Uracil

Template:Short description Template:Distinguish Template:Chembox Uracil (Template:IPAc-en) (symbol U or Ura) is one of the four nucleotide bases in the nucleic acid RNA. The others are adenine (A), cytosine (C), and guanine (G). In RNA, uracil binds to adenine via two hydrogen bonds. In DNA, the uracil nucleobase is replaced by thymine (T). Uracil is a demethylated form of thymine.

Uracil is a common and naturally occurring pyrimidine derivative.<ref name="Garrett1">Template:Cite book</ref> The name "uracil" was coined in 1885 by the German chemist Robert Behrend, who was attempting to synthesize derivatives of uric acid.<ref>Template:Cite journal</ref> Originally discovered in 1900 by Alberto Ascoli, it was isolated by hydrolysis of yeast nuclein;<ref>Template:Cite journal</ref> it was also found in bovine thymus and spleen, herring sperm, and wheat germ.<ref name="brown1"/> It is a planar, unsaturated compound that has the ability to absorb light.<ref name="Horton1"/>

Uracil that was formed extraterrestrially has been detected in the Murchison meteorite,<ref name="Murch_base" /> in near-Earth asteroid Ryugu,<ref name="Oba 2023" /> and possibly on the surface of the moon Titan.<ref name="Clark 2012"/> It has been synthesized under cold laboratory conditions similar to outer space, from pyrimidine embedded in water ice and exposed to ultraviolet light.<ref name="Nuevo 2009" />

Properties

In RNA, uracil base-pairs with adenine and replaces thymine during DNA transcription. Methylation of uracil produces thymine.<ref name="madsci1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In DNA, the evolutionary substitution of thymine for uracil may have increased DNA stability and improved the efficiency of DNA replication (discussed below). Uracil pairs with adenine through hydrogen bonding. When base pairing with adenine, uracil acts as both a hydrogen bond acceptor and a hydrogen bond donor. In RNA, uracil binds with a ribose sugar to form the ribonucleoside uridine. When a phosphate attaches to uridine, uridine 5′-monophosphate is produced.<ref name="Horton1">Template:Cite book</ref>

Uracil undergoes amide-imidic acid tautomeric shifts because any nuclear instability the molecule may have from the lack of formal aromaticity is compensated by the cyclic-amidic stability.<ref name="brown1"/> The amide tautomer is referred to as the lactam structure, while the imidic acid tautomer is referred to as the lactim structure. These tautomeric forms are predominant at pH 7. The lactam structure is the most common form of uracil.

File:Uracil tautomers.png

Uracil tautomers: Amide or lactam structure (left) and imide or lactim structure (right)

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Uracil also recycles itself to form nucleotides by undergoing a series of phosphoribosyltransferase reactions.<ref name="Garrett1"/> Degradation of uracil produces the substrates β-alanine, carbon dioxide, and ammonia.<ref name = "Garrett1"/>

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Oxidative degradation of uracil produces urea and maleic acid in the presence of H₂O₂ and Fe²⁺ or in the presence of diatomic oxygen and Fe²⁺.

Uracil is a weak acid. The first site of ionization of uracil is not known.<ref name="Zorbach1">Template:Cite book</ref> The negative charge is placed on the oxygen anion and produces a pK_a of less than or equal to 12. The basic pK_a = −3.4, while the acidic pK_a = 9.38₉. In the gas phase, uracil has four sites that are more acidic than water.<ref name="Lee1">Template:Cite journal</ref>

In DNA

Uracil is rarely found in DNA, and this may have been an evolutionary change to increase genetic stability. This is because cytosine can deaminate spontaneously to produce uracil through hydrolytic deamination. Therefore, if there were an organism that used uracil in its DNA, the deamination of cytosine (which undergoes base pairing with guanine) would lead to formation of uracil (which would base pair with adenine) during DNA synthesis. Uracil-DNA glycosylase excises uracil bases from double-stranded DNA. This enzyme would therefore recognize and cut out both types of uracil – the one incorporated naturally, and the one formed due to cytosine deamination, which would trigger unnecessary and inappropriate repair processes.<ref>Template:Cite journal</ref>

This problem is believed to have been solved in terms of evolution, that is by "tagging" (methylating) uracil. Methylated uracil is identical to thymine. Hence the hypothesis that, over time, thymine became standard in DNA instead of uracil. So cells continue to use uracil in RNA, and not in DNA, because RNA is shorter-lived than DNA, and any potential uracil-related errors do not lead to lasting damage. Apparently, either there was no evolutionary pressure to replace uracil in RNA with the more complex thymine, or uracil has some chemical property that is useful in RNA, which thymine lacks. Uracil-containing DNA still exists, for example in:

• DNA of several phages<ref>Template:Cite journal</ref>
• Endopterygote development
• Hypermutations during the synthesis of vertebrate antibodies.Template:Citation needed

Synthesis

Biological

Template:See also Organisms synthesize uracil, in the form of uridine monophosphate (UMP), by decarboxylating orotidine 5'-monophosphate (orotidylic acid). In humans this decarboxylation is achieved by the enzyme UMP synthase. In contrast to the purine nucleotides, the pyrimidine ring (orotidylic acid) that leads uracil is synthesized first and then linked to ribose phosphate, forming UMP.<ref name="Loffler 2004">Template:Cite book</ref>

Laboratory

There are many laboratory synthesis of uracil available. The first reaction is the simplest of the syntheses, by adding water to cytosine to produce uracil and ammonia:<ref name = "Garrett1"/>

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The most common way to synthesize uracil is by the condensation of malic acid with urea in fuming sulfuric acid:<ref name="brown1"/>

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Uracil can also be synthesized by a double decomposition of thiouracil in aqueous chloroacetic acid.<ref name="brown1"/>

Photodehydrogenation of 5,6-diuracil, which is synthesized by beta-alanine reacting with urea, produces uracil.<ref name="Chittenden1">Template:Cite journal</ref>

Prebiotic

In 2009, NASA scientists reported having produced uracil from pyrimidine and water ice by exposing it to ultraviolet light under space-like conditions.<ref name="Nuevo 2009">Template:Cite journal</ref> This suggests a possible natural original source for uracil.<ref name="NASA-20091105">Template:Cite news</ref> In 2014, NASA scientists reported that additional complex DNA and RNA organic compounds of life, including uracil, cytosine and thymine, have been formed in the laboratory under outer space conditions, starting with ice, pyrimidine, ammonia, and methanol, which are compounds found in astrophysical environments.<ref name="Nuevo 2014">Template:Cite journal</ref> Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), a carbon-rich chemical found in the Universe, may have been formed in red giants or in interstellar dust and gas clouds.<ref name="NASA-20150303">Template:Cite news</ref>

Based on ¹²C/¹³C isotopic ratios of organic compounds found in the Murchison meteorite, it is believed that uracil, xanthine, and related molecules can also be formed extraterrestrially.<ref name="Murch_base">Template:Cite journal</ref> Data from the Cassini mission, orbiting in the Saturn system, suggests that uracil is present in the surface of the moon Titan.<ref name="Clark 2012">Template:Cite journal</ref> In 2023, uracil was observed in a sample from 162173 Ryugu, a near-Earth asteroid, with no exposure to Earth's biosphere, giving further evidence for synthesis in space.<ref name="Oba 2023">Template:Cite journal</ref>

Reactions

File:Uridin.svg

Chemical structure of uridine

Uracil readily undergoes regular reactions including oxidation, nitration, and alkylation. While in the presence of phenol (PhOH) and sodium hypochlorite (NaOCl), uracil can be visualized in ultraviolet light.<ref name="brown1">Template:Cite book</ref> Uracil also has the capability to react with elemental halogens because of the presence of more than one strongly electron donating group.<ref name="brown1"/>

Uracil readily undergoes addition to ribose sugars and phosphates to partake in synthesis and further reactions in the body. Uracil becomes uridine, uridine monophosphate (UMP), uridine diphosphate (UDP), uridine triphosphate (UTP), and uridine diphosphate glucose (UDP-glucose). Each one of these molecules is synthesized in the body and has specific functions.

When uracil reacts with anhydrous hydrazine, a first-order kinetic reaction occurs and the uracil ring opens up.<ref name="Kochetkov1"/> If the pH of the reaction increases to > 10.5, the uracil anion forms, making the reaction go much more slowly. The same slowing of the reaction occurs if the pH decreases, because of the protonation of the hydrazine.<ref name="Kochetkov1"/> The reactivity of uracil remains unchanged, even if the temperature changes.<ref name="Kochetkov1">Template:Cite book</ref>

Uses

Uracil's use in the body is to help carry out the synthesis of many enzymes necessary for cell function through bonding with riboses and phosphates.<ref name="Garrett1"/> Uracil serves as allosteric regulator and coenzyme for reactions in animals and in plants.<ref name = "Brown2"/> UMP controls the activity of carbamoyl phosphate synthetase and aspartate transcarbamoylase in plants, while UDP and UTP regulate CPSase II activity in animals. UDP-glucose regulates the conversion of glucose to galactose in the liver and other tissues in the process of carbohydrate metabolism.<ref name="Brown2"/> Uracil is also involved in the biosynthesis of polysaccharides and the transportation of sugars containing aldehydes.<ref name="Brown2">Template:Cite book</ref> Uracil is important for the detoxification of many carcinogens, for instance those found in tobacco smoke.<ref name="Olson">Template:Cite journal</ref> Uracil is also required to detoxify many drugs such as cannabinoids (THC)<ref name="Mazur">Template:Cite journal</ref> and morphine (opioids).<ref name="DeGregori">Template:Cite journal</ref> It can also slightly increase the risk for cancer in unusual cases in which the body is extremely deficient in folate.<ref name = "Mashiyama1"/> The deficiency in folate leads to increased ratio of deoxyuridine monophosphates (dUMP)/deoxythymidine monophosphates (dTMP) and uracil misincorporation into DNA and eventually low production of DNA.<ref name="Mashiyama1">Template:Cite journal</ref>

Uracil can be used for drug delivery and as a pharmaceutical. When elemental fluorine reacts with uracil, they produce 5-fluorouracil. 5-Fluorouracil is an anticancer drug (antimetabolite) used to masquerade as uracil during the nucleic acid replication process.<ref name="Garrett1"/> Because 5-fluorouracil is similar in shape to, but does not undergo the same chemistry as, uracil, the drug inhibits RNA transcription enzymes, thereby blocking RNA synthesis and stopping the growth of cancerous cells.<ref name="Garrett1"/> Uracil can also be used in the synthesis of caffeine.<ref>Template:Cite journal</ref> Uracil has also shown potential as a HIV viral capsid inhibitor.<ref>Template:Cite journal</ref> Uracil derivatives have antiviral, anti-tubercular and anti-leishmanial activity.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Uracil can be used to determine microbial contamination of tomatoes. The presence of uracil indicates lactic acid bacteria contamination of the fruit.<ref>Template:Cite journal</ref> Uracil derivatives containing a diazine ring are used in pesticides.<ref name = "Pozharskii1"/> Uracil derivatives are more often used as antiphotosynthetic herbicides, destroying weeds in cotton, sugar beet, turnips, soya, peas, sunflower crops, vineyards, berry plantations, and orchards.<ref name="Pozharskii1">Template:Cite book</ref> Uracil derivatives can enhance the activity of antimicrobial polysaccharides such as chitosan.<ref>Template:Cite journal</ref>

In yeast, uracil concentrations are inversely proportional to uracil permease.<ref>Template:Cite journal</ref>

Mixtures containing uracil are also commonly used to test reversed-phase HPLC columns. As uracil is essentially unretained by the non-polar stationary phase, this can be used to determine the dwell time (and subsequently dwell volume, given a known flow rate) of the system.

External links

• Uracil MS Spectrum