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Guanine

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Definition

noun

plural: guanines

gua·nine, [ˈɡwɑː.niːn]


(biochemistry) A purine nucleobase that complementary pairs with cytosine in DNA and RNA, and has a chemical formula of C5H5N5O



Details

Overview

Nucleic acids such as DNA and RNA are polymers of monomeric nucleotides. Each nucleotide is made up of phosphoric acid, sugar (5-carbon), and nitrogenous base (or nucleobase). There are five nucleobases that serve as fundamental units of the genetic code: (1) adenine, (2) guanine, (3) cytosine, (4) thymine, and (5) uracil. These nucleobases may be classified into purines and pyrimidines.


Properties

Guanine is a purine nucleobase with a chemical formula of C5H5N5O. Purines are heterocyclic aromatic organic compounds. As a purine, adenine is comprised of two carbon rings: a pyrimidine ring and an imidazole ring. Guanine occurs in both DNA and RNA. It complementary base pairs with cytosine in DNA and RNA via three hydrogen bonds.


Guanine vs. Adenine

Both guanine and adenine are purine nucleobases. Guanine can be distinguished from adenine by its amine group at position 2 and a carbonyl group at position 6 of its heterocyclic aromatic (pyrimidine) ring. As mentioned earlier, guanine complementary pairs with cytosine by three hydrogen bonds in both DNA and RNA molecules. In contrast, adenine complementary pairs with thymine in DNA and with uracil in RNA by two hydrogen bonds.


Common biological reactions

Biosynthesis

Guanine, similar to adenine, is derived from the nucleotide inosine monophosphate (IMP) since purines are synthesized as ribonucleotides and not as free nucleobases. 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). GMP, though, is formed when IMP has to be first oxidized to xanthosine monophosphate (XMP) using NAD.



Guanine nucleosides

Guanosine and deoxyguanosine are the nucleosides of guanine. The guanosine (i.e. guanine bound to a ribose sugar) that is phosphorylated with three phosphoric acid groups becomes guanosine triphosphate (GTP), which serves as one of the nucleotide monomeric units that build up RNA. It may also act as a secondary messenger in signal transduction. The deoxyguanosine (i.e. guanine bound to a deoxyribose sugar) that is phosphorylated with three phosphoric acid groups becomes deoxyguanosine triphosphate (dGTP), one of the nucleotides that build up DNA molecules.


Degradation and Salvage

Guanine is degraded as follows: guanine (via guanase enxyme) » xanthine (via xanthine oxidase enzyme) » uric acid. In brief, guanine is converted into xanthine by deamination. Xanthine, in turn, is converted into uric acid by the enzyme xanthine oxidase. In humans and other vertebrates, the exogenous purines such as adenine and guanine 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. Guanine and hypoxanthine may be salvaged from catabolism and re-used by the catalytic activity of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT).



Biological functions

Guanine is one of the five primary (or canonical) nucleobases; the others are adenine, cytosine, thymine, and uracil. They are the fundamental nucleobases that make up the genetic code. Nucleic acids such as DNA and RNA molecules contain the genetic code for a particular protein based on the sequence of nucleobases. Nucleic acids hold an important role in cellular functions, heredity, and survival of an organism.


Apart from this, guanine has been associated with camouflage, display, and vision. It has been found to occur in iridocytes, which are specialized skin cells of fish.[1]


Health effects

Uric acid is the metabolic end product of purine metabolism, including guanine. 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.


Trivia

Etymologically, the name guanine is derived from the Spanish gano, meaning bird or bat droppings, since it was first obtained from guano. Guanine crystals are used in cosmetics as they furnish iridescent effect or shimmering, pearly luster.


The isolation and naming of the five nucleobases, including cytosine, is attributed to Albrecht Kossel (a German biochemist). From 1885 to 1901 he and his students discovered them through chemical analyses of the nucleic acids.[1]


Supplementary

Etymology

  • guano + -ine

IUPAC name

  • 2-amino-9H-purin-6(1H)-one

Chemical formula

  • C5H5N5O

Abbreviation

  • G

Also called

  • 1,9-dihydro-6H-purin-6-one
  • 2-amino-6-hydroxypurine
  • 2-aminohypoxanthine


Further reading

See also


Reference

  1. Gur, D., Palmer, B., Weiner, S., & Addadi, L. (2017). "Light manipulation by guanine crystals in organisms: biogenic scatterers, mirrors, multilayer reflectors and photonic crystals". Advanced Functional Materials 27, (6): 1603514. doi:10.1002/adfm.201603514
  2. The Editors of Encyclopedia Britannica. (2018). Albrecht Kossel | German biochemist. In Encyclopædia Britannica. Retrieved from https://www.britannica.com/biography/Albrecht-Kossel



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