are two nitrogen atoms.

There are two members in the group of purines, which are adenine and guanine, whose structures are presented in the following figure:

Both adenine and guanine are part of DNA and RNA. In addition, both fulfill many other additional functions within the cell, since they are part of energy storage molecules as well as neurotransmitters and other cellular messengers.

pyrimidines

Pyrimidines are a group of three nitrogenous bases whose fundamental structure is based on the pyrimidine ring, an aromatic heterocycle that has two nitrogens as part of a six-membered ring.

The group of pyrimidines is made up of cytosine, thymine and uracil. Despite being formed from pyrimidine, the inclusion of one or two carbonyl groups directly attached to the ring removes one or two double bonds on the ring, thus breaking the aromaticity of the pyrimidine.

Cytosine is only found in DNA, and in RNA its place is taken by uracil, while thymine can be found in both DNA and RNA.

Differences between purines and pyrimidines

Purines and pyrimidines have different fundamental structures.

As mentioned before, both groups of nitrogenous bases come from two different types of aromatic rings, which are purine and pyrimidine. Although both rings are aromatic heterocycles, they are very different, particularly since one is monocyclic while the other, the purine, is a bicycle.

Purines are involved in the storage and use of energy; pyrimidines do not

The main source of energy for many enzyme-catalyzed reactions comes from the hydrolysis of adenosine triphosphate, or ATP, which contains an adenine ring. Another compound that serves a similar function is GTP, which contains guanine instead of adenine, but is still a purine.

On the other hand, cAMP is a second messenger of great importance that contains adenine. Similarly, two very important cofactors, FAD and NAD also contain adenine. These cofactors are necessary for the functioning of many enzymes and they are also part of the electron transport chain in the process of cellular respiration.

Purines retain all or part of the purine aromaticity while pyrimidines are not aromatic.

Adenine maintains aromaticity in both rings of the purine backbone while guanine maintains aromaticity in the five-membered ring but not the six-membered ring. However, the three pyrimidines have carbonyl groups that form part of the ring, which breaks the aromaticity of the system.

They differ in the biosynthetic pathway

Both purines and pyrimidines are synthesized de novo , that is, from scratch, in the cytoplasm, but the way they are synthesized is very different.

Purines are mainly synthesized in the liver by adding carbon atoms directly to ribose 5-phosphate. This means that purines are not synthesized in free form but directly as nucleotides. The two fused ring system is built from the amino acids aspartate, glycine, and glutamine, as well as bicarbonate and formate ions.

In the case of pyrimidines, on the other hand, they are synthesized in different tissues of the human body and the ring is synthesized in free form from carbamoyl phosphate and aspartate and is modified by the action of various enzymes. Then, the respective pyrimidine is attached to a ribose 5-phosphate to obtain the nucleotide that will ultimately become part of RNA or DNA.

They differ in the breakdown products

These two classes of nitrogenous bases differ not only in the way they are synthesized, but also in the way they are metabolized. Purine catabolism produces uric acid as the main product. Instead, pyrimidines are further broken down into ammonia and carbon dioxide.

References

Aliouche, HB (2019, January 25). Purine Biosynthesis. Retrieved from https://www.news-medical.net/life-sciences/Purine-Biosynthesis.aspx

Comparative Table PURINES AND PYRIMIDINES – Docsity. (nd). Retrieved from https://www.docsity.com/es/cuadro-parativo-purinas-y-pyrimidinas/5423720/

Difference between Purines and Pyrimidines. (2018, December 19). Retrieved from https://www.diferenciasentre.info/purinas-pirimidinas/

Kumari, A. (2018). Pyrimidine de novo Synthesis. Sweet Biochemistry , 101–103. Retrieved from https://doi.org/10.1016/B978-0-12-814453-4.00020-0

Stewart, D. (2019, October 15). Elements found in biomolecules. Retrieved from https://www.portalsalud.com/elementos-encontrados-en-las-biomoleculas_13110282/