The Difference Between Purines and Pyrimidines

Purine and pyrimidine nitrogenous bases.
Purine and pyrimidine nitrogenous bases. chromatos / Getty Images

Purines and pyrimidines are two types of aromatic heterocyclic organic compounds. In other words, they are ring structures (aromatic) that contain nitrogen as well as carbon in the rings (heterocyclic). Both purines and pyrimidines are similar to the chemical structure of the organic molecule pyridine (C5H5N). Pyridine, in turn, is related to benzene (C6H6), except one of the carbon atoms is replaced by a nitrogen atom.

Purines and pyrimidines are important molecules in organic chemistry and biochemistry because they are the basis for other molecules (e.g., caffeine, theobromine, theophylline, thiamine) and because they are key components of the nucleic acids dexoyribonucleic acid (DNA) and ribonucleic acid (RNA).

Pyrimidines

A pyrimidine is an organic ring consisting of six atoms: 4 carbon atoms and 2 nitrogen atoms. The nitrogen atoms are placed in the 1 and 3 positions around the ring. Atoms or groups attached to this ring distinguish pyrimidines, which include cytosine, thymine, uracil, thiamine (vitamin B1), uric acid, and barbituates. Pyrimidines function in DNA and RNA, cell signaling, energy storage (as phosphates), enzyme regulation, and to make protein and starch.

Purines

A purine contains a pyrimidine ring fused with an imidazole ring (a five-member ring with two non-adjacent nitrogen atoms). This two-ringed structure has nine atoms forming the ring: 5 carbon atoms and 4 nitrogen atoms. Different purines are distinguished by the atoms or functional groups attached to the rings.

Purines are the most widely occurring heterocyclic molecules that contain nitrogen. They are abundant in meat, fish, beans, peas, and grains. Examples of purines include caffeine, xanthine, hypoxanthine, uric acid, theobromine, and the nitrogenous bases adenine and guanine. Purines serve much the same function as pyrimidines in organisms. They are part of DNA and RNA, cell signaling, energy storage, and enzyme regulation. The molecules are used to make starch and proteins.

Bonding Between Purines and Pyrimidines

While purines and pyrimidines include molecules that are active on their own (as in drugs and vitamins), they also form hydrogen bonds between each other to link the two strands of the DNA double helix and to form complementary molecules between DNA and RNA. In DNA, the purine adenine bonds to the pyrimidine thymine and the purine guanine bonds to the pyrimidine cytosine. In RNA, adenine bonds to uracil and guanine still bonds with cytosine. Approximately equal amounts of purines and pyrimidines are required to form either DNA or RNA.

It's worth noting there are exceptions to the classic Watson-Crick base pairs. In both DNA and RNA, other configurations occur, most often involving methylated pyrimidines. These are called "wobble pairings."

Comparing and Contrasting Purines and Pyrimidines

The purines and pyrimidines both consist of heterocyclic rings. Together, the two sets of compounds make up the nitrogenous bases. Yet, there are distinct differences between the molecules. Obviously, because purines consist of two rings rather than one, they have a higher molecular weight. The ring structure also affects the melting points and solubility of the purified compounds.

The human body synthesizes (anabolism) and breaks down (catabolism) the molecules differently. The end product of purine catabolism is uric acid, while the end products of pyrimidine catabolism are ammonia and carbon dioxide. The body does not make the two molecules in the same location, either. Purines are synthesized primarily in the liver, while a variety of tissues make pyrimidines.

Here is a summary of the essential facts about purines and pyrimidines:

Purine Pyrimidine
Structure Double ring (one is a pyrimidine) Single ring
Chemical Formula C5H4N4 C4H4N2
Nitrogenous Bases Adenine, guanine Cytosine, uracil, thymine
Uses DNA, RNA, vitamins, drugs (e.g., barbituates), energy storage, protein and starch synthesis, cell signaling, enzyme regulation DNA, RNA, drugs (e.g., stimulants), energy storage, protein and starch synthesis, enzyme regulation, cell signaling
Melting Point 214 °C (417 °F) 20 to 22 °C (68 to 72 °F)
Molar Mass 120.115 g·mol−1 80.088 g mol−1
Solubility (Water) 500 g/L Miscible
Biosynthesis Liver Various tissues
Catabolism Product Uric acid Ammonia and carbon dioxide

Sources

  • Carey, Francis A. (2008). Organic Chemistry (6th ed.). Mc Graw Hill. ISBN 0072828374.
  • Guyton, Arthur C. (2006). Textbook of Medical Physiology. Philadelphia, PA: Elsevier. p. 37. ISBN 978-0-7216-0240-0.
  • Joule, John A.; Mills, Keith, eds. (2010). Heterocyclic Chemistry (5th ed.). Oxford: Wiley. ISBN 978-1-405-13300-5.
  • Nelson, David L. and Michael M Cox (2008). Lehninger Principles of Biochemistry (5th ed.). W.H. Freeman and Company. p. 272. ISBN 071677108X.
  • Soukup, Garrett A. (2003). "Nucleic Acids: General Properties." eLS. American Cancer Society. doi:10.1038/npg.els.0001335 ISBN 9780470015902.