Purines and pyrimidines are two types of nitrogenous bases that form the structural foundation of nucleic acids like DNA and RNA. Though they both serve similar functions within the cell, their chemical structures, properties, and roles vary considerably. Purines consist of a two-ring structure (a six-membered and a five-membered nitrogen-containing ring fused together), while pyrimidines have a single six-membered nitrogen-containing ring. Here are details of these essential biological molecules, including their chemical structure, molecular formula, examples, functions, properties, dietary sources, and synthesis.
Purines
Chemical Structure
Purines are composed of a pyrimidine ring fused with an imidazole ring. The molecule consists of four nitrogen atoms and six carbon atoms forming two rings—a nine-membered double-ring system.
Molecular Formula
The general molecular formula for purines is C5H4N4.
Examples
Adenine and guanine are the two primary purines found in nucleic acids (DNA and RNA).
Functions
Purines are essential for a variety of cellular processes, including DNA and RNA synthesis, energy storage and transfer through molecules like ATP, and signal transduction as secondary messengers (cAMP).
Properties
Purines are relatively larger than pyrimidines and have a higher molecular weight. They are soluble in water and are relatively stable at the physiological pH.
Word Origin
The term “purine” was first introduced in the late 19th century and is derived from the word “purum,” which is Latin for “pure,” and the suffix “-ine,” which is common for chemical compounds. Alternatively, the name means “pure urine”, since the source material for the purification was uric acid from urine. Its discovery is credited to Emil Fischer, a German chemist, who pioneered work on purines and was awarded the Nobel Prize in Chemistry in 1902 for his contributions to the field.
Dietary Sources
Dietary sources of purines include meats, seafood, beans, and certain vegetables like spinach and asparagus.
Synthesis
In humans, purine synthesis occurs through the purine biosynthetic pathway, where the molecule inosine monophosphate (IMP) serves as the precursor. The synthesis is highly regulated and takes place in the cytoplasm of cells.
Lab synthesis is via Traube purine synthesis.
Pyrimidines
Chemical Structure
Pyrimidines consist of a single six-membered ring with two nitrogen atoms and four carbon atoms.
Molecular Formula
The general molecular formula for pyrimidines is C4H4N2.
Examples
Cytosine (both DNA and RNA), thymine (in DNA), and uracil (in RNA) are the primary pyrimidines.
Functions
Like purines, pyrimidines also contribute to the structure of DNA and RNA. They are essential for protein synthesis, as a component of RNA, and participate in cellular energy conversion.
Properties
Pyrimidines are smaller than purines and are less complex in structure. They have a lower molecular weight, lower melting point, and are less soluble in water compared to purines.
Word Origin
The term “pyrimidine” comes from the Greek word “pyr,” which means “fire,” and the suffix “-imidine,” which refers to certain chemical compounds. The name “pyrimidine” reflects the compound’s origin in the breakdown of organic materials at high temperatures (hence, “pyr” for fire). Just like purines, pyrimidines were first identified and isolated in the late 19th century.
Dietary Sources
Fish, chicken, whole grain breads, and dairy products are excellent sources of pyrimidines.
Synthesis
Pyrimidine synthesis also takes place in the cytoplasm and involves the creation of the orotic acid as an intermediate compound. Unlike purines, pyrimidines originate as bases that later attach to the ribose sugar.
In the laboratory, pyrimidine synthesis occurs via the Bigineli reaction.
Comparison Table: Purines vs. Pyrimidines
| Parameter | Purines | Pyrimidines |
|---|---|---|
| Chemical Structure | Two rings (six-membered and five-membered) | Single six-membered ring |
| Molecular Formula | C5H4N4 | C4H4N2 |
| Examples | Adenine, Guanine | Cytosine, Thymine, Uracil |
| Functions | DNA/RNA structure, energy storage, signal transduction | DNA/RNA structure, protein synthesis, energy conversion |
| Properties | Larger, more complex, higher molecular weight, higher melting point (214 °C), soluble in water | Smaller, less complex, lower molecular weight, lower melting point (20-22 °C), less soluble in water |
| Dietary Sources | Meats, seafood, beans, certain vegetables | Fish, chicken, whole grains, dairy products |
| Synthesis Location | Cytoplasm | Cytoplasm |
Additional Facts About Purines and Pyrimidines
- Purine bases are degraded into uric acid in humans, which can lead to conditions like gout when present in high levels.
- Pyrimidines degrade into simpler compounds like beta-alanine and are generally less problematic in terms of accumulation and degradation.
- In DNA and RNA, purines always bond with pyrimidines. The ratio of the two types of compounds is constant within the molecule. Adenine pairs with thymine (DNA) or uracil (RNA) with two hydrogen bonds. Guanine bonds with cytosine (DNA or RNA) with three hydrogen bonds.
- The balance between purines and pyrimidines is crucial for cellular processes, and any imbalance potentially leads to diseases and conditions like cancer.
References
- Carey, Francis A. (2008). Organic Chemistry (6th ed.). Mc Graw Hill. ISBN 0072828374.
- Guyton, Arthur C. (2006). Textbook of Medical Physiology. Philadelphia, PA: Elsevier. 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. ISBN 071677108X.
- Soukup, Garrett A. (2003). “Nucleic Acids: General Properties.” eLS. American Cancer Society. doi:10.1038/npg.els.0001335 ISBN 9780470015902.