Purines, Pyrimidines, Nucleosides, Nucleotides

• Purine and pyrimidine bases and their major derivatives, nucleosides and nucleotides, are the building blocks of nucleic acids.
• There are two main types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
• A nucleotide, the building block of nucleic acid, consists of a nitrogenous base (purine or pyrimidine), a pentose sugar (ribose or deoxyribose), and a phosphate group.

Nitrogenous Bases

• Nitrogenous bases are heterocyclic ring compounds containing carbon and nitrogen.
• There are two classes based on their abundance in cells: major bases and minor bases.
• Major bases are present in large amounts and include purines (adenine and guanine) and pyrimidines (cytosine, thymine, and uracil).
• Adenine and guanine are present in both DNA and RNA.
• Cytosine is present in both DNA and RNA.
• Thymine is present in DNA only.
• Uracil is present in RNA only.
• Minor bases are present in trace amounts and are always methylated, such as 5-methyl cytosine and N7-methylguanine.
• Minor bases serve important functions such as oligonucleotide recognition, regulating the half-life of RNAs, and protecting host DNA from digestion by DNases.
• Some bases exist in a free state inside cells, such as xanthine, hypoxanthine, and uric acid.
• Methylated xanthines include caffeine (1, 3, 7 trimethyl xanthine) in coffee, theophylline (1, 3 dimethyl xanthine) in tea, and theobromine (3, 7 dimethyl xanthine) in coca.

Nucleosides

• Nucleosides comprise a base and a sugar (ribose or deoxyribose).
• Sugars bind bases via a covalent, β-N-glycosidic bond that links the C₁ of the sugar with N-9 of a purine or N-1 of a pyrimidine.
• Numbering of the sugar atoms employs a prime (e.g., 3' or 5') to distinguish sugar atoms from those of the base.

Nucleotides

• A nucleotide consists of a base, sugar, and phosphate.
• Phosphate is attached to C-5 of the sugar.

Tautomerism

• Purines and pyrimidines exist in two isoforms: keto (or lactam) or enol (lactim) form, and amino (-NH₂) or imino (=NH) form.
• Tautomerism refers to the change from one form to the other.
• The keto and amino forms are common at physiological pH.

Syn and Anti-Conformers

• Nucleotides exist as stable, non-interconvertible syn and anti conforms.
• The β-N-glycosidic bond is a very firm bond with limited rotation.
• Interconversion from one form to another occurs only by rupture and reformation of the glycosidic bond.
• The anti-conformer predominates.

Bases, Nucleosides, and Nucleotides Nomenclature

• Adenine (A) becomes Adenosine, which becomes Adenosine monophosphate (AMP)
• Guanine (G) becomes Guanosine, which becomes Guanosine monophosphate (GMP)
• Xanthine (X) becomes Xanthosine, which becomes Xanthosine monophosphate (XMP)
• Hypoxanthine (I) becomes Inosine, which becomes Inosine monophosphate (IMP)
• Cytosine (C) becomes Cytidine, which becomes Cytidine monophosphate (CMP)
• Uracil (U) becomes Uridine, which becomes Uridine monophosphate (UMP)
• Thymine (T) becomes Thymidine, which becomes Thymidine monophosphate (TMP)

Functions of Nucleotides

• Both purine and pyrimidine nucleotides enter the structure of nucleic acids like DNA and RNA.
• Purine nucleotides enter the structure of high energy stores ATP and GTP and intracellular signals like cAMP and cGMP.
• Purine nucleotides enter the structure of many coenzymes such as NAD, NADP, FAD, and FMN, which act as hydrogen carriers.
• CoASH acts as an acid carrier.
• The active methyl donor is S-adenosyl methionine (SAM).
• The active sulfate donor is 3'-phosphoadenosine-5'-phosphosulfate (PAPS).
• ADP regulates the rate of oxidative phosphorylation in mitochondria.
• Pyrimidine nucleotides enter the structure of high energy intermediates like Uridine diphosphate glucose (UDP-glucose).
• UDP-glucose is used in glycogenesis and the uronic acid pathway.
• Pyrimidine nucleotides are high energy stores for CTP and UTP.

Learn about purine and pyrimidine bases, major derivatives, nucleosides, and nucleotides, the building blocks of nucleic acids. Explore the two main types of nucleic acids: DNA and RNA, and the components of a nucleotide: a nitrogenous base, a pentose sugar, and a phosphate group.