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A heterocyclic aromatic compound with a chemical structure comprised of an imidazole ring fused to a pyrimidine ring, and makes up nucleic acids (e.g. DNA and RNA) and certain alkaloids (e.g. caffeine and theophylline)
A nucleobase is a nitrogen-containing compound that when attached to a pentose sugar ribose or deoxyribose forms nucleotide. Nucleotide is the monomeric unit of nucleic acids, such as DNA and RNA. In two-stranded nucleic acids like DNA, the nucleobases are paired. The two nucleobases that are complementary are connected by a hydrogen bond. The nucleobases can be grouped into two major types: purines and pyrimidines.
Purines vs. Pyrimidines
While both purines and pyrimidines are heterocyclic aromatic compounds, they can be differed from each other based on the chemical structure. A purine has two carbon rings whereas a pyrimidine has one carbon ring. The purine has a pyrimidine ring fused to an imidazole ring. The pyrimidine has only a pyrimidine ring. Thus, the purine has four nitrogen atoms whereas the pyrimidine has two.
Purines include adenine and guanine whereas pyrimidines include cytosine, thymine, and uracil. These five nitrogenous bases are regarded as primary or canonical since they are the fundamental units of the genetic code. The nucleobases that make up the nucleic acid are used to distinguish DNA from RNA molecules. In DNA, thymine complementary pairs with adenine whereas in RNA, uracil matches with adenine. The thymine differs from uracil in having a methyl group, which the uracil lacks. The pairings of nucleobases C-G and A-T (or A-U in RNA) are referred to as base complements.
Properties of Purines
Purine is a heterocyclic aromatic organic compound with a chemical formula of C5H4N4. Its chemical structure is comprised of a pyrimidine ring with an imidazole ring fused to it, thus, has two carbon rings and a total of four nitrogen atoms. The pyrimidine ring of purines contains two nitrogen atoms that are located at positions 1 and 3 of the ring (similar to those of pyrimidines). The imidazole ring attached to the pyrimidine ring has two nitrogen atoms that are located at positions 7 and 9. The molar mass of purine is 120.115 g/mol and its melting point is at 214 °C.
Adenine vs. Guanine
Adenine is a purine that complementary pairs with thymine in DNA and with uracil in RNA by two hydrogen bonds. Guanine is a purine that complementary pairs with cytosine in both DNA and RNA. Adenine can be distinguished from guanine by its amine group at position 6 and the presence of an additional double bond between N-1 and C-6 in its heterocyclic aromatic (pyrimidine) ring.
Endogenous purines and Exogenous purines
Purines are present in all cells. In general, purines and pyrimidines occur in the same amounts inside the cell. Purines that are manufactured biologically are referred to as endogenous purines. Apart from biosynthesis, purines may also be obtained from dietary sources, and in such case are referred to as exogenous purines. Meat and meat products, especially internal organs, are high in purines. Anchovies, mackerel, scallops, and sardines have high-purine content. Red meat, pork, beef, poultry, and other seafood have relatively moderate amounts of purines. Some of the plant-based purine source includes mushrooms, spinach, asparagus, cauliflower, beans, lentils, and wheat bran.
Common biological reactions
Purines are ubiquitous in nature. They are produced inside the cell. In humans, purines are biosynthesized mainly in the liver. Ribonucleotides (i.e. nucleobases attached to ribose 5-phosphate) are precursors to nucleobases.
Purines are derived from the nucleotide inosine monophosphate (IMP) since purines are synthesized as ribonucleotides and not as free nucleobases (as opposed to pyrimidines that are synthesized first as a free base). IMP, in turn, is produced from a pre-existing ribose phosphate that forms mainly from the amino acids glycine, glutamine, and aspartic acid. Ribose 5-phosphate reacts with ATP to produce 5-Phosphoribosyl-1-pyrophosphate (PRPP).
PRRP has a role in both purine and pyrimidine synthesis; it is also involved in NAD and NADP formation and salvage pathways. PRRP though becomes committed particularly to purine biosynthesis when PRRP is converted into 5-phosphoribosyl amine by having the pyrophosphate of PRRP replaced by the amide group of glutamine.
In humans, the biosynthesis of purines occurs in the cytosol of the liver cell. IMP is then converted into either adenosine monophosphate (AMP) or guanosine monophosphate (GMP).
Purines guanine and adenine may be degraded as follows:
- Guanine (via guanase) » xanthine (via xanthine oxidase) » uric acid
- Adenosine »» inosine (via purine nucleoside phosphorylase) » hypoxanthine (via xanthine oxidase) » xanthine (via xanthine oxidase) » uric acid
In humans and other vertebrates, the exogenous purines are degraded in the liver. As a result of purine degradation, uric acid is produced as a waste product. The uric acid is released from the liver into the bloodstream through which it reaches the kidney. It is then excreted from the body via the urine.
Purines from catabolism may be salvaged and re-used as follows:
- Adenine is salvaged by the enzyme adenine phosphoribosyltransferase (APRT)
- Guanine and hypoxanthine are salvaged by the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
Purines as one of the major groups of nucleobases are important structural components of nucleic acids. Nucleic acids such as DNA and RNA molecules contain the genetic information important for all cellular functions and heredity. Apart from the nucleic acids, nucleobases are also important components of certain proteins and starches. Thus, their functions are not just to serve as structural constituents of DNA and RNA but they are also involved in the regulation of enzymes and cell signaling. Examples of purines other than the two purine bases mentioned above are hypoxanthine, xanthine, theobromine, caffeine, uric acid, and isoguanine.
Uric acid is the metabolic end product of purine metabolism. In the diet, purines are found in high amounts in liver, kidney, and other internal organs. They are also present in meat, seafood, cauliflower, beans, and mushrooms but in moderate amounts. Hyperuricemia is the condition when there is too much uric acid level in the body. Too much uric acid from a high-purine diet could eventually lead to gout (inflammation in the joint) and kidney stones. Thus, people with such conditions are advised to eat a rather low-purine diet. It is also further advised to restrain from, or avoid consuming, alcohol and saturated fats because they obstruct the proper metabolism of purines.
- Purine 5-nucleotidase
- Purine ribonucleoside
- Purine-restricted diet
- Purinergic receptor
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