CYTOSINE: STRUCTURE, FUNCTIONS, PROPERTIES, SYNTHESIS - BIOLOGY - 2025

The cytosine is a pyrimidine nucleobase type, serving for the biosynthesis of cytidine-5'-monophosphate and deoxycytidine 5'-monophosphate. These compounds serve for the biosynthesis, respectively, of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA stores genetic information and RNA has various functions.

In living things, cytosine is not found free, but commonly forms ribonucleotides or deoxyribonucleotides. Both types of compound have a phosphate group, a ribose, and a nitrogen base.

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The carbon 2 of ribose has a hydroxyl group (-OH) in ribonucleotides, and a hydrogen atom (-H) in deoxyribonucleotides. Depending on the number of phosphate groups present, there is cytidine-5′-monophosphate (CMP), cytidine-5′-diphosphate (CDP) and cytidine-5′-triphosphate (CTP).

The deoxygenated equivalents are called deoxycytidin-5′-monophosphate (dCMP), deoxycytidin-5′-diphosphate (dCDP), and deoxycytidine-5′-triphosphate (dCTP).

Cytosine, in its various forms, participates in different functions, such as DNA and RNA biosynthesis, glycoprotein biosynthesis, and regulation of gene expression.

Structure and properties

Cytosine, 4-amino-2-hydroxypyrimidine, has the empirical formula C ₄ H ₅ N ₃ O, whose molecular weight is 111.10 g / mol, and is purified as a white powder.

The structure of cytosine is a planar aromatic heterocyclic ring. The wavelength of maximum absorbance (ʎ _max) is 260 nm. The melting temperature of cytosine exceeds 300ºC.

To form a nucleotide, cytosine is covalently attached, through nitrogen 1, via an N-beta-glycosidic bond to the 1 ′ carbon of ribose. The 5 ′ carbon is esterified with a phosphate group.

Biosynthesis

The nucleotide biosynthesis of pyrimidines has a common pathway, consisting of six enzyme-catalyzed steps. The pathway begins with carbamoyl phosphate biosynthesis. In prokaryotes there is only one enzyme: carbamoyl phosphate synthase. This is responsible for the synthesis of pyrimidines and glutamine. In eukaryotes, there are carbamoyl phosphate synthase I and II, which are responsible, respectively, for the biosynthesis of glutamine and pyrimidines.

The second step consists of the formation of N-carbamoylaspartate, from carboyl phosphate and aspartate, a reaction catalyzed by aspartate transcabamoylase (ATCase).

The third step is the synthesis of L-dihydrorotate, which causes the closure of the pyrimidine ring. This step is catalyzed by dihydrootase.

The fourth step is the formation of orotate, which is a redox reaction catalyzed by dihydroorotate dehydrogenase.

The fifth step consists of the formation of orotidylate (OMP) using phosphoribosyl pyrophosphate (PRPP) as a substrate, and orotate phosphoribosyl transferase as a catalyst.

The sixth step is the formation of uridylate (uridin-5′-monophosphate, UMP), a reaction catalyzed by an OMP-decarboxylase.

The next steps consist of the kinase-catalyzed phosphorylation of UMP to form UTP, and the transfer of an amino group from glutamine to UTP to form CTP, a reaction catalyzed by CTP synthetase.

Regulation of biosynthesis

In mammals, regulation occurs at the level of carbamoyl phosphate synthase II, an enzyme found in the cytosol, whereas carbamoyl phosphate synthase I is mitochondrial.

Carbamoyl phosphate synthase II is regulated by negative feedback. Its regulators, UTP and PRPP, are, respectively, inhibitor and activator of this enzyme.

In non-hepatic tissues, carbamoyl phosphate synthase II is the only source of carbamoyl phosphate. While in the liver, under conditions of excess ammonia, carbamoyl phosphate synthase I produces, in the mitochondria, carbamoyl phosphate, which is transported to the cytosol, from where it enters the pyrimidine biosynthesis pathway.

Another point of regulation is OMP-decarboxylase, which is regulated by competitive inhibition. Its reaction product, UMP, competes with OMP for the binding site on OMP-decarboxylase.

Pyrimidines, like cytosine, are recycled

The recycling of pyrimidines has the function of reusing pyrimidines without the need for de novo biosynthesis, and avoiding the degradative pathway. The recycling reaction is catalyzed by pyrimimidine phosphoribosyltransferase. The general reaction is as follows:

Pyrimidine + PRPP -> pyrimidine nucleoside 5′-monophosphate + PPi

In vertebrates, pyrimimidine phosphoribosyltransferase is found in erythrocytes. The substrate pyrimidines for this enzyme are uracil, thymine, and orotate. Cytosine is indirectly recycled from uridine-5′-monophosphate.

Role in DNA biosynthesis

During DNA replication, the information contained in DNA is copied into DNA by a DNA polymerase.

RNA biosynthesis requires deoxynucleotide triphosphate (dNTP), namely: deoxythymidine triphosphate (dTTP), deoxycytidine triphosphate (dCTP), deoxyadenine triphosphate (dATP) and deoxyguanine triphosphate (dGTP). The reaction is:

(DNA) _{n residues} + dNTP -> (DNA) _{n + 1} residue + PPi

The hydrolysis of inorganic pyrophosphate (PPi) provides the energy for RNA biosynthesis.

Role in stabilizing the structure of DNA

In the DNA double helix, a one-stranded purine is linked to the opposite-stranded pyrimidine by hydrogen bonds. Thus, cytosine is always linked to guanine by three hydrogen bonds: adenine is linked to thymine by two hydrogen bonds.

Hydrogen bonds are broken when a solution of purified native DNA, at pH 7, is subjected to temperatures above 80 ºC. This causes the DNA double helix to form two separate strands. This process is known as denaturation.

The temperature at which 50% of DNA is denatured is known as the melting temperature (Tm). DNA molecules whose ratio of guanine and cytosine is higher than that of thymine and adenine have higher Tm values ​​than those whose base ratio is inverse.

The above described constitutes the experimental proof that a greater number of hydrogen bonds better stabilize the native DNA molecules.

Function of cytosine-rich regions in DNA

Recently, it was found that DNA from the nucleus of human cells can adopt interspersed motif (iM) structures. These structures occur in regions rich in cytosine.

The iM structure consists of four strands of DNA, unlike classic double-stranded DNA which has two strands. More specifically, two parallel duplex chains are interspersed in an antiparallel orientation, and are held together by a pair of hemiprotonated cytosines (C: C ⁺).

In the human genome, iM structures are found in regions such as promoters and telomeres. The number of iM structures is higher during the G1 / S phase of the cell cycle, in which transcription is high. These regions are protein recognition sites involved in the activation of the transcriptional machinery.

On the other hand, in the regions rich in consecutive guanine base pairs (C), the DNA tends to adopt the A-helix shape, under dehydrating conditions. This shape is typical of RNA and DNA-RNA double bands during transcription and replication, and at certain times when DNA is bound to proteins.

Consecutive base regions of cytosine have been shown to create an electropositive patch in the major cleft of DNA. Thus, these regions are believed to bind to proteins, predisposing certain genomic regions to genetic fragility.

Role in RNA biosynthesis

During transcription, the information contained in DNA is copied into RNA by an RNA polymerase. RNA biosynthesis requires nucleoside triphosphate (NTP), namely: cytidine triphosphate (CTP), uridine triphosphate (UTP), adenine triphosphate (ATP) and guanine triphosphate (GTP). The reaction is:

(RNA) _{n residues} + NTP -> (RNA) _{n + 1} residue + PPi

The hydrolysis of inorganic pyrophosphate (PPi) provides the energy for RNA biosynthesis.

Role in glycoprotein biosynthesis

The sequential transfer of hexoses to form oligosaccharides, O-linked to proteins, occurs from nucleotide precursors.

In vertebrates, the last step of O-linked oligosaccharide biosynthesis consists of the addition of two sialic acid residues (N-acetylneuraminic) from a cytidine-5′-monophosphate (CMP) precursor. This reaction occurs in the trans Golgi sac.

Cytosine and cancer chemotherapeutic treatments

Tetrahydrofolate acid (FH4) is a source of -CH ₃ groups, and is necessary for the biosynthesis of dTMP from dUMP. In addition, FH2 is formed. The reduction of FH2 to FH4 ​​requires a reductase of folate and NADPH. Some folate reductase inhibitors, such as aminopterin and methotrexate, are used in cancer treatments.

Methotrexan is a competitive inhibitor. Folate reductase binds with 100 times more affinity to this inhibitor than to its substrate. Aminopterin works in a similar way.

The inhibition of folate reductase indirectly hinders the biosynthesis of dTMP, and therefore that of dCTP. Direct inhibition occurs by inhibitors of thymidylate synthetase enzyme, which catalyzes dTMP from dUMP. These inhibitors are 5-fluorouracil and 5-fluoro-2-deoxyuridine.

For example, 5-fluoroacyl is not itself an inhibitor but is first converted, in the recycling pathway, to deoxyuridine mphosphate d (FdUMP), which binds to and inhibits thymidylate synthetase.

Substances analogous to glutamine, azaserine and acivicin, inhibit glutamine amidotransferase. Azarin was one of the first substances discovered to act as a suicide inactivator.

References

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