Ribonucleotide Definition
Ribonucleotide refers to a nucleotide that is the structural unit of nucleic acids and ribose forms the sugar component in that.
What is Ribonucleotide?
A nucleotide is the fundamental unit of the biomolecules DNA and RNA. Other biomolecules include proteins, fats, and carbohydrates. The genetic regulation occurs at the nucleic acid level, where they regulate and express molecular processes that result in inheritance. A nucleotide is formed by a pentose sugar (ribose or deoxyribose), nitrogenous bases (purines or pyrimidines), and a phosphate group. The nucleotides in RNA and DNA differ at the level of pentose sugar. Ribose sugar is incorporated in RNA, while deoxyribose forms part of nucleotides in DNA.
Ribonucleotide Properties
The nitrogenous nucleobases include guanine (G), adenine (A), uracil (U), and cytosine (C). The ribonucleotides can be classified based on the number of phosphates they comprise like monophosphate, diphosphate, and triphosphate. Ribonucleotides commonly seen include ADP, CMP, UTP, CTP, UMP, UTP, GTP, GMP, and ATP.
A nucleotide without a phosphate group is known as a nucleoside.
Ribonucleotide Biosynthesis
The nucleotides are generated by the de novo pathway and recycled by the salvage pathway. Inosine monophosphate (IMP) is the precursor of purines (guanine and adenine) and they are generated as ribonucleotides, unlike pyrimidines.
Glycine, aspartic acid, and glycine produce ribose phosphate that helps in synthesizing IMP. In reaction with ATP, Ribose 5-phosphate produces PRPP that is involved in both types of nitrogenous base synthesis along with the production of NADP and NAD. When PRRP gets its pyrophosphate replaced converted into by having the PRRP replaced by the amide group of glutamine it forms 5-phosphoribosyl amine and it is now committed to purine biosynthesis. This process happens in the liver cell cytoplasm, where IMP is further converted to GMP or AMP.
The biosynthesis of pyrimidines (cytosine and uracil) begins with carbamoyl phosphate that is formed by ATP, water molecules, bicarbonate, and glutamine in the presence of the enzyme carbamoyl phosphate synthetase II. Then, aspartate transcarbamylase catalysis is converted to carbamoyl aspartate that is then converted to dihydroorotate. Diihydroorotate is oxidized to ororate by dihydroorotate dehydrogenase.
Carbon 2 of pyrimidine ring is derived from bicarbonate ion, N3 from glutamine, and all rest of atoms from aspartate. After this OMP is formed by the reaction of ribose phosphate, PRPP, and orotate. It yields UMP by decarboxylation. Eventually, with kinases and ATPs, UDP and UTP are formed that can be converted to CTP by CTP synthase. Pyrimidine biosynthesis occurs in tissues of the thymus, spleen, and GI tract.
Ribonucleotide Biology
Guanine is degraded in a reaction catalyzed by guanase into xanthine. Xanthine is converted in the reaction catalyzed by xanthine oxidase and is finally converted ultimately in uric acid.
Adenosine in the presence of purine nucleoside phosphorylase is converted to inosine. It is converted in 2 steps to xanthine catalyzed by xanthine oxidase and then eventually into uric acid.
The degradation of exogenous purines in vertebrates occurs in the liver resulting in the production of uric acid that is excreted in the urine.
Purines can also be recovered from a salvage pathway. In this pathway enzyme APRT helps in salvaging Adenine. HGPRT enzyme is involved in the salvaging of hypoxanthine and Guanine.
The Pyrimidine salvage pathway also helps in recycling nucleobases. Cytosine is deaminated into uracil that is converted into uridine on reaction with ribose-1-phosphate. It is then converted into UMP.
On reaction with ribose-1-phosphate uracil can be recycled to form UMP in the presence of uridine monophosphate (UMP).
Ribonucleotide Importance
Ribonucleotides are the basic fundamental units of RNA. Ribonucleotides can be reduced by ribonucleotide reductase to form the precursors of deoxyribonucleotides that form DNA. They are also involved in cell signaling and cell regulation like AMP, along with nucleic acid biosynthesis.
AMP, in particular, may be phosphorylation yields ATP the energy currency of a cell that stores energy in its high-energy phosphate bonds. This stored chemical energy is released when it is hydrolyzed for different metabolic processes and is converted into ADP.
References
- Therapeutic Potential of NAD-Boosting Molecules: The In Vivo Evidence. Cell Metab . 2018 Mar 6;27(3):529-547.
- Ribonucleotide incorporation into DNA during DNA replication and its consequences. Crit Rev Biochem Mol Biol . 2021 Feb;56(1):109-124.
- Ribonucleotide discrimination by translesion synthesis DNA polymerases. Crit Rev Biochem Mol Biol . 2018 Aug;53(4):382-402.