Difference Between Purines and Pyrimidines

Main Difference – Purines vs Pyrimidines

Purines and pyrimidines are the two types of nitrogenous bases found as the building blocks of nucleic acids of both DNA and RNA. Equal amounts of purines and pyrimidines are found in cells. Both purines and pyrimidines are heterocyclic, aromatic organic compounds which are involved in the synthesis of proteins and starch, regulation of enzymes and cell signaling. Two types of purines and three types of pyrimidines are found in the nucleic acid structure. Adenine and guanine are the two purines and cytosine, thymine and uracil are the three pyrimidines. The main difference between purines and pyrimidines is that purines contain a six-membered nitrogen-containing ring fused to an imidazole ring whereas pyrimidines contain only a six-membered nitrogen-containing ring.

This article looks at,

1. What are Purines
      – Definition, Structure, Properties
2. What are Pyrimidines
      – Definition, Structure, Properties
3. What is the difference between Purines and Pyrimidines

What are Purines

Purines are heterocyclic organic compounds containing a six-membered ring with two nitrogen atoms, which is fused to an imidazole ring. They are the most commonly found nitrogen-containing heterocyclic rings in nature. Purines are most commonly found in meat products like liver and kidney. The structure of purine is shown in figure 1.

Figure 1: Purine Structure

Purines are repeatedly occurring building blocks of DNA and RNA. Adenine and guanine are the purines found in DNA and RNA. Other common nuclear bases of purine are hypozanthine, xanthine, theobromine, caffeine, uric acid, and isoguanine. In addition to building the nucleic acids, purines form important biomolecules in the cell like ATP, GTP, NAD, cyclic AMP and coenzyme A. ATP is the major energy currency of the cell. GTP is used as an energy source during protein synthesis. NAD is a coenzyme involved in the redox reactions during metabolism like glycolysis. Cyclic AMP is a second messenger involved in the cAMP dependent pathway of signal transduction. Coenzyme A is an acetyl group carrier involved in the citric acid cycle. It forms acetyl-CoA. Purines are also capable of functioning as neurotransmitters, activating purinergic receptors. The main purine-derived nucleobases, adenine, and guanine are shown in figure 2.

Figure 2: Purines

Purines are synthesized as nucleosides, which are attached to ribose sugars. Both de novo and salvage pathways are involved in the biosynthesis of purines. Inosine monophosphate (IMP) is the precursor of both adenine and guanine in the de novo pathway. Guanine and hypoxanthine are sequentially converted into xanthine and uric acid during purine catabolism. Uric acid is excreted from the body.

What are Pyrimidines

Pyrimidines are heterocyclic organic compounds, containing a six-membered ring with two nitrogen atoms. The structure of the ring is similar to pyridine. Three isomerizing diazine structures are involved in the formation of the nucleobase ring. In pyridazine, nitrogen atoms are found in the positions, 1 and 2 in the heterocyclic ring. In pyrimidine, nitrogen atoms are found in the positions, 1 and 3 in the heterocyclic ring. In pyrazine, nitrogen atoms are found in the positions, 1 and 4 in the heterocyclic ring. The three isomers, pyridazine, pyrimidine and pyrazine are shown in figure 3.

Figure 3: Diazine isomers
1 – Pyridazine, 2 – Pyrimidine, 3 – Pyrazine

Cytosine and thymine are the two nucleobases found in DNA. Uracil is found in RNA. While forming the double-stranded structure of nucleic acids, pyrimidines form hydrogen bonds with complementary purines in the process called complementary base pairing. Cytosine forms three hydrogen bonds with guanine and thymine forms two hydrogen bonds with adenine in DNA. In RNA, uracil forms two hydrogen bonds with adenine instead of thymine. Cytosine, thymine, and uracil are shown in figure 4.

Figure 4: Pyrimidines

Pyrimidines are synthesized using both de novo and salvage pathways inside the cell. Uridine monophosphate (UMP) is the precursor producing in the de novo pathway, which is involved in the synthesis of uracil, cytosine, and thymine. Pyrimidines are catabolized into urea, carbon dioxide and water.

Difference Between Purines and Pyrimidines

Structure

Purines: Purines are heterocyclic aromatic organic compounds, consisting of a pyrimidine ring fused to an imidazole ring.

Pyrimidines: Pyrimidines are heterocyclic aromatic organic compounds.

Nucleobases

Purines: Adenine, guanine, hypoxanthine, and xanthine are the nucleobases found in purines.

Pyrimidines: Cytosine, thymine, uracil and orotic acid are the nucleobases found in pyrimidines.

Chemical Composition

Purines: Purines contain two carbon-nitrogen rings and four nitrogen atoms since they are composed of a pyrimidine ring, which is fused to an imidazole ring.

Pyrimidines: Pyrimidines contain a single carbon-nitrogen ring and 2 nitrogen atoms.

Chemical Formula

Purines: Chemical formula of purine is C₅H₄N₄.

Pyrimidines: Chemical formula of pyrimidine is C₄H₄N₂.

Melting Point/ Boiling Point

Purines: Purines contain comparatively high melting and boiling points.

Pyrimidines: Pyrimidines contain comparatively low melting and boiling points.

Synthesis in Lab

Purines: Purines are synthesized by Traube Purine Synthesis.

Pyrimidines: Pyrimidines are synthesized by Biginelli Reaction.

Catabolism

Purines: Purine catabolism produces uric acid.

Pyrimidines: Pyrimidine catabolism produces beta amino acids, carbon dioxide, and ammonia.

Conclusion

Purines and pyrimidines are the two repeating building blocks in nucleic acids involved in the storage of genetic information in the cell required for the development, functioning and reproduction of organisms. Adenine and guanine are the purines and cytosine, thymine and uracil are the pyrimidines found in nucleic acids. RNA contains uracil, instead of thymine. While forming the double-stranded structure of nucleic acids, adenine forms hydrogen bonds with thymine or uracil and guanine form hydrogen bonds with cytosine. Purines have other functions in the cell like serving as energy sources. Both purines and pyrimidines are synthesized in the cell either by de novo or salvage pathways. However, the main difference between purines and pyrimidines is in the structure of the nucleobases which are shared by them. 

Reference:
1.Fort, Ray. Purines and Pyrimidines. N.p., n.d. Web. 28 Apr. 2017.
2.”Purine and Pyrimidine Metabolism.” PURINES AND PYRIMIDINES. N.p., n.d. Web. 28 Apr. 2017.

Image Courtesy:
1. “9H-Purine” By NEUROtiker (talk) – Own work (Public Domain) via Commons Wikimedia
2. “Blausen 0323 DNA Purines” By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia
3. “Diazine isomers” By Luigi Chiesa. Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia
4. “Blausen 0324 DNA Pyrimidines” Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work (CC BY 3.0) via Commons Wikimedia